INSERT INTO `node_revision__body` VALUES ('paper',0,409,601,'und',0,'<p align=\"center\" style=\"margin-top: 0.42cm; orphans: 0; widows: 0; page-break-after: avoid\">\r\n<br/>\r\n<br/>\r\n\r\n</p>\r\n\r\n	<p class=\"western\" align=\"justify\">This document aims to give a\r\n	reasonable explanation of most of the core mechanisms that make\r\n	Yaffs work. \r\n	</p>\r\n	<p class=\"western\" align=\"justify\">This document should serve as a\r\n	first port of call for understanding Yaffs.</p>\r\n\r\n<h1 class=\"western\">1 Purpose</h1>\r\n<p class=\"western\">The purpose of this document is to give a\r\nreasonable explanation of most of the core mechanisms that make Yaffs\r\nwork. As of the time of writing this, the Yaffs2 code base includes\r\napproximately 19,000 lines of code making a very detailed discussion\r\nimpractical.</p>\r\n<p class=\"western\">This document should serve as a first port of call\r\nfor understanding Yaffs. Further detailed questions and discussions\r\nare welcomed on the Yaffs discussion list.</p>\r\n<p class=\"western\">This document is updated periodically. Yaffs\r\nmechanism design changes from time to time and this document will not\r\nalways reflect the most up to date code. \r\n</p>\r\n<h1 class=\"western\">2 What is Yaffs?</h1>\r\n<p class=\"western\">Yaffs stands for <i>Yet Another Flash File System</i>,\r\na term coined by Charles Manning in 2001 when suggesting that the\r\nalready cluttered flash file system space could do with yet another\r\noffering – this time a flash file system designed from the ground\r\nup to work with NAND flash.</p>\r\n<p class=\"western\">The first release of Yaffs was developed during\r\nthe end of 2001 and the beginning of 2002. By mid-2002, Yaffs was\r\nbeing trialled in various products.  In May 2002,  Yaffs was\r\nannounced and a few more projects started playing with it. In June,\r\nporting to other OSs started. In September 2002, Yaffs was announced\r\non linuxdevices.com which started a far wider uptake of Yaffs.</p>\r\n<p class=\"western\">Yaffs1, the original version of Yaffs, supported\r\n512-byte page NAND devices with a SmartMedia-like memory layout.</p>\r\n<p class=\"western\">Yaffs2 is a follow-up to Yaffs1 extending Yaffs1\r\nto support larger and different devices with different constraints.</p>\r\n<p class=\"western\">Yaffsis highly portable and has been used in many\r\ndifferent products and applications as varied as sewing machines,\r\npoint of sale, phones and aerospace. Yaffs has been used with\r\nmultiple different operating systems. Native support is available for\r\nLinux, WindowsCE and eCOS while Yaffs Direct Interface provides a\r\nlayer that can be used in other applications such as RTOSs. Yaffs has\r\nbeen used with multiple different CPUs and compilers.</p>\r\n<p class=\"western\">Yaffs has also been used as a NOR file system and\r\neven as a RAM file system.</p>\r\n<h1 class=\"western\">3 Design and coding strategies</h1>\r\n<p class=\"western\">Yaffs is designed to work in multiple environments\r\nwhich drives the need for portability. Portability also improves the\r\ndebugging and testing since it is easier to develop and debug code in\r\nan application environment than within an OS kernel. This generally\r\nallows Yaffs to progress faster with less programming resources and\r\nprovides more varied testing. Primary strategies that improve\r\nportability include:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">No OS-specific features used in main code\r\n	body.</p>\r\n	<li/>\r\n<p class=\"western\">No compiler-specific features used in the\r\n	main code body.</p>\r\n	<li/>\r\n<p class=\"western\">Abstract types and functions used to allow\r\n	Unicode or ASCII operation.</p>\r\n</ul>\r\n\r\n<p class=\"western\" style=\"page-break-before: auto; page-break-after: avoid\">\r\nSimplicity is another key goal. Complexity is limited to those\r\nsituations that provide significant improvements to performance or\r\nutility. Simplicity improves robustness and the ease of integration\r\nand development. Primary strategies that improve simplicity are:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Single threaded model. Yaffs is locked on a\r\n	per-partition basis at a high level. This is simpler than tracking\r\n	lower-level locking. Yaffs Direct Interface uses a single lock for\r\n	all partitions.</p>\r\n</ul>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Log structure makes for a simpler garbage\r\n	collector and allocation methodology.</p>\r\n	<li/>\r\n<p class=\"western\">Abstractions that build layered code which\r\n	is easier to understand, port and debug.</p>\r\n</ul>\r\n<h1 class=\"western\">4 Terminology: Yaffs vs Yaffs1 vs Yaffs2</h1>\r\n<p class=\"western\">Before we embark on our quest to understand what\r\nYaffs is, a terminology explanation is in order.</p>\r\n<p class=\"western\">There are two versions of Yaffs code: the original\r\nYaffs code-base and the current Yaffs2 code-base. The Yaffs2\r\ncode-base supports functionality provided by the Yaffs code-base as\r\nwell as being extended with new functionality to provide extra modes\r\nof operation, most importantly those required to work with larger and\r\nmore modern NAND parts. The Yaffs2 code-base supports the Yaffs1 mode\r\nof operation through a backward compatibility mode.</p>\r\n<p class=\"western\">Unless explicitly mentioned, this text refers to\r\nthe Yaffs2 code-base.</p>\r\n<p class=\"western\">This text uses three terms Yaffs, Yaffs1 and\r\nYaffs2 depending on various modes of operation.</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Yaffs1\r\n	is a simpler mode of operation that uses deletion markers to track\r\n	state. </font>\r\n	</p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Yaffs2\r\n	is a more complex mode of operating that was developed to support\r\n	larger modern flash types, in particular MLC NAND, that cannot used\r\n	deletion markers.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Yaffs\r\n	means operations common to both.</font></p>\r\n</ul>\r\n<p class=\"western\">Yaffs1 originally only worked with 512-byte page\r\ndevices but has been extended to support larger flash pages such as\r\nIntel M18 flash with 1k pages.</p>\r\n<p class=\"western\">Yaffs2 was originally designed for 1k or larger\r\npage sizes but can be used with smaller pages by using in-band tags.</p>\r\n<p class=\"western\">Since the Yaffs1 mode of operation is simpler to\r\nunderstand than the Yaffs2 mode of operation,  Yaffs1 will be\r\ndescribed first and readers interested in Yaffs2 operation should\r\nfirst fully acquaint themselves with the Yaffs1 mode of operation. It\r\nis suggested that this document be read sequentially rather than\r\nbrowsing it.</p>\r\n<h1 class=\"western\">5 Objects</h1>\r\n<p class=\"western\">In Yaffs-speak, an Object is anything that is\r\nstored in the file system. These are:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Regular data files</p>\r\n	<li/>\r\n<p class=\"western\">Directories</p>\r\n	<li/>\r\n<p class=\"western\">Hard-links</p>\r\n	<li/>\r\n<p class=\"western\">Symbolic links</p>\r\n	<li/>\r\n<p class=\"western\">Special objects (pipes, devices etc).</p>\r\n</ul>\r\n<p class=\"western\">All objects are identified by a unique integer\r\nobject id (<i><b>obj_id</b></i><i><b>)</b></i>.</p>\r\n<p class=\"western\">In the standard POSIX model, as used by Linux,\r\nthere are inodes and directory entries (dentries). \r\n</p>\r\n<p class=\"western\">An inode may be a regular file, directory, or\r\nspecial file.</p>\r\n<p class=\"western\">Directory entries provide the naming mechanism\r\nthat is used to locate inodes.</p>\r\n<p class=\"western\">Under POSIX, each inode may have zero, one or many\r\ndentries:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">One dentry per inode is commonly\r\n	understood.</p>\r\n	<li/>\r\n<p class=\"western\">Multiple dentries per inode allow the same\r\n	inode to be accessed via multiple names. This is achieved through\r\n	hard links.</p>\r\n	<li/>\r\n<p class=\"western\">Zero dentries per inode is less often\r\n	understood. This happens when an inode is unlinked while there is\r\n	still a handle open to the inode. The unlinking deletes the dentry\r\n	but the inode still exists. As soon as the handle is closed the\r\n	inode is deleted too.</p>\r\n</ul>\r\n<h1 class=\"western\">6 The Yaffs NAND model</h1>\r\n<p class=\"western\">The memory in NAND flash is arranged in <i><b>pages.</b></i>\r\nA page is the unit of allocation and programming.  In Yaffs, the unit\r\nof allocation is the<i> </i><i><b>chunk</b></i>. Typically a chunk\r\nwill be the same as a NAND page, but there is flexibility to use\r\nchunks which map to multiple pages (eg. A system may have two NAND\r\nchips in parallel requiring 2x2048 = 4096 byte chunks) . This\r\ndistinction gives a lot of flexibility in how the system can be\r\nconfigured.  In this text the term page and chunk are synonymous\r\nunless stated otherwise.</p>\r\n<p class=\"western\">Many, typically 32 to 128 but as many as a few\r\nhundred, chunks form a <i><b>block</b></i>. A block is the unit of\r\nerasure. \r\n</p>\r\n<p class=\"western\">NAND flash may be shipped with bad blocks and\r\nfurther blocks may go bad during the operation of the device. Thus,\r\nYaffs is aware of bad blocks and needs to be able to detect and mark\r\nbad blocks.</p>\r\n<p class=\"western\">NAND flash also typically requires the use of some\r\nsort of error detection and correction code (ECC). Yaffs can either\r\nuse existing ECC logic or provide its own.</p>\r\n<h1 class=\"western\">7 How Yaffs1 stores files</h1>\r\n<p class=\"western\">Yaffs1 has a modified log structure, while Yaffs2\r\n(see below) has a true log structure. A true log structured file\r\nsystem only ever writes sequentially. Yaffs1 uses deletion markers,\r\nwhich breaks the sequential write rule. Yaffs2 does not use deletion\r\nmarkers and is thus a true log structured file system.</p>\r\n<p class=\"western\">Instead of writing data in locations specific to\r\nthe files, the file system data is written in the form of a\r\nsequential log, The entries in the log are all one chunk in size and\r\ncan hold one of two types of chunk:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><i><b>Data chunk</b></i>: A chunk holding\r\n	regular data file contents.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Object Header</b></i>: A descriptor\r\n	for an object (directory, regular data file, hard link, soft link,\r\n	special descriptor,...). This holds details such as the identifier\r\n	for the parent directory, object name, etc.</p>\r\n</ul>\r\n<p class=\"western\">Each chunk has <i><b>tags</b></i> associated with\r\nit. The tags comprise the following important fields (others being\r\nignored for now):</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><i><b>Object Id (</b></i><i><b>obj_id</b></i><i><b>)</b></i>:\r\n	Identifies which object the chunk belongs to.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Chunk Id (</b></i><i><b>chunk_id</b></i><i><b>)</b></i>:\r\n	Identifies where in the file this chunk belongs. A chunk_id of zero\r\n	signifies that this chunk contains an objectHeader. chunk_id==1\r\n	signifies the first chunk in the file (ie. At file offset 0),\r\n	chunk_id==2  is the next chunk and so on.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Deletion Marker (</b></i><i><b>is_deleted</b></i><i><b>)</b></i><i><b>:</b></i>\r\n	(Yaffs1 only)  Shows that this chunk is no longer in use.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Byte Count</b></i><b>: </b><b>(n_bytes)</b>:\r\n	Number of bytes of data if this is a data chunk.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Serial Number </b></i><i><b>(ser</b></i><i><b>ia</b></i><i><b>l_number)</b></i>:\r\n	 (Yaffs1 only) Serial number used to differentiate chunks with the\r\n	same obj_id and chunk_id.</p>\r\n</ul>\r\n\r\n<p class=\"western\" style=\"page-break-before: always\">Now let\'s bring\r\nthat all together in an explanation. This is a simplified explanation\r\nand skips over issues such as Soft Deletion (mentioned later in the\r\ndocument). For explanation purposes, we shall consider usage of\r\nfictitious NAND type that has 4 chunks per block, starting with an\r\nempty (erased) flash.</p>\r\n\r\n<p class=\"western\">First we\'ll create a file.</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"37\">\r\n	<col width=\"56\">\r\n	<col width=\"67\">\r\n	<col width=\"60\">\r\n	<col width=\"64\">\r\n	<col width=\"271\">\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">Block</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">Chunk</p>\r\n		</td>\r\n		<td width=\"67\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">obj_id</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">chunk_id</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">Deletion</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" height=\"26\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"67\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Object Header for this file (length 0)</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Next we write a few chunks of data to the file.</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"37\">\r\n	<col width=\"48\">\r\n	<col width=\"60\">\r\n	<col width=\"64\">\r\n	<col width=\"56\">\r\n	<col width=\"290\">\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Block</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Chunk</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">obj_id</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">chunk_id</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deletion</p>\r\n		</td>\r\n		<td width=\"290\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"290\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Object Header for t<font size=\"3\" style=\"font-size: 12pt\">his\r\n			file (length 0)</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"290\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">First chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"290\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Second chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"290\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Third chunk of data</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Next we close the file. This writers a new object\r\nheader for the file. Notice how the previous object header is\r\ndeleted.</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"37\">\r\n	<col width=\"48\">\r\n	<col width=\"60\">\r\n	<col width=\"68\">\r\n	<col width=\"60\">\r\n	<col width=\"282\">\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Block</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Chunk</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">obj_id</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">chunk_id</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deletion</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsoleted object header <font size=\"2\" style=\"font-size: 10pt\">(length\r\n			0)</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">First chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Second chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Third chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">New object header <font size=\"2\" style=\"font-size: 10pt\">(length\r\n			n)</font></p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\" style=\"page-break-before: always\">Let\'s now open\r\nthe file for read/write, overwrite part of the first chunk in the\r\nfile and close the file. The replaced data and object header chunks\r\nbecome deleted.</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\" style=\"page-break-inside: avoid\">\r\n	<col width=\"37\">\r\n	<col width=\"48\">\r\n	<col width=\"64\">\r\n	<col width=\"64\">\r\n	<col width=\"71\">\r\n	<col width=\"271\">\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Block</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Chunk</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">obj_id</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">chunk_id</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deletion</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete first chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Second chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Third chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">New first chunk of file</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">New object header</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Now let\'s resize the file to zero by opening the\r\nfile with O_TRUNC and closing the file. This writes a new object\r\nheader with length 0 and the contents are pruned making the data\r\nchunks deleted.</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"37\">\r\n	<col width=\"48\">\r\n	<col width=\"64\">\r\n	<col width=\"64\">\r\n	<col width=\"71\">\r\n	<col width=\"271\">\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Block</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Chunk</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">obj_id</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">chunk_id</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deletion</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete first chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Second chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Third chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Deleted first chunk of file</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Delted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsoleted object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">New object header <font size=\"2\" style=\"font-size: 10pt\">(length\r\n			0).</font></p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Note how all the pages in block 1 are now marked\r\nas deleted. This means that block 1 no longer contains useful\r\ninformation so we can now erase block 1 and reuse the space.</p>\r\n<p class=\"western\">We will now rename the file. To do this we write a\r\nnew object header for the file</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\" style=\"page-break-inside: avoid\">\r\n	<col width=\"38\">\r\n	<col width=\"42\">\r\n	<col width=\"58\">\r\n	<col width=\"60\">\r\n	<col width=\"60\">\r\n	<col width=\"297\">\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Block</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Chunk</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">obj_id</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">chunk_id</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Erased</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Erased</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Erased</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Erased</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Deleted first chunk of file</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsoleted object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"3\" sdnum=\"2057;\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">New object header showing new name</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Block 2 now contains only deleted chunks and can\r\nbe erased and reused.</p>\r\n<p class=\"western\">Note how the tags tell us:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Which\r\n	chunks belong to which object</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">The\r\n	position of the chunks within a file,</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Which\r\n	chunks are current</font></p>\r\n</ul>\r\n<ul>\r\n	<p class=\"western\"><font size=\"3\" style=\"font-size: 12pt\">With that\r\n	information we can recreate the state of the files regardless of the\r\n	placement of the chunks in NAND. This means that should the system\r\n	fail (crash, power fail,...) at any point during the sequences\r\n	above, we are still able to recover the file system state up to that\r\n	point.</font></p>\r\n</ul>\r\n<p class=\"western\"><font size=\"3\" style=\"font-size: 12pt\">Note that\r\nthere are no file allocation tables or similar structures stored in\r\nthe flash. This reduces the amount of writing and erasing (improving\r\nwrite performance) while also improving robustness. Corruption of\r\nfile allocation tables and similar structures is a common failure\r\nmechanism for embedded file systems. The lack of such structures\r\nmakes Yaffs more robust. Or, to put it another way, you can\'t corrupt\r\nwhat you don\'t store.</font></p>\r\n<h1 class=\"western\">8 Garbage collection</h1>\r\n<p class=\"western\">As we have shown so far, when a block is made up\r\nonly of deleted chunks, that block can be erased and reused. However,\r\nconsider what will happen if we have many blocks which each have a\r\nfew current chunks in them. Clearly we cannot erase any of the\r\ncurrent blocks or we will destroy data. What we need to do is copy\r\nthe useful data chunks off a block, deleting the originals and\r\nallowing the block to be erased and reused. This process is referred\r\nto as <i><b>garbage collection</b></i>.</p>\r\n<p class=\"western\">The Yaffs garbage collection process works as\r\nfollows:</p>\r\n<ol>\r\n	<li/>\r\n<p class=\"western\">Find a block worth collecting (according\r\n	the the heuristics below). If none found, then exit.</p>\r\n	<li/>\r\n<p class=\"western\">Iterate through the chunks in the block. If\r\n	the chunk is in use then create a new copy and delete the original.\r\n	Fix up the RAM data structures to reflect the change.</p>\r\n</ol>\r\n<p class=\"western\">Once all chunks in this block have been deleted,\r\nthe block can be erased ready for reuse.</p>\r\n<p class=\"western\">Note that although step 2 deletes the original\r\nchunk when it gets copied there is not much point in actually\r\nprogramming the deletion marker (is_deleted flag) in flash since\r\nwe\'re about to erase the block anyway. Thus Yaffs does not program\r\nthe deletion marker in these cases. If power is lost part way through\r\nthe operation we can still tell the difference between the original\r\nand the copy by looking at the serial number (not that it really\r\nmatters anyway since the two have the same contents so either can be\r\nused).</p>\r\n<p class=\"western\">The heuristics to determine whether a block is\r\nworth collecting are as follows:</p>\r\n<ol>\r\n	<li/>\r\n<p class=\"western\">If there are many erased blocks available,\r\n	then Yaffs does not need to work hard, but will instead only attempt\r\n	to perform garbage collection of blocks with very few chunks in use.\r\n	This is termed <i><b>passive</b></i> garbage collection.</p>\r\n	<li/>\r\n<p class=\"western\">If, however, there are very few erased\r\n	blocks, Yaffs will work much harder to recover more space and will\r\n	garbage collect blocks with more chunks in use. This is termed\r\n	<i><b>aggressive</b></i> garbage collection.</p>\r\n</ol>\r\n<p class=\"western\">OK, the passive/aggressive names are a bit silly,\r\nbut they were contrived around midnight... give me a break!</p>\r\n<p class=\"western\">If garbage collection is aggressive, we need\r\nerased space urgency and the whole block is collected in a single\r\ngarbage collection cycle. If the collection is passive then only a\r\nfew chunks are collected at a time thus spreading the effort over\r\nmany garbage collection cycles. This is done to reduce garbage\r\ncollection load and improve responsiveness.</p>\r\n<p class=\"western\">The rationale behind the above heuristics is to\r\ndelay garbage collection when possible to reduce the amount of\r\ncollection that needs to be performed, thus increasing average system\r\nperformance. Yet there is a conflicting goal of trying to spread the\r\ngarbage collection so that it does not all happen at the same causing\r\nfluctuations in file system throughput. These conflicting goals make\r\ngarbage tuning quite challenging.</p>\r\n<p class=\"western\">All flash file systems need some sort of garbage\r\ncollector to reclaim space. The garbage collector is often a\r\ndetermining factor in the performance of the flash file system. Many\r\nfile systems have to do a significant amount of effort in a single\r\ncollection sweep.</p>\r\n<p class=\"western\">Yaffs only deals with one block at a time which\r\nconstrains the amount of effort in a garbage collection cycle, thus\r\nreducing the “stall time”.  Under normal conditions that garbage\r\ncollection is performed a few chunks at a time, further reducing\r\n“stall time”.</p>\r\n<h1 class=\"western\">9 Yaffs1 serial numbers</h1>\r\n<p class=\"western\">In Yaffs1, each chunk is marked with a 2-bit\r\nserial number that is incremented every time a chunk with the same\r\ntags value is replaced (eg. When a chunk is overwritten or when\r\ncopied by garbage collection).</p>\r\n<p class=\"western\">Now let\'s suppose first that we did not have a\r\nserial number. \r\n</p>\r\n<p class=\"western\">Let\'s consider a rename where an object header\r\nchunk must be replaced with a new one with the new name. Yaffs could\r\ndo the following:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Delete\r\n	the old chunk.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Write\r\n	the new chunk. </font>\r\n	</p>\r\n</ul>\r\n<p class=\"western\">If the system fails after the delete, the system\r\ncould end up with no chunks matching the relevant tags values. This\r\ncould cause files to be lost (end up in the lost+found directory).\r\nThus, we have to write the new chunk before deleting the old chunk.</p>\r\n<p class=\"western\">But now consider what might happen if we did this:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">W<font face=\"Liberation Serif, serif\">rite\r\n	new chunk.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Delete\r\n	old chunk.</font></p>\r\n</ul>\r\n<p class=\"western\">If the system failed after the write we now have\r\ntwo chunks that match the tags. How can we tell which one is current?</p>\r\n<p class=\"western\">The current chunk is determined by inspecting the\r\nserial number.  Since the serial number is only incremented by one\r\nbefore the old chunk is deleted, a 2-bit serial number is sufficient.\r\nValid pairs are:</p>\r\n<table width=\"86\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"35\">\r\n	<col width=\"33\">\r\n	<tr valign=\"top\">\r\n		<td width=\"35\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Old</p>\r\n		</td>\r\n		<td width=\"33\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">New</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"35\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">00</p>\r\n		</td>\r\n		<td width=\"33\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">01</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"35\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">01</p>\r\n		</td>\r\n		<td width=\"33\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">10</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"35\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">10</p>\r\n		</td>\r\n		<td width=\"33\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">11</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"35\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">11</p>\r\n		</td>\r\n		<td width=\"33\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">00</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Thus, by inspecting the serial number attached to\r\nthe old and new tags the current chunk can be determined and the old\r\nchunk can be deleted.</p>\r\n<h1 class=\"western\">10 Yaffs2 NAND model</h1>\r\n<p class=\"western\">Yaffs2 is an extension of Yaffs designed to\r\nfulfill a new set of objectives to work with newer NAND types\r\nincluding:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Zero overwrites. Yaffs1 only ever\r\n	overwrites a single byte in the spare area of the chunks to set the\r\n	deletion marker. More modern NAND devices are less tolerant of\r\n	overwrites and Yaffs2 performs no overwrites at all.</p>\r\n	<li/>\r\n<p class=\"western\">Sequential chunk writing within a block.\r\n	More modern NAND reliability specifications tend to assume\r\n	sequential writing. Since Yaffs2 does not write deletion markers,\r\n	the writing of chunks within a block is strictly sequential.</p>\r\n	<li/>\r\n<p class=\"western\">Some flash technologies – such as MLC –\r\n	only work properly if pages are written sequentially. This is a\r\n	fundamental requirement of how many MLC types perform the bit\r\n	overlaying.</p>\r\n</ul>\r\n<p class=\"western\">Since Yaffs2 performs less writes, there is\r\npotential for Yaffs2 to have improved write performance.</p>\r\n<p class=\"western\">Yaffs2 cannot use a deletion marker because that\r\nwould violate the zero overwrites mandate and alternate mechanisms\r\nare provided to determine which chunks are current and which are\r\ndeleted. These mechanisms are:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><i><b>Sequence Number </b></i><i><b>(seq_number)</b></i>:\r\n	As each block is allocated, the file system\'s sequence number is\r\n	incremented and each chunk in the block is marked with that sequence\r\n	number. The sequence number thus provides a way of organising the\r\n	log in chronological order.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Shrink Header Marker </b></i><i><b>(shrink_header)</b></i>:\r\n	The shrink header marker is used to mark object headers that are\r\n	written to shrink the size of a data file. This is explained more in\r\n	the accompanying text.</p>\r\n</ul>\r\n\r\n<p class=\"western\" align=\"center\"><i><b>Note: The Yaffs2 sequence\r\nnumber is not the same as the Yaffs1 serial number!</b></i></p>\r\n\r\n<p class=\"western\">Note: We still refer to chunk deletion in Yaffs2.\r\nThe chunks are marked as deleted in the RAM data structures used by\r\ngarbage collection etc, but there is no deletion marker written to\r\nflash.</p>\r\n<h2 class=\"western\">10.1 Sequence Number</h2>\r\n<ul>\r\n	<p class=\"western\">The sequence number allows the chronological\r\n	order to be determined which allows Yaffs to determine the sequence\r\n	of events and restore the file system state. This is achieved by\r\n	scanning <i><b>backwards</b></i> chronologically (ie. From the\r\n	highest sequence number backwards to the lowest sequence number).\r\n	Thus:</p>\r\n	<li/>\r\n<p class=\"western\">Since we\'re scanning backwards, the most\r\n	recently written – and thus current – chunk matching an\r\n	obj_id:chunk_id pair will be encountered first and all subsequent\r\n	matching chunks must be obsolete and treated as deleted.</p>\r\n	<li/>\r\n<p class=\"western\">The file length in object headers are used\r\n	to trim back resized files. If an object header is encountered, then\r\n	subsequent data chunks that lie beyond that file length are clearly\r\n	obsolete and treated as deleted. Note that both current and obsolete\r\n	object header sizes must be used to obtain an accurate\r\n	reconstruction.</p>\r\n</ul>\r\n<h2 class=\"western\">10.2 Shrink Header</h2>\r\n<p class=\"western\">Explaining the purpose of the shrink header marker\r\nis a bit convoluted. The purpose of a shrink header marker is to show\r\nthat this object header indicates that a file has shrunken in size,\r\nand to prevent those object headers from being deleted by garbage\r\ncollection.  To understand this, it is easiest to consider what would\r\nhappen if the shrink header markers did not exist.</p>\r\n<p class=\"western\">For simplification we\'ll assume no garbage\r\ncollection happens during this part of the exercise. It is safe to\r\nmake that assumption since it does not alter the reasoning.</p>\r\n<p class=\"western\">Consider a file that has been through the\r\nfollowing:</p>\r\n<p class=\"code-western\"><font face=\"Liberation Mono, monospace\"><font size=\"2\" style=\"font-size: 10pt\"><b>#define\r\nMB (1024 * 1024)<br/>\r\n</b></font></font><br/>\r\n\r\n</p>\r\n<p class=\"code-western\"><code class=\"western\"><font face=\"Liberation Mono, monospace\"><font size=\"2\" style=\"font-size: 9pt\">h\r\n= open(“foo”,O_CREAT| O_RDWR, S_IREAD|S_IWRITE); /* create file\r\n*/</font></font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\"><br/>\r\nwrite(h,data,5*</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">MB</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">);\r\n/* write 5 MB of data /<br/>\r\ntruncate(h,1*</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">MB</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">);\r\n /* truncate to 1MB */<br/>\r\nlseek(h,2*</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">MB</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">,\r\nSEEK_SET); /* set the file access position to 2MB\r\n*/<br/>\r\nwrite(h,data,1*</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">MB</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">);\r\n/* write 1MB of data */<br/>\r\nclose(h);</font></code><br/>\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">The file will now be 3MB in length, but there will\r\nbe a “hole” between 1MB and 2MB. According to POSIX (and to aid\r\nsecurity etc) that “hole” should always read back as zeros.</p>\r\n<p class=\"western\">Yaffs2 would represent that in NAND with the\r\nfollowing sequence of chunks:</p>\r\n<ol>\r\n	<li/>\r\n<p class=\"western\">Object header for file creation (file\r\n	length = 0)</p>\r\n	<li/>\r\n<p class=\"western\">5MB worth of data chunks (0 to 5MB)</p>\r\n	<li/>\r\n<p class=\"western\">Object header for truncation (file length =\r\n	1MB)</p>\r\n	<li/>\r\n<p class=\"western\">1MB worth of data chunks (2MB to 3MB)</p>\r\n	<li/>\r\n<p class=\"western\">Object header for close (file length = 3MB)</p>\r\n	<p class=\"western\">At this stage, only the the following data chunks\r\n	are current:</p>\r\n</ol>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">The first 1MB of data created in step 2\r\n	above.</p>\r\n	<li/>\r\n<p class=\"western\">The 1MB of data created in step 4 above.</p>\r\n	<li/>\r\n<p class=\"western\">The object header created in step 5 above.</p>\r\n</ul>\r\n<p class=\"western\">But Yaffs must remember the truncation that\r\nhappened in step 3, otherwise Yaffs will “forget”  that there is\r\na hole, which would cause the old data to be visible again. Thus, if\r\nYaffs creates a hole it first writes a shrink header to indicate the\r\nstart of the hole and a regular object header to mark the end of the\r\nhole. \r\n</p>\r\n<p class=\"western\">The shrink header modifies the behavior of the\r\ngarbage collector to ensure that  these shrink headers are not erased\r\nuntil it is safe to do so. This unfortunately has a negative impact\r\non garbage collection in systems which make extensive use of files\r\nwith holes.</p>\r\n<p class=\"western\">Shrink headers are also used to indicate that a\r\nfile has been deleted and that the record of the file deletion is not\r\nlost.</p>\r\n<h2 class=\"western\">10.3 Yaffs2 extended tags</h2>\r\n<p class=\"western\">Yaffs2 extends the tags in object headers with\r\nextra fields to improve mount scanning performance. The operation of\r\nextended tags is described later in this document.</p>\r\n<h1 class=\"western\">11 Bad block handling NAND error handling</h1>\r\n<p class=\"western\">NAND flash is designed to be very low cost and\r\nvery high density. In order to get high yields, and thus keep costs\r\ndown, NAND flash is typically shipped with a few bad blocks and some\r\nfurther blocks are expected to go bad during the use of the device.\r\nThus, any flash file system without an effective bad block handling\r\npolicy is unsuitable for NAND flash.</p>\r\n<p class=\"western\">Yaffs1 uses the Smart-Media style of bad block\r\nmarking. This checks the sixth byte (byte 5) of the spare area. In a\r\ngood block this should be 0xFF. A factory-marked bad block should be\r\n0x00. If Yaffs1 determines that a block has gone bad it is marked bad\r\nwith 0x59 (\'Y\'). Using a distinctive marker allows Yaffs-marked bad\r\nblocks to be differentiated from factory-marked bad blocks.</p>\r\n<p class=\"western\">Yaffs2 mode is designed to support a wider range\r\nof devices and spare-area layouts. Thus, Yaffs2 does not decide which\r\nbytes to mark but instead calls driver functions to determine whether\r\nthe block is bad, or to mark it bad.</p>\r\n<p class=\"western\">So, when does Yaffs mark a block bad? Yaffs will\r\nmark a block bad if a read or write operation fails or if three ECC\r\nerrors are detected. A block that is marked bad is retired from use,\r\nthus improving the robustness of the file system. This policy is\r\nsuitable for SLC flash. MLC policies are under development.</p>\r\n<p class=\"western\">Further, NAND flash cells can sometimes be\r\ndisturbed (corrupted) by NAND activity and charge loss.  These errors\r\nare corrected by error correction codes (ECC) which may be\r\nimplemented in hardware, software drivers, or within Yaffs itself.\r\nAgain, any flash file system lacking effective ECC handling is\r\nunsuitable for NAND flash.</p>\r\n<p class=\"western\">Yaffs1 mode can use either the built in ECC or use\r\nECC provided by the driver or hardware.</p>\r\n<p class=\"western\">Since Yaffs2 mode is designed for a wider range of\r\ndevices, it does not provide ECC internally but requires that the\r\ndriver provide the ECC.</p>\r\n<p class=\"western\">The ECC code supplied with Yaffs (yaffs_ecc.c) is\r\nthe fastest C code implementation of a Smart Media compatible ECC\r\nalgorithm that we are aware of. This code will correct any single bit\r\nerror within a 256-byte data block and detect 2 errors per 256-byte\r\ndata block. This is sufficient error correction to provide very high\r\nreliability on most SLC-type NAND flash.</p>\r\n<p class=\"western\">For a more complete description of failure\r\nhandling, please refer to the Yaffs error mitigation document.</p>\r\n<h1 class=\"western\">12 RAM structures</h1>\r\n<p class=\"western\">While it is theoretically possible to use a log\r\nstructured file system with very few RAM structures, this would\r\nprovide reduce performance. Thus, significant RAM data structures are\r\nrequired to store enough information to provide sufficiently high\r\nperformance. \r\n</p>\r\n<p class=\"western\">Before we jump in, it is a good idea to first\r\nfigure out what purpose the RAM structures serve. The main structures\r\nare defined in yaffs_guts.h and their main purposes, are :</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Device/partition: This is named <i><b>yaffs_dev</b></i><span style=\"font-style: normal\"><span style=\"font-weight: normal\">.\r\n	</span></span>This is required to hold information relating to a\r\n	Yaffs “partition” or “mount point”. Using these, rather than\r\n	globals, allows Yaffs to simultaneously support multiple partitions\r\n	and partitions of different types. In fact, almost other data\r\n	structures mentioned here are part of, or accessed via, the\r\n	structure.</p>\r\n	<li/>\r\n<p class=\"western\">NAND Block Information: This is named\r\n	<i><b>yaffs_block_info </b></i>and holds the current state of the\r\n	NAND blocks. Each yaffs_dev has an array of these.</p>\r\n	<li/>\r\n<p class=\"western\">NAND Chunk Information: This is a bitfield\r\n	attached to the yaffs_dev that holds the current in-use state of\r\n	each chunk in the system. There is one bit per chunk in the\r\n	partition.</p>\r\n	<li/>\r\n<p class=\"western\">Object: This is named <i><b>yaffs_obj</b></i>\r\n	and holds the state of an object (see section 3). There is one of\r\n	these for each object in the file system, where an object is one of\r\n	a regular file, directory, hard link, symbolic link or special link.\r\n	There are different variants of yaffs_Objects to reflect the\r\n	different data required for these different object types. Each\r\n	object is uniquely identified by an obj_id.</p>\r\n	<li/>\r\n<p class=\"western\">File structure: For each file object, Yaffs\r\n	holds a tree which provides the mechanism to find the data chunks in\r\n	a file. The tree consists of nodes called <i><b>yaffs_tnode</b></i>\r\n	(tree nodes).</p>\r\n	<li/>\r\n<p class=\"western\">Directory structure: The directory\r\n	structure allows objects to be found by name. The directory\r\n	structure is built from a doubly-linked list that is rooted in the\r\n	directory and binds together the sibling objects within a directory.</p>\r\n	<li/>\r\n<p class=\"western\">Object number hash table: The object number\r\n	hash table provides a mechanism to find an object from its obj_id.\r\n	The obj_id is hashed to select a hash bucket. Each hash bucket has a\r\n	doubly-linked list of objects that belong in this hash bucket, as\r\n	well as a count of the objects in the bucket.</p>\r\n	<li/>\r\n<p class=\"western\">Cache: Yaffs provides a read/write cache\r\n	that significantly improves performance for short operations. The\r\n	size of the cache can be set at run-time.</p>\r\n</ul>\r\n<p class=\"western\">The usage of these structures is described in more\r\ndetail below.</p>\r\n<p class=\"western\">Yaffs makes significant use of doubly-linked\r\nlists. These data structures are very useful because they provide\r\nrapid insertion, removal and traversal. They do however come at the\r\nexpense of requiring two pointers rather than just one that would be\r\nrequired by a singly-linked list.</p>\r\n<h2 class=\"western\">12.1 Yaffs objects (struct yaffs_obj)</h2>\r\n<p class=\"western\">Each object (file, directory,...) in the system is\r\nrepresented by a yaffs_obj structure \r\n</p>\r\n<p class=\"western\">The yaffs_obj\'s main function is to store most of\r\nthe object metadata and type-specific information. This means that\r\nalmost all the information about an object can be accessed\r\nimmediately without reading flash.</p>\r\n<p class=\"western\">The metadata includes, amongst others:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">obj_id:\r\n	The number used to identify the object.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">parent:\r\n	A pointer to the parent directory. Not all objects have a parent.\r\n	The root director and special directories for unlinked and deleted\r\n	files do not have parents so this is NULL.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">short_name:\r\n	If the name is short enough to fit in a small fixed-sized array\r\n	(default 16 characters) then it is stored here otherwise it must be\r\n	fetched from flash each time it is requested.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">type:\r\n	Type of object.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Permissions,\r\n	time, ownership and other attributes</font></p>\r\n</ul>\r\n<p class=\"western\"><font size=\"3\" style=\"font-size: 12pt\">Depending\r\non the object type, yaffs_obj also stores:</font></p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">A\r\n	tnode tree, file extents etc for data files</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">A\r\n	directory chain for directories</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">An\r\n	equivalent object for hard links</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">A\r\n	string for symbolic links</font></p>\r\n</ul>\r\n<h2 class=\"western\">12.2 Look-up by obj_id</h2>\r\n<p class=\"western\">The purpose of this mechanism is to rapidly access\r\nan object by its device and obj_id. This is required during garbage\r\ncollection, scanning and similar operations.</p>\r\n<p class=\"western\">Each yaffs_dev uses a hash table for this purpose.\r\nThe hash table has 256 “buckets” (tunable), each holding a\r\ndoubly-linked list of the yaffs_objs in that bucket.</p>\r\n<p class=\"western\">The hash table\'s hash function is just masking the\r\nleast significant bits of the obj_id. This is a very cheap function\r\nthat provides reasonable “spread”.</p>\r\n<p class=\"western\">Yaffs makes an attempt to assign obj_ids in a way\r\nthat keeps the number of objects in each hash bucket quite low, thus\r\npreventing any one hash entry getting too long.</p>\r\n<p class=\"western\">This mechanism is also charged with assigning new\r\n(unique) obj_id values.</p>\r\n<h2 class=\"western\">12.3 Directory structure</h2>\r\n<p class=\"western\">The purpose of the directory structure is to\r\nrapidly access an object by name. This is required to perform file\r\noperations such as opening, renaming etc.</p>\r\n<p class=\"western\"><font size=\"3\" style=\"font-size: 12pt\">The Yaffs\r\ndirectory structure is formed by a tree of objects of type\r\nYAFFS_OBJECT_TYPE_DIRECTORY. These objects have a doubly linked node\r\nof children.</font></p>\r\n<p class=\"western\">Each yaffs_obj also has a doubly linked list node\r\ncalled sibling which chain together the siblings in a directory.</p>\r\n<p class=\"western\">Each Yaffs partition has a root directory.</p>\r\n<p class=\"western\">Each yaffs partition also has some special purpose\r\n“fake” directories which are not stored in NAND but are instead\r\ncreated on mounting:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">lost+found: This directory is used as a\r\n	place to store any lost file parts that cannot be placed in the\r\n	normal directory tree.</p>\r\n	<li/>\r\n<p class=\"western\">Unlinked and deleted: These are\r\n	pseudo-directories that are not in the directory tree. Placing\r\n	objects in these directories gives them a status of being unlinked\r\n	or deleted.</p>\r\n</ul>\r\n<p class=\"western\" align=\"left\">Each object has a name that is used\r\nto look up the object within the directory, Thus, each name within a\r\ndirectory must be unique. This rule does not apply to objects in the\r\nunlinked and deleted pseudo-directories since we never use names to\r\nlook up unlinked/deleted files and there may be many files of the\r\nsame name that have been deleted.</p>\r\n<p class=\"western\">Name resolution is sped up by two mechanisms:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">short_names\r\n	are stored directly in the yaffs_obj so that they don\'t have to be\r\n	loaded from flash.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Each\r\n	object has a “name sum”. This is a simple hashing of the name\r\n	which speeds up the matching process because it is cheaper to\r\n	compare the hash than compare the whole name string.</font></p>\r\n</ul>\r\n<p class=\"western\"><img src=\"data:image/png;base64,[encoded data, 22776 characters]\" name=\"Object2\" align=\"left\" width=\"412\" height=\"355\">\r\n  <br clear=\"left\"/>\r\n</img>\r\n<br/>\r\n<br/>\r\n\r\n</p>\r\n\r\n<p class=\"western\">The above directory structure represents the\r\nfollowing directory tree</p>\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"119\">\r\n	<col width=\"468\">\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Root directory</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/a</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Directory containing c,d,e</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/b</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Empty directory</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/lost+found</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Empty directory</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/a/c</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Directory</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/a/d</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Data file</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/a/e</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Data file</font></p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n<p class=\"western\" align=\"left\" style=\"margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">If a Yaffs partition has been somehow corrupted\r\nthen it is possible for an object to be created during scanning but\r\nwith no object header (which contains the object name) being found.\r\nThe object would then not have a name. In these cases, Yaffs creates\r\na pseudonym from the object number of the form objnnnn so that the\r\nobject can be placed in the directory structure. \r\n</p>\r\n\r\n<h2 class=\"western\" style=\"page-break-before: always\">12.4 Hard links</h2>\r\n<p class=\"western\">In POSIX file systems, the objects (inodes in\r\nLinux terminology, vnode in BSD-speak) and the object names (dentry\r\nin Linux terminology)  are independent in that an object can have\r\nzero, one or more names.</p>\r\n<p class=\"western\">Having files with zero links is caused by\r\nconstructs such as:</p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">h\r\n= open(“foo”,O_CREAT | O_RDWR, S_IREAD | S_IWRITE); /* create\r\nfile “foo” */</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">unlink(“foo”);	/*\r\nunlink it </font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">to\r\nremove</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">\r\ndirectory </font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">entry\r\n</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">*/</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">/*\r\nwe still have a handle to the file so can still access it via the\r\nhandle */</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">read(h,buffer,100);	</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">write(h,buffer,100);</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">close(h);\r\n/* as soon as the handle is closed the file is deleted */</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">Yaffs does not store separate object and name\r\nrecords because that would waste space and incur extra\r\nreading/writing. Instead, Yaffs is optimised for the most typical\r\ncase of one name per object. That is, Yaffs stores one name per\r\nyaffs_obj and uses “cheats” to achieve other behavior:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Objects that have zero names (ie unlinked\r\n	objects) are handled by renaming the object into a fake directory\r\n	that is used to designate unlinked objects.</p>\r\n	<li/>\r\n<p class=\"western\">Multiple names object are handled by\r\n	designating one of the objects the main object and having multiple\r\n	hard link pseudo-objects that refer to it.</p>\r\n	<li/>\r\n<p class=\"western\">The inode number (vnid in BSD-speak) is a\r\n	per-file system unique number that identifies the object. The Yaffs\r\n	obj_id of the main object is presented to the user. That is, if the\r\n	inode number is queried from a hardlink object then the obj_id of\r\n	the equivalent object is returned.</p>\r\n</ul>\r\n<p class=\"western\">The linkage between hard link pseudo-objects and\r\ntheir equivalent object is through two fields:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Each object has a hard_links doubly linked\r\n	list node that links the list of hard links to that object.</p>\r\n	<li/>\r\n<p class=\"western\">Each hard link has a equiv_obj pointer to\r\n	the “equivalent object”.</p>\r\n</ul>\r\n\r\n<p class=\"western\" style=\"page-break-before: always\">The usage of\r\nthis structure is straight forward except for one interesting case\r\nwhen hard links are being deleted. Consider the following sequence of\r\nevents:</p>\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"50\">\r\n	<col width=\"80\">\r\n	<col width=\"449\">\r\n	<tr valign=\"top\">\r\n		<td width=\"50\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">1</font></p>\r\n		</td>\r\n		<td width=\"80\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\"># touch a</font></p>\r\n		</td>\r\n		<td width=\"449\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Create file a</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"50\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">2</font></p>\r\n		</td>\r\n		<td width=\"80\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\"># ln a b</font></p>\r\n		</td>\r\n		<td width=\"449\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Create a hard link b with\r\n			equivalent object a.</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"50\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">3</font></p>\r\n		</td>\r\n		<td width=\"80\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\"># rm a</font></p>\r\n		</td>\r\n		<td width=\"449\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Remove a. The object must\r\n			still exist under the name b</font></p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n<p class=\"western\" align=\"left\" style=\"margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">In step 3, Yaffs cannot just delete the equivalent\r\nobject since that would leave a “hanging” hard link with no\r\nequivalent object. \r\n</p>\r\n<p class=\"western\">We also cannot just change the type of one of the\r\nhardlink objects because that would cause the object\'s obj_id to\r\nchange. This would be unacceptable since it would break any mechanism\r\nthat use the obj_id (of which there are many within and outside of\r\nYaffs).</p>\r\n<p class=\"western\">Thus, if an object is deleted and it has hard\r\nlinks to it then we do the following:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Select one of the object\'s hard links.</p>\r\n	<li/>\r\n<p class=\"western\">Rename the object to the name in the hard\r\n	link.</p>\r\n	<li/>\r\n<p class=\"western\">Delete the hard link pseudo-object.</p>\r\n</ul>\r\n<h2 class=\"western\">12.5 Symbolic links and special objects</h2>\r\n<p class=\"western\">These object types just store values that are\r\nneeded to provide a full POSIX file system.</p>\r\n<p class=\"western\">A symbolic link object saves a symbolic link\r\nstring that is used to provide the POSIX symbolic string mechanism.</p>\r\n<p class=\"western\">A special object is used to store entries for\r\ndevice numbers, pipes and other similar special objects.</p>\r\n<h2 class=\"western\">12.6 File objects</h2>\r\n<p class=\"western\">The yaffs_Objects have a type and a variant\r\nportion which is a union of different data required to hold the state\r\nof different object types.</p>\r\n<p class=\"western\">The file objects have type YAFFS_OBJECT_TYPE_FILE,\r\nand an associated file variant. This stores the following main\r\nvalues:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">file_size:\r\n	file size</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">top_level:\r\n	depth of the Tnode tree</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">top:\r\n	pointer to top of Tnode tree</font></p>\r\n</ul>\r\n<p class=\"western\">The meaning of the file size is quite obvious.\r\nThis is the current length of the file. Note that a file might have\r\nholes in it caused by seeking forward past the extents of the file.</p>\r\n<p class=\"western\">The top and top_level work together to control the\r\nTnode trees. \r\n</p>\r\n<h3 class=\"western\" style=\"page-break-before: always\">12.6.1 Tnode\r\ntree</h3>\r\n<p class=\"western\">Each file has a tnode tree to provide the mapping\r\nfrom file position to actual NAND chunk address. top is a pointer to\r\nthe top of the tnode tree</p>\r\n\r\n<p class=\"western\"><img src=\"data:image/png;base64,[encoded data, 20236 characters]\" name=\"Object1\" align=\"left\" width=\"385\" height=\"378\">\r\n  <br clear=\"left\"/>\r\n</img>\r\n<br/>\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">The tnode tree is made up of tnodes (tree nodes)\r\narranged in levels, each of which holds either:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">At levels greater than 0, a tnode has 8\r\n	pointers to other tnodes in the next level down.</p>\r\n	<li/>\r\n<p class=\"western\">At level 0, a tnode has 16 NAND chunk Ids\r\n	which identify the chunk\'s location in RAM.</p>\r\n</ul>\r\n<p class=\"western\">These numbers are powers of 2 which makes for\r\nsimple look-ups by just applying bitmasks. The code that traverses\r\nthis tree is in yaffs_find_tnode_0().</p>\r\n<p class=\"western\">As the file size grows, more tnodes are added and\r\nas the levels fill, more levels are added. If the file is truncated\r\nor resized downwards, then the tree is pruned. If a chunk in a file\r\nis replaced (eg. data is overwritten or copied by garbage collection)\r\nthen the tnode tree must be updated to reflect the change.</p>\r\n<p class=\"western\">Tnodes typically make up the bulk of the Yaffs\'\r\nRAM usage.</p>\r\n<h1 class=\"western\">13 How various mechanisms work</h1>\r\n<p class=\"western\">Having looked at the Yaffs data structures, we now\r\nconsider how some of the mechanisms in Yaffs operates. This is most\r\neasily done by walking through some operations that Yaffs performs.</p>\r\n<h2 class=\"western\">13.1 Block and chunk management</h2>\r\n<h3 class=\"western\">13.1.1 Block states</h3>\r\n<p class=\"western\">Yaffs tracks the state of each block and chunk in\r\nthe partition. This information is first built up during scanning (or\r\nrecovered from a checkpoint). A block may be in one of the following\r\nstates (see definition of  enum yaffs_block_state):</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"146\">\r\n	<col width=\"441\">\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">UNKNOWN</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">The block state is not yet known.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">NEEDS_SCANNING</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">During pre-scanning it has been determined that\r\n			this block has something on it and needs scanning.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">SCANNING</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This block is currently being scanned.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">EMPTY</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This block has nothing in it (is erased) and\r\n			may be used for allocation. The block can get to this state by\r\n			being erased.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">ALLOCATING</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This is the block currently being used for\r\n			allocation by the chunk allocator.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">FULL</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">All the chunks in this block have been\r\n			allocated and at least one chunk contains useful information that\r\n			has not yet been deleted.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">DIRTY</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">All the chunks in this block have been\r\n			allocated and all have been deleted. The block might have been in\r\n			the FULL or COLLECTING state before this. This block can now be\r\n			erased and returned to EMPTY state.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">CHECKPOINT</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This block has checkpoint data in it. When the\r\n			checkpoint is invalidated this block will be erased and returned\r\n			to the EMPTY state.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">COLLECTING</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This block is being garbage collected. As soon\r\n			as all live data has been inspected this block becomes DIRTY.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">DEAD</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This block has been retired or was marked bad.\r\n			This is a terminal state.</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n<p class=\"western\"><img src=\"data:image/png;base64,[encoded data, 42088 characters]\" name=\"Object3\" align=\"left\" width=\"575\" height=\"510\">\r\n  <br clear=\"left\"/>\r\n</img>\r\n<br/>\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">The regular runtime states are; EMPTY, ALLOCATING,\r\nFULL, COLLECTING and DIRTY.</p>\r\n<p class=\"western\">The checkpoint state is used for checkpoint\r\nblocks.</p>\r\n<p class=\"western\">The DEAD state is the terminal state for blocks\r\nthat were retired because they were either marked bad or an error was\r\ndetected.</p>\r\n<h3 class=\"western\">13.1.2 Block and chunk allocation</h3>\r\n<p class=\"western\">An available chunk must be allocated before it can\r\nbe written. The chunk allocation mechanism is provided by\r\nyaffs_alloc_chunk() and is very simple. Each yaffs_dev has a current\r\nblock that it is allocating from. This is termed the <i><b>allocation\r\nblock </b></i><i><b>(alloc_block)</b></i>. Chunks are allocated\r\nsequentially from the allocation block. As the chunks are allocated,\r\nblock and chunk management information is updated, including the\r\nblock\'s pages_in_use count and the chunk use bitmap.</p>\r\n<p class=\"western\">When the block is fully allocated, another empty\r\nblock is selected to become the allocation block. The blocks are\r\nselected by searching upwards from the previous allocation block.</p>\r\n<p class=\"western\">In older versions of Yaffs, the Yaffs scanning\r\nwould identify the allocation block being used when the file system\r\nwas last shut down and would continue allocation from that point.\r\nThis had the benefit of continuing to use the same block and slightly\r\nreduce garbage creation. However, doing this could potentially cause\r\nproblems due to attempting to write a block that had been in the\r\nmiddle of a write when power was lost. Thus Yaffs now always starts\r\nallocation on a new block after a mount. This slightly increases the\r\namount of garbage (typically by an insignificant amount) but improves\r\nrobustness.</p>\r\n<h3 class=\"western\">13.1.3 A word about wear leveling</h3>\r\n<p class=\"western\">Since flash memory blocks can only endure a\r\nlimited number of writes and erases ( termed the flash endurance), a\r\nflash file system needs to ensure that the same few blocks are not\r\noverwritten continuously causing those blocks to wear out while the\r\nother blocks are hardly used at all.. Instead it is important that\r\nthe file system spreads out the wear.</p>\r\n<p class=\"western\">This is extremely important in a file system such\r\nas FAT which uses a limited set of logical blocks to store the file\r\nallocation table entries which get modified frequently. If FAT is\r\nused with a static logical to physical block mapping then the file\r\nallocation table blocks will wear out far before other blocks causing\r\npremature device failure. Thus, for FAT, it is very important that\r\nthe logical to physical mapping gets stirred up a bit to cause the\r\nfile allocation tables to be written to different physical blocks,\r\nthus leveling out the wear.</p>\r\n<p class=\"western\">Wear leveling is far less of a concern in a log\r\nstructured file system such as Yaffs since we always write to the end\r\nof the log and thus don\'t keep on overwriting the same blocks.</p>\r\n<p class=\"western\">There are basically two ways to achieve wear\r\nleveling:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Use a\r\n	specifically written set of functions that actively perform wear\r\n	leveling.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Allow\r\n	a degree of wear leveling to happen as a side effect of other\r\n	activity.</font></p>\r\n</ul>\r\n<p class=\"western\">Yaffs takes the second approach.</p>\r\n<p class=\"western\">Firstly, being log structured, Yaffs inherently\r\nspreads out the wear by performing all writes in sequence. That means\r\nthat we don\'t single out a few blocks for a specific purpose and\r\nwrite them more than others.</p>\r\n<p class=\"western\">Secondly, blocks are allocated serially from the\r\nerased blocks in the partition. Thus the process of erasing (after\r\ndeletion of garbage collection) and then allocating blocks does tend\r\nto spread the block use and provides a level of wear leveling.</p>\r\n<p class=\"western\">Thus, even though there is no code to specifically\r\nperform wear leveling it does happen as a side effect of other\r\nactivity.</p>\r\n<p class=\"western\">This strategy performs well and has survived many\r\naccelerated lifetime tests in both simulations and on real NAND-based\r\nproducts.</p>\r\n<h3 class=\"western\">13.1.4 Management of yaffs_tnode and yaffs_obj\r\nstructures</h3>\r\n<p class=\"western\">yaffs_tnodes and yaffs_objs are dynamically\r\ncreated and released as they are needed. \r\n</p>\r\n<p class=\"western\">The allocation and management could be done by\r\njust using the standard memory allocator (malloc, kmalloc etc), but\r\nthis could cause performance problems, particularly in embedded\r\nsystems which tend to have rather simplistic memory allocators. \r\n</p>\r\n<p class=\"western\">The allocation and deletion of many small\r\nstructures can cause memory fragmentation causing out-of-memory and\r\nslowdown problems.</p>\r\n<p class=\"western\">To prevent these problems, Yaffs manages these\r\nobjects internally. Yaffs creates many structures (100 or so) at a\r\ntime and then allocates them singly from a free list. When structures\r\nare released they are placed on the free list for reuse.</p>\r\n<h2 class=\"western\">13.2 Internal cache</h2>\r\n<p class=\"western\">This describes the standard Yaffs internal cache\r\nimplementation. Alternative caches have been devised for other\r\npurposes and at some future point Yaffs might be extended to support\r\npluggable cache policies.</p>\r\n<p class=\"western\">The main purpose of the internal cache is to\r\nreduce NAND access for  badly behaved applications which perform many\r\nsmall reads and writes. Believe it or not, but there are applications\r\nout there that read or write files a few bytes at a time:</p>\r\n<p class=\"code-western\"><code class=\"western\">while(...)<br/>\r\n	write(h,buf,1);</code></p>\r\n<p class=\"western\" style=\"margin-left: 1.25cm\"><br/>\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">This sort of behavior would be terrible for\r\nperformance and flash memory life if each read or write operation\r\nresulted in a flash access. Yaffs provides an internal cache to\r\nmitigate against this sort of behaviour.</p>\r\n<p class=\"western\">The Yaffs internal cache is called a “Short Op\r\nCache” (short operation cache) and only comes in to play for\r\noperations which are not chunk aligned. The caching mechanism is very\r\nsimple and was primarily written for less sophisticated OSs that lack\r\ntheir own file caching layer. The cache is still beneficial in OSs\r\nlike Linux, which provide a caching layer, because it caches both the\r\nread and write path. Thus, while short reads will be serviced by the\r\nLinux page cache, the short op cache does address non-chunk aligned\r\nwrites.</p>\r\n<p class=\"western\">Page aligned reads/writes bypass the cache unless\r\nthat chunk is already in the cache, except when inband tags are used.\r\nInband tags always require the cache because the cache is used to\r\nbuild up the buffer where the tags are attached to the data chunk\r\nbeing written.</p>\r\n<p class=\"western\">The cache is stored in an array of cache entries.\r\nThe number of cache entries is set in the yaffs_dev parameters\r\nn_caches and the cache is configured during initialisation. n_caches\r\nof zero disables the cache. The cache management algorithms are\r\nsimplistic and probably don\'t scale well to very large numbers. This\r\nmeans that it is probably best to stick with around 5 to 20 cache\r\nentries.</p>\r\n<p class=\"western\">Each cache entry holds:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">obj_id, chunk id, nbytes (the tag info of\r\n	the chunk being cached)</p>\r\n	<li/>\r\n<p class=\"western\">The actual data bytes</p>\r\n	<li/>\r\n<p class=\"western\">Cache slot status (last used counter,\r\n	dirty,...)</p>\r\n</ul>\r\n<p class=\"western\">Finding an unused cache entry is a simple\r\noperation. We just iterate through the cache entries searching for\r\none that is not assigned. If all cache entries are busy then a cache\r\npush-out is performed to free up a cahce entry.</p>\r\n<p class=\"western\">Whenever a cache entry is accessed (read or\r\nwritten) the last-use counter is is set to an incrementing number.\r\nThis counter value is used to drive the least-recently-used (LRU)\r\nalgorithm.</p>\r\n<p class=\"western\">Cache entries which have been modified and not yet\r\nwritten back to flash are marked with a dirty flag. This is tells us\r\nwhether we need to write out the data during a flush or push-out.</p>\r\n<p class=\"western\">A push-out happens whenever the cache is full and\r\nwe need to push an entry out of the cache to make space to load a new\r\nentry. Yaffs uses the LRU algorithm to select the LRU cache entry,\r\nwrite it out if needed and then assign that for use.</p>\r\n<p class=\"western\">Whenever a file is flushed (file flush or close),\r\nthe file\'s dirty entries are written out. At certain times (eg.\r\nbefore an unmount or at sync()) the entire cache is flushed.</p>\r\n<p class=\"western\">This caching policy is gives a useful performance\r\nimprovement with very simple algorithms (ie good bang-for-bucks).</p>\r\n<p class=\"western\">Apart from its normal caching function, the short\r\nop cache also does much of the heavy lifting for inband tags support\r\nsince all the inband tags accesses are not chunk aligned and must\r\ntypically be re-aligned. The cache  makes this easy.</p>\r\n<h2 class=\"western\">13.3 Scanning</h2>\r\n<p class=\"western\">Scanning is the process whereby file system state\r\nis rebuilt from scratch. This happens on mounting a Yaffs partition\r\nif no checkpoint data is available or if the mounting was told to\r\nignore the checkpoint data.</p>\r\n<p class=\"western\">Scanning involves reading the tags from all the\r\nactive chunks in the system so can take quite a while, but probably\r\nless time than you\'d expect.</p>\r\n<p class=\"western\">The scanning process is different for Yaffs1 vs\r\nYaffs2 because of the differences mentioned before.</p>\r\n<h3 class=\"western\">13.3.1 Yaffs1 scanning</h3>\r\n<p class=\"western\">Yaffs1 scanning is rather straight forward because\r\nYaffs1 uses deletion markers.</p>\r\n<p class=\"western\">All we really need to do is read the tags off all\r\nthe chunks in the partition and determine if the chunk is active.\r\nOrder is not important and blocks could be read in any order. If the\r\ndeletion marker has not been set then it is active and we need to add\r\nthe chunk to the file system state as follows:</p>\r\n<p class=\"western\">If the chunk is a data chunk (chunk_id &gt; 0)\r\nthen the chunk must be added to the relevant file\'s tnode tree. If\r\nthe object does not exist, then we create it in lost+found, although\r\nit has no name and other file details yet.</p>\r\n<p class=\"western\">If the chunk is an object header (chunk_id == 0)\r\nthen the object must be created (or it might already exist in lost +\r\nfound and we need to fix up its details and slot it into the correct\r\npart of the directory tree.</p>\r\n<p class=\"western\">If at any time during scanning we find that we are\r\nreading a second chunk with the same obj_id:chunk_id as a previously\r\nencountered chunk then the 2-bit serial number is used to resolve the\r\ndispute.</p>\r\n<p class=\"western\">Once the scanning is complete we still need to fix\r\nup a few things: \r\n</p>\r\n<p class=\"western\">Hard links must be connected up properly (it is\r\nmore efficient to defer this until after the scanning has completed).</p>\r\n<p class=\"western\">Also, objects which were marked as unlinked must\r\nbe deleted since there are no longer handles open to these objects.</p>\r\n<h3 class=\"western\">13.3.2 Yaffs2 scanning</h3>\r\n<p class=\"western\">Yaffs2 has no deletion markers which makes the\r\nscanning significantly more complex.</p>\r\n<p class=\"western\">The first thing that needs to be done is to\r\npre-scan the blocks to determine the sequence number for the blocks.\r\nThe list of blocks and sequence numbers is then sorted to make a\r\nchronologically ordered list.</p>\r\n<p class=\"western\">Yaffs then scans <i><b>backwards</b></i> through\r\nthese blocks (ie in reverse chronological order). Thus, the first\r\nencountered  obj_id:chunk_id pair is the current one and subsequent\r\nchunks with those tags are out of date.</p>\r\n<p class=\"western\" style=\"margin-left: 1.25cm\">We read the tags for\r\neach chunk in each block. \r\n</p>\r\n<p class=\"western\" style=\"margin-left: 1.25cm\">If it is a data chunk\r\n( ie. chunk_id &gt; 0) then:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">If a chunk with the same obj_id:chunk_id\r\n	has already been found then this is not the current chunk. Delete\r\n	this one.</p>\r\n	<li/>\r\n<p class=\"western\">Else if the object is marked as deleted, or\r\n	corrupted, then delete it.</p>\r\n	<li/>\r\n<p class=\"western\">Else if the object does not yet exist then\r\n	this chunk was written since the last object header write for this\r\n	object so this chunk must be the last written chunk in the file and\r\n	must have happened just before an unclean shutdown with the file\r\n	still open. We can create the file and use the chunk_id and nbytes\r\n	to set the file extents.</p>\r\n	<li/>\r\n<p class=\"western\">Else if the object does exist and the chunk\r\n	lies beyond the object\'s file scan size (extents) then it is a\r\n	truncated chunk and can be deleted.</p>\r\n	<li/>\r\n<p class=\"western\">Else it is placed into the file\'s tnode\r\n	tree.</p>\r\n</ul>\r\n<p class=\"western\" style=\"margin-left: 1.25cm\">If it is an object\r\nheader (ie. chunk_id == 0) then:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">If the object is in the deleted directory\r\n	then mark the object as deleted and set the object\'s shrink size to\r\n	zero.</p>\r\n	<li/>\r\n<p class=\"western\">Else if an object header has not yet been\r\n	found for this object, then this is the most current object header\r\n	and holds the most current name etc.  If no data chunks have yet\r\n	been found for this chunk then the size in the object header is also\r\n	the file extents. The file size is used to set the file scan size\r\n	since at the time it was written the file did not extend past that\r\n	point and we can safely ignore further data chunks for that file\r\n	that extend past that point.</p>\r\n	<li/>\r\n<p class=\"western\">Else just use the file size as the scan\r\n	size if it is smaller than the current file scan size.</p>\r\n	<li/>\r\n<p class=\"western\">Else this an obsolete object header. It is\r\n	deleted.</p>\r\n</ul>\r\n<p class=\"western\">A bit more explanation is probably needed to\r\ndescribe the file scan size. The whole purpose of this is to\r\ndetermine data chunks that should be ignored due to the file being\r\ntruncated.</p>\r\n<h2 class=\"western\">13.4 Checkpoint</h2>\r\n<p class=\"western\">Mount scanning takes quite a lot of time and slows\r\nmounting, though perhaps by less than you\'d think. Checkpointing is a\r\nmechanism to speed the mounting by taking a snapshot of the Yaffs\r\nruntime state at unmount or sync() and then reconstituting the\r\nruntime state on re-mounting. The speed up is dramatic.</p>\r\n<p class=\"western\">The actual checkpoint mechanism is quite simple. A\r\n“stream” of data is written to a set of blocks which are marked\r\nas holding checkpoint data and the important runtime state is written\r\nto the stream.</p>\r\n<p class=\"western\">Not all state needs to be saved, only the state\r\nneeded to reconstitute the structure of the runtime data. For\r\nexample, file metadata need not be saved since it is very easy to\r\nload that through lazy-loading.</p>\r\n<p class=\"western\">The checkpoint stores data in the following order:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Start\r\n	marker, including checkpoint format id.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">yaffs_dev\r\n	information</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Block\r\n	information</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Chunk\r\n	flags</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Objects,\r\n	including file structure</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">End\r\n	marker</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Checksum</font></p>\r\n</ul>\r\n<p class=\"western\">The checkpoint validity is ensured by the\r\nfollowing mechanisms:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">The\r\n	data is stored using whatever ECC mechanisms are in place.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">The\r\n	start marker includes a checkpoint version so that an obsolete\r\n	checkpoint will not be read if the checkpoint code changes.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Data\r\n	is written to the stream as structured records, each with a type and\r\n	size.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">The\r\n	end marker must be read when expected.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Two\r\n	checksums are maintained across the whole checkpoint data set.</font></p>\r\n</ul>\r\n<p class=\"western\">If any checks fail, the checkpoint is ignored and\r\nthe state is re-scanned.</p>\r\n<p class=\"western\">The astute reader might notice that block state\r\nchanges as we\'re writing checkpoint blocks, so how do we manage to\r\nre-constitute the block states correctly? This is achieved by the\r\nfollowing:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">As we\r\n	write the checkpoint, the checkpoint block allocation selects EMPTY\r\n	blocks. We don\'t change the state as we\'re writing the checkpoint so\r\n	their state is written to the checkpoint as EMPTY or CHECKPOINT, it\r\n	does not really matter which.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">After\r\n	reading the checkpoint, we know which blocks have been used to store\r\n	checkpoint data. We update those blocks to reflect the CHECKPOINT\r\n	state.</font></p>\r\n</ul>\r\n<p class=\"western\">A checkpoint is invalidated by any modifications\r\n(writes, erases,...) thus any modification path also checks to see if\r\nthere is a checkpoint and erases it.</p>\r\n<p class=\"western\">The regular Yaffs block allocator and garbage\r\ncollection logic also needs to be aware of the approximate size of\r\nthe checkpoint so that these can ensure enough spare space is\r\nreserved to store a checkpoint. There is a calculation that updates\r\nthe value whenever the number of objects etc changes.</p>\r\n<h2 class=\"western\">13.5 Extended tags and packed tags</h2>\r\n<p class=\"western\">Extended tags is an abstraction of a way to store\r\nmore information in the tags, allowing for faster scan times. Packed\r\nTags is a physical implementation of the storage of extended tags.</p>\r\n<p class=\"western\">When we\'re scanning we really only need to build\r\nup the structure of the file system. In other words, rebuild the\r\ndirectory relationships, tnode trees and such. Much of the detail\r\nsuch as file names, permissions etc can be lazy loaded if and when\r\nneeded.</p>\r\n<p class=\"western\">This required storing extra information, such as\r\nthe parent directory and object type, in the tags of object headers.\r\nBut how do we do this without enlarging the tags size?</p>\r\n<p class=\"western\">Object headers always have a chunk_id of zero and\r\nn_bytes has no meaning because it is not a data chunk. By using a\r\nsingle bit to identify object headers we suddenly open up a whole lot\r\nof space that we can stuff full of useful data. By being clever as to\r\nwhat data we use, we can pack in the important data in most\r\ncircumstances.</p>\r\n<p class=\"western\">Sometimes, for example very large files, the\r\npacking will not work because there are insufficient bits to hold the\r\nfield so we just write regular tags instead. That\'s OK since there\r\ncan only be few such files in the system so the performance reduction\r\nonly impacts very few files and the system level impact is\r\nunnoticeable. A simple, cheap, strategy that gives speed up 99% of\r\nthe time is a very handy one.</p>\r\n<p class=\"western\">Other details are lazy-loaded on demand. This will\r\nhappen when the object is looked up (eg. by a file open or searching\r\nfor the file in the directory).</p>\r\n<h2 class=\"western\">13.6 Block summaries</h2>\r\n<p class=\"western\">Scanning requires that the tags for each chunk are\r\nread so that they can be inspected. Reading the tags from each chunk\r\ncan be time consuming. Thus, block summaries were added in 2011.</p>\r\n<p class=\"western\">Block summaries write all the tags data for chunks\r\nin the block into the last few (typically only one) chunk in a block.\r\nThis allows all the tags for chunks in that block to be read in one\r\nhit. Depending on the system throughput (CPU, RAM and flash speeds),\r\nthis can speed up mount time dramatically.</p>\r\n<p class=\"western\">Note that the block summary does not replace the\r\ntags attached to each chunk. Both are written. Thus, if the whole\r\nblock is not written (and the block summary is thus not written), the\r\nchunk tags can still be read and used.</p>\r\n<p class=\"western\">If a block summary is not available for a block,\r\nthen the old mechanism is still used to scan the tags on each chunk.</p>\r\n<h2 class=\"western\">13.7 Inband tags</h2>\r\n<p class=\"western\">Thus far we have considered chunks where there are\r\nsufficient unallocated bytes in the spare (or out of band) area to\r\nstore the tags. \r\n</p>\r\n<p class=\"western\">But what happens if we don\'t have enough space in\r\nthe spare area to store the tags? Inband tags to the rescue! \r\n</p>\r\n<p class=\"western\">With inband tags we store the tags in the flash\r\ndata area alongside the data. This gives us far more flexibility in\r\nthe types of storage that Yaffs can support.</p>\r\n<p class=\"western\">There are three major penalties with inband tags:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Due to the loss of chunk alignment, all\r\n	transactions must pass through the short op cache. This costs an\r\n	extra memcpy().</p>\r\n	<li/>\r\n<p class=\"western\"></p>\r\n	<li/>\r\n<p class=\"western\">Reading tags during scanning etc can\r\n	involve reading the whole page, making this slower. The impact is\r\n	minimised by using checkpointing or block summaries.</p>\r\n</ul>\r\n<p class=\"western\">Thus, it is better, performance wise, to only use\r\ninband tags when regular tags will not work.</p>\r\n<h2 class=\"western\">13.8 Soft deletion</h2>\r\n<p class=\"western\">Soft deletion is a mechanism that was added to\r\nspeed up file deletion in Yaffs1 mode.</p>\r\n<p class=\"western\">Looking back to the description of how files are\r\nstored in Yaffs1, you can see that deleting a file involves writing\r\nthe deletion marker of every data chunk in the file. This can take\r\nquite some time for large files.</p>\r\n<p class=\"western\">The soft deletion mechanism was added to avoid\r\nwriting all those deletion markers. Instead of writing the deletion\r\nmarkers, soft deletion instead tracks the number of chunks in each\r\nblock that belong to deleted files and leaves the clean up to the\r\ngarbage collector.</p>\r\n<p class=\"western\">When the garbage collector looks at a block it can\r\nsee what current chunks (ie chunks not yet deleted) belong to deleted\r\nfiles and does not copy those, thus effectively deleting the chunk.\r\nOnce all the chunks associated with a deleted file have cleaned up by\r\nthe garbage collector the file header object can also be deleted.</p>\r\n<p class=\"western\">Yaffs2 mode does not write deletion markers and\r\nthus does not use soft deletion.</p>\r\n','This document gives a reasonable explanation of most of the core mechanisms that make Yaffs work. As of the time of writing this, the Yaffs2 code base includes approximately 19,000 lines of code making a very detailed discussion impractical. This document should serve as a first port of call for understanding Yaffs. Further detailed questions and discussions are welcomed on the Yaffs discussion list.','raw_html'),('paper',0,409,612,'und',0,'<p align=\"center\" style=\"margin-top: 0.42cm; orphans: 0; widows: 0; page-break-after: avoid\">\r\n<br/>\r\n<br/>\r\n\r\n</p>\r\n\r\n	<p class=\"western\" align=\"justify\">This document aims to give a\r\n	reasonable explanation of most of the core mechanisms that make\r\n	Yaffs work. \r\n	</p>\r\n	<p class=\"western\" align=\"justify\">This document should serve as a\r\n	first port of call for understanding Yaffs.</p>\r\n\r\n<h1 class=\"western\">1 Purpose</h1>\r\n<p class=\"western\">The purpose of this document is to give a\r\nreasonable explanation of most of the core mechanisms that make Yaffs\r\nwork. As of the time of writing this, the Yaffs2 code base includes\r\napproximately 19,000 lines of code making a very detailed discussion\r\nimpractical.</p>\r\n<p class=\"western\">This document should serve as a first port of call\r\nfor understanding Yaffs. Further detailed questions and discussions\r\nare welcomed on the Yaffs discussion list.</p>\r\n<p class=\"western\">This document is updated periodically. Yaffs\r\nmechanism design changes from time to time and this document will not\r\nalways reflect the most up to date code. \r\n</p>\r\n<h1 class=\"western\">2 What is Yaffs?</h1>\r\n<p class=\"western\">Yaffs stands for <i>Yet Another Flash File System</i>,\r\na term coined by Charles Manning in 2001 when suggesting that the\r\nalready cluttered flash file system space could do with yet another\r\noffering – this time a flash file system designed from the ground\r\nup to work with NAND flash.</p>\r\n<p class=\"western\">The first release of Yaffs was developed during\r\nthe end of 2001 and the beginning of 2002. By mid-2002, Yaffs was\r\nbeing trialled in various products.  In May 2002,  Yaffs was\r\nannounced and a few more projects started playing with it. In June,\r\nporting to other OSs started. In September 2002, Yaffs was announced\r\non linuxdevices.com which started a far wider uptake of Yaffs.</p>\r\n<p class=\"western\">Yaffs1, the original version of Yaffs, supported\r\n512-byte page NAND devices with a SmartMedia-like memory layout.</p>\r\n<p class=\"western\">Yaffs2 is a follow-up to Yaffs1 extending Yaffs1\r\nto support larger and different devices with different constraints.</p>\r\n<p class=\"western\">Yaffsis highly portable and has been used in many\r\ndifferent products and applications as varied as sewing machines,\r\npoint of sale, phones and aerospace. Yaffs has been used with\r\nmultiple different operating systems. Native support is available for\r\nLinux, WindowsCE and eCOS while Yaffs Direct Interface provides a\r\nlayer that can be used in other applications such as RTOSs. Yaffs has\r\nbeen used with multiple different CPUs and compilers.</p>\r\n<p class=\"western\">Yaffs has also been used as a NOR file system and\r\neven as a RAM file system.</p>\r\n<h1 class=\"western\">3 Design and coding strategies</h1>\r\n<p class=\"western\">Yaffs is designed to work in multiple environments\r\nwhich drives the need for portability. Portability also improves the\r\ndebugging and testing since it is easier to develop and debug code in\r\nan application environment than within an OS kernel. This generally\r\nallows Yaffs to progress faster with less programming resources and\r\nprovides more varied testing. Primary strategies that improve\r\nportability include:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">No OS-specific features used in main code\r\n	body.</p>\r\n	<li/>\r\n<p class=\"western\">No compiler-specific features used in the\r\n	main code body.</p>\r\n	<li/>\r\n<p class=\"western\">Abstract types and functions used to allow\r\n	Unicode or ASCII operation.</p>\r\n</ul>\r\n\r\n<p class=\"western\" style=\"page-break-before: auto; page-break-after: avoid\">\r\nSimplicity is another key goal. Complexity is limited to those\r\nsituations that provide significant improvements to performance or\r\nutility. Simplicity improves robustness and the ease of integration\r\nand development. Primary strategies that improve simplicity are:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Single threaded model. Yaffs is locked on a\r\n	per-partition basis at a high level. This is simpler than tracking\r\n	lower-level locking. Yaffs Direct Interface uses a single lock for\r\n	all partitions.</p>\r\n</ul>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Log structure makes for a simpler garbage\r\n	collector and allocation methodology.</p>\r\n	<li/>\r\n<p class=\"western\">Abstractions that build layered code which\r\n	is easier to understand, port and debug.</p>\r\n</ul>\r\n<h1 class=\"western\">4 Terminology: Yaffs vs Yaffs1 vs Yaffs2</h1>\r\n<p class=\"western\">Before we embark on our quest to understand what\r\nYaffs is, a terminology explanation is in order.</p>\r\n<p class=\"western\">There are two versions of Yaffs code: the original\r\nYaffs code-base and the current Yaffs2 code-base. The Yaffs2\r\ncode-base supports functionality provided by the Yaffs code-base as\r\nwell as being extended with new functionality to provide extra modes\r\nof operation, most importantly those required to work with larger and\r\nmore modern NAND parts. The Yaffs2 code-base supports the Yaffs1 mode\r\nof operation through a backward compatibility mode.</p>\r\n<p class=\"western\">Unless explicitly mentioned, this text refers to\r\nthe Yaffs2 code-base.</p>\r\n<p class=\"western\">This text uses three terms Yaffs, Yaffs1 and\r\nYaffs2 depending on various modes of operation.</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Yaffs1\r\n	is a simpler mode of operation that uses deletion markers to track\r\n	state. </font>\r\n	</p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Yaffs2\r\n	is a more complex mode of operating that was developed to support\r\n	larger modern flash types, in particular MLC NAND, that cannot used\r\n	deletion markers.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Yaffs\r\n	means operations common to both.</font></p>\r\n</ul>\r\n<p class=\"western\">Yaffs1 originally only worked with 512-byte page\r\ndevices but has been extended to support larger flash pages such as\r\nIntel M18 flash with 1k pages.</p>\r\n<p class=\"western\">Yaffs2 was originally designed for 1k or larger\r\npage sizes but can be used with smaller pages by using in-band tags.</p>\r\n<p class=\"western\">Since the Yaffs1 mode of operation is simpler to\r\nunderstand than the Yaffs2 mode of operation,  Yaffs1 will be\r\ndescribed first and readers interested in Yaffs2 operation should\r\nfirst fully acquaint themselves with the Yaffs1 mode of operation. It\r\nis suggested that this document be read sequentially rather than\r\nbrowsing it.</p>\r\n<h1 class=\"western\">5 Objects</h1>\r\n<p class=\"western\">In Yaffs-speak, an Object is anything that is\r\nstored in the file system. These are:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Regular data files</p>\r\n	<li/>\r\n<p class=\"western\">Directories</p>\r\n	<li/>\r\n<p class=\"western\">Hard-links</p>\r\n	<li/>\r\n<p class=\"western\">Symbolic links</p>\r\n	<li/>\r\n<p class=\"western\">Special objects (pipes, devices etc).</p>\r\n</ul>\r\n<p class=\"western\">All objects are identified by a unique integer\r\nobject id (<i><b>obj_id</b></i><i><b>)</b></i>.</p>\r\n<p class=\"western\">In the standard POSIX model, as used by Linux,\r\nthere are inodes and directory entries (dentries). \r\n</p>\r\n<p class=\"western\">An inode may be a regular file, directory, or\r\nspecial file.</p>\r\n<p class=\"western\">Directory entries provide the naming mechanism\r\nthat is used to locate inodes.</p>\r\n<p class=\"western\">Under POSIX, each inode may have zero, one or many\r\ndentries:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">One dentry per inode is commonly\r\n	understood.</p>\r\n	<li/>\r\n<p class=\"western\">Multiple dentries per inode allow the same\r\n	inode to be accessed via multiple names. This is achieved through\r\n	hard links.</p>\r\n	<li/>\r\n<p class=\"western\">Zero dentries per inode is less often\r\n	understood. This happens when an inode is unlinked while there is\r\n	still a handle open to the inode. The unlinking deletes the dentry\r\n	but the inode still exists. As soon as the handle is closed the\r\n	inode is deleted too.</p>\r\n</ul>\r\n<h1 class=\"western\">6 The Yaffs NAND model</h1>\r\n<p class=\"western\">The memory in NAND flash is arranged in <i><b>pages.</b></i>\r\nA page is the unit of allocation and programming.  In Yaffs, the unit\r\nof allocation is the<i> </i><i><b>chunk</b></i>. Typically a chunk\r\nwill be the same as a NAND page, but there is flexibility to use\r\nchunks which map to multiple pages (eg. A system may have two NAND\r\nchips in parallel requiring 2x2048 = 4096 byte chunks) . This\r\ndistinction gives a lot of flexibility in how the system can be\r\nconfigured.  In this text the term page and chunk are synonymous\r\nunless stated otherwise.</p>\r\n<p class=\"western\">Many, typically 32 to 128 but as many as a few\r\nhundred, chunks form a <i><b>block</b></i>. A block is the unit of\r\nerasure. \r\n</p>\r\n<p class=\"western\">NAND flash may be shipped with bad blocks and\r\nfurther blocks may go bad during the operation of the device. Thus,\r\nYaffs is aware of bad blocks and needs to be able to detect and mark\r\nbad blocks.</p>\r\n<p class=\"western\">NAND flash also typically requires the use of some\r\nsort of error detection and correction code (ECC). Yaffs can either\r\nuse existing ECC logic or provide its own.</p>\r\n<h1 class=\"western\">7 How Yaffs1 stores files</h1>\r\n<p class=\"western\">Yaffs1 has a modified log structure, while Yaffs2\r\n(see below) has a true log structure. A true log structured file\r\nsystem only ever writes sequentially. Yaffs1 uses deletion markers,\r\nwhich breaks the sequential write rule. Yaffs2 does not use deletion\r\nmarkers and is thus a true log structured file system.</p>\r\n<p class=\"western\">Instead of writing data in locations specific to\r\nthe files, the file system data is written in the form of a\r\nsequential log, The entries in the log are all one chunk in size and\r\ncan hold one of two types of chunk:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><i><b>Data chunk</b></i>: A chunk holding\r\n	regular data file contents.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Object Header</b></i>: A descriptor\r\n	for an object (directory, regular data file, hard link, soft link,\r\n	special descriptor,...). This holds details such as the identifier\r\n	for the parent directory, object name, etc.</p>\r\n</ul>\r\n<p class=\"western\">Each chunk has <i><b>tags</b></i> associated with\r\nit. The tags comprise the following important fields (others being\r\nignored for now):</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><i><b>Object Id (</b></i><i><b>obj_id</b></i><i><b>)</b></i>:\r\n	Identifies which object the chunk belongs to.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Chunk Id (</b></i><i><b>chunk_id</b></i><i><b>)</b></i>:\r\n	Identifies where in the file this chunk belongs. A chunk_id of zero\r\n	signifies that this chunk contains an objectHeader. chunk_id==1\r\n	signifies the first chunk in the file (ie. At file offset 0),\r\n	chunk_id==2  is the next chunk and so on.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Deletion Marker (</b></i><i><b>is_deleted</b></i><i><b>)</b></i><i><b>:</b></i>\r\n	(Yaffs1 only)  Shows that this chunk is no longer in use.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Byte Count</b></i><b>: </b><b>(n_bytes)</b>:\r\n	Number of bytes of data if this is a data chunk.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Serial Number </b></i><i><b>(ser</b></i><i><b>ia</b></i><i><b>l_number)</b></i>:\r\n	 (Yaffs1 only) Serial number used to differentiate chunks with the\r\n	same obj_id and chunk_id.</p>\r\n</ul>\r\n\r\n<p class=\"western\" style=\"page-break-before: always\">Now let\'s bring\r\nthat all together in an explanation. This is a simplified explanation\r\nand skips over issues such as Soft Deletion (mentioned later in the\r\ndocument). For explanation purposes, we shall consider usage of\r\nfictitious NAND type that has 4 chunks per block, starting with an\r\nempty (erased) flash.</p>\r\n\r\n<p class=\"western\">First we\'ll create a file.</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"37\">\r\n	<col width=\"56\">\r\n	<col width=\"67\">\r\n	<col width=\"60\">\r\n	<col width=\"64\">\r\n	<col width=\"271\">\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">Block</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">Chunk</p>\r\n		</td>\r\n		<td width=\"67\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">obj_id</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">chunk_id</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">Deletion</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\" style=\"font-weight: normal\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" height=\"26\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"67\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Object Header for this file (length 0)</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Next we write a few chunks of data to the file.</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"37\">\r\n	<col width=\"48\">\r\n	<col width=\"60\">\r\n	<col width=\"64\">\r\n	<col width=\"56\">\r\n	<col width=\"290\">\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Block</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Chunk</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">obj_id</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">chunk_id</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deletion</p>\r\n		</td>\r\n		<td width=\"290\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"290\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Object Header for t<font size=\"3\" style=\"font-size: 12pt\">his\r\n			file (length 0)</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"290\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">First chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"290\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Second chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"56\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"290\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Third chunk of data</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Next we close the file. This writers a new object\r\nheader for the file. Notice how the previous object header is\r\ndeleted.</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"37\">\r\n	<col width=\"48\">\r\n	<col width=\"60\">\r\n	<col width=\"68\">\r\n	<col width=\"60\">\r\n	<col width=\"282\">\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Block</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Chunk</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">obj_id</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">chunk_id</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deletion</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsoleted object header <font size=\"2\" style=\"font-size: 10pt\">(length\r\n			0)</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">First chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Second chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Third chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"68\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"282\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">New object header <font size=\"2\" style=\"font-size: 10pt\">(length\r\n			n)</font></p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\" style=\"page-break-before: always\">Let\'s now open\r\nthe file for read/write, overwrite part of the first chunk in the\r\nfile and close the file. The replaced data and object header chunks\r\nbecome deleted.</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\" style=\"page-break-inside: avoid\">\r\n	<col width=\"37\">\r\n	<col width=\"48\">\r\n	<col width=\"64\">\r\n	<col width=\"64\">\r\n	<col width=\"71\">\r\n	<col width=\"271\">\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Block</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Chunk</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">obj_id</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">chunk_id</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deletion</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete first chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Second chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Third chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">New first chunk of file</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">New object header</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Now let\'s resize the file to zero by opening the\r\nfile with O_TRUNC and closing the file. This writes a new object\r\nheader with length 0 and the contents are pruned making the data\r\nchunks deleted.</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"37\">\r\n	<col width=\"48\">\r\n	<col width=\"64\">\r\n	<col width=\"64\">\r\n	<col width=\"71\">\r\n	<col width=\"271\">\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Block</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Chunk</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">obj_id</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">chunk_id</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deletion</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete first chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Second chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Third chunk of data</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Deleted first chunk of file</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Delted</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsoleted object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"37\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"48\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"64\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"71\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"271\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">New object header <font size=\"2\" style=\"font-size: 10pt\">(length\r\n			0).</font></p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Note how all the pages in block 1 are now marked\r\nas deleted. This means that block 1 no longer contains useful\r\ninformation so we can now erase block 1 and reuse the space.</p>\r\n<p class=\"western\">We will now rename the file. To do this we write a\r\nnew object header for the file</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\" style=\"page-break-inside: avoid\">\r\n	<col width=\"38\">\r\n	<col width=\"42\">\r\n	<col width=\"58\">\r\n	<col width=\"60\">\r\n	<col width=\"60\">\r\n	<col width=\"297\">\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Block</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Chunk</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">obj_id</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">chunk_id</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">Comment</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Erased</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Erased</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Erased</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"1\" sdnum=\"2057;\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\"><br/>\r\n\r\n			</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Erased</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">1</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Deleted first chunk of file</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsoleted object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"2\" sdnum=\"2057;\">\r\n			<p class=\"western\">2</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Deleted</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">Obsolete object header</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"38\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\" sdval=\"3\" sdnum=\"2057;\">\r\n			<p class=\"western\">3</p>\r\n		</td>\r\n		<td width=\"42\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"58\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">500</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">0</p>\r\n		</td>\r\n		<td width=\"60\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Live</p>\r\n		</td>\r\n		<td width=\"297\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">New object header showing new name</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Block 2 now contains only deleted chunks and can\r\nbe erased and reused.</p>\r\n<p class=\"western\">Note how the tags tell us:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Which\r\n	chunks belong to which object</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">The\r\n	position of the chunks within a file,</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Which\r\n	chunks are current</font></p>\r\n</ul>\r\n<ul>\r\n	<p class=\"western\"><font size=\"3\" style=\"font-size: 12pt\">With that\r\n	information we can recreate the state of the files regardless of the\r\n	placement of the chunks in NAND. This means that should the system\r\n	fail (crash, power fail,...) at any point during the sequences\r\n	above, we are still able to recover the file system state up to that\r\n	point.</font></p>\r\n</ul>\r\n<p class=\"western\"><font size=\"3\" style=\"font-size: 12pt\">Note that\r\nthere are no file allocation tables or similar structures stored in\r\nthe flash. This reduces the amount of writing and erasing (improving\r\nwrite performance) while also improving robustness. Corruption of\r\nfile allocation tables and similar structures is a common failure\r\nmechanism for embedded file systems. The lack of such structures\r\nmakes Yaffs more robust. Or, to put it another way, you can\'t corrupt\r\nwhat you don\'t store.</font></p>\r\n<h1 class=\"western\">8 Garbage collection</h1>\r\n<p class=\"western\">As we have shown so far, when a block is made up\r\nonly of deleted chunks, that block can be erased and reused. However,\r\nconsider what will happen if we have many blocks which each have a\r\nfew current chunks in them. Clearly we cannot erase any of the\r\ncurrent blocks or we will destroy data. What we need to do is copy\r\nthe useful data chunks off a block, deleting the originals and\r\nallowing the block to be erased and reused. This process is referred\r\nto as <i><b>garbage collection</b></i>.</p>\r\n<p class=\"western\">The Yaffs garbage collection process works as\r\nfollows:</p>\r\n<ol>\r\n	<li/>\r\n<p class=\"western\">Find a block worth collecting (according\r\n	the the heuristics below). If none found, then exit.</p>\r\n	<li/>\r\n<p class=\"western\">Iterate through the chunks in the block. If\r\n	the chunk is in use then create a new copy and delete the original.\r\n	Fix up the RAM data structures to reflect the change.</p>\r\n</ol>\r\n<p class=\"western\">Once all chunks in this block have been deleted,\r\nthe block can be erased ready for reuse.</p>\r\n<p class=\"western\">Note that although step 2 deletes the original\r\nchunk when it gets copied there is not much point in actually\r\nprogramming the deletion marker (is_deleted flag) in flash since\r\nwe\'re about to erase the block anyway. Thus Yaffs does not program\r\nthe deletion marker in these cases. If power is lost part way through\r\nthe operation we can still tell the difference between the original\r\nand the copy by looking at the serial number (not that it really\r\nmatters anyway since the two have the same contents so either can be\r\nused).</p>\r\n<p class=\"western\">The heuristics to determine whether a block is\r\nworth collecting are as follows:</p>\r\n<ol>\r\n	<li/>\r\n<p class=\"western\">If there are many erased blocks available,\r\n	then Yaffs does not need to work hard, but will instead only attempt\r\n	to perform garbage collection of blocks with very few chunks in use.\r\n	This is termed <i><b>passive</b></i> garbage collection.</p>\r\n	<li/>\r\n<p class=\"western\">If, however, there are very few erased\r\n	blocks, Yaffs will work much harder to recover more space and will\r\n	garbage collect blocks with more chunks in use. This is termed\r\n	<i><b>aggressive</b></i> garbage collection.</p>\r\n</ol>\r\n<p class=\"western\">OK, the passive/aggressive names are a bit silly,\r\nbut they were contrived around midnight... give me a break!</p>\r\n<p class=\"western\">If garbage collection is aggressive, we need\r\nerased space urgency and the whole block is collected in a single\r\ngarbage collection cycle. If the collection is passive then only a\r\nfew chunks are collected at a time thus spreading the effort over\r\nmany garbage collection cycles. This is done to reduce garbage\r\ncollection load and improve responsiveness.</p>\r\n<p class=\"western\">The rationale behind the above heuristics is to\r\ndelay garbage collection when possible to reduce the amount of\r\ncollection that needs to be performed, thus increasing average system\r\nperformance. Yet there is a conflicting goal of trying to spread the\r\ngarbage collection so that it does not all happen at the same causing\r\nfluctuations in file system throughput. These conflicting goals make\r\ngarbage tuning quite challenging.</p>\r\n<p class=\"western\">All flash file systems need some sort of garbage\r\ncollector to reclaim space. The garbage collector is often a\r\ndetermining factor in the performance of the flash file system. Many\r\nfile systems have to do a significant amount of effort in a single\r\ncollection sweep.</p>\r\n<p class=\"western\">Yaffs only deals with one block at a time which\r\nconstrains the amount of effort in a garbage collection cycle, thus\r\nreducing the “stall time”.  Under normal conditions that garbage\r\ncollection is performed a few chunks at a time, further reducing\r\n“stall time”.</p>\r\n<h1 class=\"western\">9 Yaffs1 serial numbers</h1>\r\n<p class=\"western\">In Yaffs1, each chunk is marked with a 2-bit\r\nserial number that is incremented every time a chunk with the same\r\ntags value is replaced (eg. When a chunk is overwritten or when\r\ncopied by garbage collection).</p>\r\n<p class=\"western\">Now let\'s suppose first that we did not have a\r\nserial number. \r\n</p>\r\n<p class=\"western\">Let\'s consider a rename where an object header\r\nchunk must be replaced with a new one with the new name. Yaffs could\r\ndo the following:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Delete\r\n	the old chunk.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Write\r\n	the new chunk. </font>\r\n	</p>\r\n</ul>\r\n<p class=\"western\">If the system fails after the delete, the system\r\ncould end up with no chunks matching the relevant tags values. This\r\ncould cause files to be lost (end up in the lost+found directory).\r\nThus, we have to write the new chunk before deleting the old chunk.</p>\r\n<p class=\"western\">But now consider what might happen if we did this:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">W<font face=\"Liberation Serif, serif\">rite\r\n	new chunk.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Delete\r\n	old chunk.</font></p>\r\n</ul>\r\n<p class=\"western\">If the system failed after the write we now have\r\ntwo chunks that match the tags. How can we tell which one is current?</p>\r\n<p class=\"western\">The current chunk is determined by inspecting the\r\nserial number.  Since the serial number is only incremented by one\r\nbefore the old chunk is deleted, a 2-bit serial number is sufficient.\r\nValid pairs are:</p>\r\n<table width=\"86\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"35\">\r\n	<col width=\"33\">\r\n	<tr valign=\"top\">\r\n		<td width=\"35\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">Old</p>\r\n		</td>\r\n		<td width=\"33\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">New</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"35\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">00</p>\r\n		</td>\r\n		<td width=\"33\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">01</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"35\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">01</p>\r\n		</td>\r\n		<td width=\"33\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">10</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"35\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">10</p>\r\n		</td>\r\n		<td width=\"33\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">11</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"35\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">11</p>\r\n		</td>\r\n		<td width=\"33\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">00</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n\r\n<p class=\"western\">Thus, by inspecting the serial number attached to\r\nthe old and new tags the current chunk can be determined and the old\r\nchunk can be deleted.</p>\r\n<h1 class=\"western\">10 Yaffs2 NAND model</h1>\r\n<p class=\"western\">Yaffs2 is an extension of Yaffs designed to\r\nfulfill a new set of objectives to work with newer NAND types\r\nincluding:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Zero overwrites. Yaffs1 only ever\r\n	overwrites a single byte in the spare area of the chunks to set the\r\n	deletion marker. More modern NAND devices are less tolerant of\r\n	overwrites and Yaffs2 performs no overwrites at all.</p>\r\n	<li/>\r\n<p class=\"western\">Sequential chunk writing within a block.\r\n	More modern NAND reliability specifications tend to assume\r\n	sequential writing. Since Yaffs2 does not write deletion markers,\r\n	the writing of chunks within a block is strictly sequential.</p>\r\n	<li/>\r\n<p class=\"western\">Some flash technologies – such as MLC –\r\n	only work properly if pages are written sequentially. This is a\r\n	fundamental requirement of how many MLC types perform the bit\r\n	overlaying.</p>\r\n</ul>\r\n<p class=\"western\">Since Yaffs2 performs less writes, there is\r\npotential for Yaffs2 to have improved write performance.</p>\r\n<p class=\"western\">Yaffs2 cannot use a deletion marker because that\r\nwould violate the zero overwrites mandate and alternate mechanisms\r\nare provided to determine which chunks are current and which are\r\ndeleted. These mechanisms are:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><i><b>Sequence Number </b></i><i><b>(seq_number)</b></i>:\r\n	As each block is allocated, the file system\'s sequence number is\r\n	incremented and each chunk in the block is marked with that sequence\r\n	number. The sequence number thus provides a way of organising the\r\n	log in chronological order.</p>\r\n	<li/>\r\n<p class=\"western\"><i><b>Shrink Header Marker </b></i><i><b>(shrink_header)</b></i>:\r\n	The shrink header marker is used to mark object headers that are\r\n	written to shrink the size of a data file. This is explained more in\r\n	the accompanying text.</p>\r\n</ul>\r\n\r\n<p class=\"western\" align=\"center\"><i><b>Note: The Yaffs2 sequence\r\nnumber is not the same as the Yaffs1 serial number!</b></i></p>\r\n\r\n<p class=\"western\">Note: We still refer to chunk deletion in Yaffs2.\r\nThe chunks are marked as deleted in the RAM data structures used by\r\ngarbage collection etc, but there is no deletion marker written to\r\nflash.</p>\r\n<h2 class=\"western\">10.1 Sequence Number</h2>\r\n<ul>\r\n	<p class=\"western\">The sequence number allows the chronological\r\n	order to be determined which allows Yaffs to determine the sequence\r\n	of events and restore the file system state. This is achieved by\r\n	scanning <i><b>backwards</b></i> chronologically (ie. From the\r\n	highest sequence number backwards to the lowest sequence number).\r\n	Thus:</p>\r\n	<li/>\r\n<p class=\"western\">Since we\'re scanning backwards, the most\r\n	recently written – and thus current – chunk matching an\r\n	obj_id:chunk_id pair will be encountered first and all subsequent\r\n	matching chunks must be obsolete and treated as deleted.</p>\r\n	<li/>\r\n<p class=\"western\">The file length in object headers are used\r\n	to trim back resized files. If an object header is encountered, then\r\n	subsequent data chunks that lie beyond that file length are clearly\r\n	obsolete and treated as deleted. Note that both current and obsolete\r\n	object header sizes must be used to obtain an accurate\r\n	reconstruction.</p>\r\n</ul>\r\n<h2 class=\"western\">10.2 Shrink Header</h2>\r\n<p class=\"western\">Explaining the purpose of the shrink header marker\r\nis a bit convoluted. The purpose of a shrink header marker is to show\r\nthat this object header indicates that a file has shrunken in size,\r\nand to prevent those object headers from being deleted by garbage\r\ncollection.  To understand this, it is easiest to consider what would\r\nhappen if the shrink header markers did not exist.</p>\r\n<p class=\"western\">For simplification we\'ll assume no garbage\r\ncollection happens during this part of the exercise. It is safe to\r\nmake that assumption since it does not alter the reasoning.</p>\r\n<p class=\"western\">Consider a file that has been through the\r\nfollowing:</p>\r\n<p class=\"code-western\"><font face=\"Liberation Mono, monospace\"><font size=\"2\" style=\"font-size: 10pt\"><b>#define\r\nMB (1024 * 1024)<br/>\r\n</b></font></font><br/>\r\n\r\n</p>\r\n<p class=\"code-western\"><code class=\"western\"><font face=\"Liberation Mono, monospace\"><font size=\"2\" style=\"font-size: 9pt\">h\r\n= open(“foo”,O_CREAT| O_RDWR, S_IREAD|S_IWRITE); /* create file\r\n*/</font></font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\"><br/>\r\nwrite(h,data,5*</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">MB</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">);\r\n/* write 5 MB of data /<br/>\r\ntruncate(h,1*</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">MB</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">);\r\n /* truncate to 1MB */<br/>\r\nlseek(h,2*</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">MB</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">,\r\nSEEK_SET); /* set the file access position to 2MB\r\n*/<br/>\r\nwrite(h,data,1*</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">MB</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 9pt\">);\r\n/* write 1MB of data */<br/>\r\nclose(h);</font></code><br/>\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">The file will now be 3MB in length, but there will\r\nbe a “hole” between 1MB and 2MB. According to POSIX (and to aid\r\nsecurity etc) that “hole” should always read back as zeros.</p>\r\n<p class=\"western\">Yaffs2 would represent that in NAND with the\r\nfollowing sequence of chunks:</p>\r\n<ol>\r\n	<li/>\r\n<p class=\"western\">Object header for file creation (file\r\n	length = 0)</p>\r\n	<li/>\r\n<p class=\"western\">5MB worth of data chunks (0 to 5MB)</p>\r\n	<li/>\r\n<p class=\"western\">Object header for truncation (file length =\r\n	1MB)</p>\r\n	<li/>\r\n<p class=\"western\">1MB worth of data chunks (2MB to 3MB)</p>\r\n	<li/>\r\n<p class=\"western\">Object header for close (file length = 3MB)</p>\r\n	<p class=\"western\">At this stage, only the the following data chunks\r\n	are current:</p>\r\n</ol>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">The first 1MB of data created in step 2\r\n	above.</p>\r\n	<li/>\r\n<p class=\"western\">The 1MB of data created in step 4 above.</p>\r\n	<li/>\r\n<p class=\"western\">The object header created in step 5 above.</p>\r\n</ul>\r\n<p class=\"western\">But Yaffs must remember the truncation that\r\nhappened in step 3, otherwise Yaffs will “forget”  that there is\r\na hole, which would cause the old data to be visible again. Thus, if\r\nYaffs creates a hole it first writes a shrink header to indicate the\r\nstart of the hole and a regular object header to mark the end of the\r\nhole. \r\n</p>\r\n<p class=\"western\">The shrink header modifies the behavior of the\r\ngarbage collector to ensure that  these shrink headers are not erased\r\nuntil it is safe to do so. This unfortunately has a negative impact\r\non garbage collection in systems which make extensive use of files\r\nwith holes.</p>\r\n<p class=\"western\">Shrink headers are also used to indicate that a\r\nfile has been deleted and that the record of the file deletion is not\r\nlost.</p>\r\n<h2 class=\"western\">10.3 Yaffs2 extended tags</h2>\r\n<p class=\"western\">Yaffs2 extends the tags in object headers with\r\nextra fields to improve mount scanning performance. The operation of\r\nextended tags is described later in this document.</p>\r\n<h1 class=\"western\">11 Bad block handling NAND error handling</h1>\r\n<p class=\"western\">NAND flash is designed to be very low cost and\r\nvery high density. In order to get high yields, and thus keep costs\r\ndown, NAND flash is typically shipped with a few bad blocks and some\r\nfurther blocks are expected to go bad during the use of the device.\r\nThus, any flash file system without an effective bad block handling\r\npolicy is unsuitable for NAND flash.</p>\r\n<p class=\"western\">Yaffs1 uses the Smart-Media style of bad block\r\nmarking. This checks the sixth byte (byte 5) of the spare area. In a\r\ngood block this should be 0xFF. A factory-marked bad block should be\r\n0x00. If Yaffs1 determines that a block has gone bad it is marked bad\r\nwith 0x59 (\'Y\'). Using a distinctive marker allows Yaffs-marked bad\r\nblocks to be differentiated from factory-marked bad blocks.</p>\r\n<p class=\"western\">Yaffs2 mode is designed to support a wider range\r\nof devices and spare-area layouts. Thus, Yaffs2 does not decide which\r\nbytes to mark but instead calls driver functions to determine whether\r\nthe block is bad, or to mark it bad.</p>\r\n<p class=\"western\">So, when does Yaffs mark a block bad? Yaffs will\r\nmark a block bad if a read or write operation fails or if three ECC\r\nerrors are detected. A block that is marked bad is retired from use,\r\nthus improving the robustness of the file system. This policy is\r\nsuitable for SLC flash. MLC policies are under development.</p>\r\n<p class=\"western\">Further, NAND flash cells can sometimes be\r\ndisturbed (corrupted) by NAND activity and charge loss.  These errors\r\nare corrected by error correction codes (ECC) which may be\r\nimplemented in hardware, software drivers, or within Yaffs itself.\r\nAgain, any flash file system lacking effective ECC handling is\r\nunsuitable for NAND flash.</p>\r\n<p class=\"western\">Yaffs1 mode can use either the built in ECC or use\r\nECC provided by the driver or hardware.</p>\r\n<p class=\"western\">Since Yaffs2 mode is designed for a wider range of\r\ndevices, it does not provide ECC internally but requires that the\r\ndriver provide the ECC.</p>\r\n<p class=\"western\">The ECC code supplied with Yaffs (yaffs_ecc.c) is\r\nthe fastest C code implementation of a Smart Media compatible ECC\r\nalgorithm that we are aware of. This code will correct any single bit\r\nerror within a 256-byte data block and detect 2 errors per 256-byte\r\ndata block. This is sufficient error correction to provide very high\r\nreliability on most SLC-type NAND flash.</p>\r\n<p class=\"western\">For a more complete description of failure\r\nhandling, please refer to the Yaffs error mitigation document.</p>\r\n<h1 class=\"western\">12 RAM structures</h1>\r\n<p class=\"western\">While it is theoretically possible to use a log\r\nstructured file system with very few RAM structures, this would\r\nprovide reduce performance. Thus, significant RAM data structures are\r\nrequired to store enough information to provide sufficiently high\r\nperformance. \r\n</p>\r\n<p class=\"western\">Before we jump in, it is a good idea to first\r\nfigure out what purpose the RAM structures serve. The main structures\r\nare defined in yaffs_guts.h and their main purposes, are :</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Device/partition: This is named <i><b>yaffs_dev</b></i><span style=\"font-style: normal\"><span style=\"font-weight: normal\">.\r\n	</span></span>This is required to hold information relating to a\r\n	Yaffs “partition” or “mount point”. Using these, rather than\r\n	globals, allows Yaffs to simultaneously support multiple partitions\r\n	and partitions of different types. In fact, almost other data\r\n	structures mentioned here are part of, or accessed via, the\r\n	structure.</p>\r\n	<li/>\r\n<p class=\"western\">NAND Block Information: This is named\r\n	<i><b>yaffs_block_info </b></i>and holds the current state of the\r\n	NAND blocks. Each yaffs_dev has an array of these.</p>\r\n	<li/>\r\n<p class=\"western\">NAND Chunk Information: This is a bitfield\r\n	attached to the yaffs_dev that holds the current in-use state of\r\n	each chunk in the system. There is one bit per chunk in the\r\n	partition.</p>\r\n	<li/>\r\n<p class=\"western\">Object: This is named <i><b>yaffs_obj</b></i>\r\n	and holds the state of an object (see section 3). There is one of\r\n	these for each object in the file system, where an object is one of\r\n	a regular file, directory, hard link, symbolic link or special link.\r\n	There are different variants of yaffs_Objects to reflect the\r\n	different data required for these different object types. Each\r\n	object is uniquely identified by an obj_id.</p>\r\n	<li/>\r\n<p class=\"western\">File structure: For each file object, Yaffs\r\n	holds a tree which provides the mechanism to find the data chunks in\r\n	a file. The tree consists of nodes called <i><b>yaffs_tnode</b></i>\r\n	(tree nodes).</p>\r\n	<li/>\r\n<p class=\"western\">Directory structure: The directory\r\n	structure allows objects to be found by name. The directory\r\n	structure is built from a doubly-linked list that is rooted in the\r\n	directory and binds together the sibling objects within a directory.</p>\r\n	<li/>\r\n<p class=\"western\">Object number hash table: The object number\r\n	hash table provides a mechanism to find an object from its obj_id.\r\n	The obj_id is hashed to select a hash bucket. Each hash bucket has a\r\n	doubly-linked list of objects that belong in this hash bucket, as\r\n	well as a count of the objects in the bucket.</p>\r\n	<li/>\r\n<p class=\"western\">Cache: Yaffs provides a read/write cache\r\n	that significantly improves performance for short operations. The\r\n	size of the cache can be set at run-time.</p>\r\n</ul>\r\n<p class=\"western\">The usage of these structures is described in more\r\ndetail below.</p>\r\n<p class=\"western\">Yaffs makes significant use of doubly-linked\r\nlists. These data structures are very useful because they provide\r\nrapid insertion, removal and traversal. They do however come at the\r\nexpense of requiring two pointers rather than just one that would be\r\nrequired by a singly-linked list.</p>\r\n<h2 class=\"western\">12.1 Yaffs objects (struct yaffs_obj)</h2>\r\n<p class=\"western\">Each object (file, directory,...) in the system is\r\nrepresented by a yaffs_obj structure \r\n</p>\r\n<p class=\"western\">The yaffs_obj\'s main function is to store most of\r\nthe object metadata and type-specific information. This means that\r\nalmost all the information about an object can be accessed\r\nimmediately without reading flash.</p>\r\n<p class=\"western\">The metadata includes, amongst others:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">obj_id:\r\n	The number used to identify the object.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">parent:\r\n	A pointer to the parent directory. Not all objects have a parent.\r\n	The root director and special directories for unlinked and deleted\r\n	files do not have parents so this is NULL.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">short_name:\r\n	If the name is short enough to fit in a small fixed-sized array\r\n	(default 16 characters) then it is stored here otherwise it must be\r\n	fetched from flash each time it is requested.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">type:\r\n	Type of object.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Permissions,\r\n	time, ownership and other attributes</font></p>\r\n</ul>\r\n<p class=\"western\"><font size=\"3\" style=\"font-size: 12pt\">Depending\r\non the object type, yaffs_obj also stores:</font></p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">A\r\n	tnode tree, file extents etc for data files</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">A\r\n	directory chain for directories</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">An\r\n	equivalent object for hard links</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">A\r\n	string for symbolic links</font></p>\r\n</ul>\r\n<h2 class=\"western\">12.2 Look-up by obj_id</h2>\r\n<p class=\"western\">The purpose of this mechanism is to rapidly access\r\nan object by its device and obj_id. This is required during garbage\r\ncollection, scanning and similar operations.</p>\r\n<p class=\"western\">Each yaffs_dev uses a hash table for this purpose.\r\nThe hash table has 256 “buckets” (tunable), each holding a\r\ndoubly-linked list of the yaffs_objs in that bucket.</p>\r\n<p class=\"western\">The hash table\'s hash function is just masking the\r\nleast significant bits of the obj_id. This is a very cheap function\r\nthat provides reasonable “spread”.</p>\r\n<p class=\"western\">Yaffs makes an attempt to assign obj_ids in a way\r\nthat keeps the number of objects in each hash bucket quite low, thus\r\npreventing any one hash entry getting too long.</p>\r\n<p class=\"western\">This mechanism is also charged with assigning new\r\n(unique) obj_id values.</p>\r\n<h2 class=\"western\">12.3 Directory structure</h2>\r\n<p class=\"western\">The purpose of the directory structure is to\r\nrapidly access an object by name. This is required to perform file\r\noperations such as opening, renaming etc.</p>\r\n<p class=\"western\"><font size=\"3\" style=\"font-size: 12pt\">The Yaffs\r\ndirectory structure is formed by a tree of objects of type\r\nYAFFS_OBJECT_TYPE_DIRECTORY. These objects have a doubly linked node\r\nof children.</font></p>\r\n<p class=\"western\">Each yaffs_obj also has a doubly linked list node\r\ncalled sibling which chain together the siblings in a directory.</p>\r\n<p class=\"western\">Each Yaffs partition has a root directory.</p>\r\n<p class=\"western\">Each yaffs partition also has some special purpose\r\n“fake” directories which are not stored in NAND but are instead\r\ncreated on mounting:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">lost+found: This directory is used as a\r\n	place to store any lost file parts that cannot be placed in the\r\n	normal directory tree.</p>\r\n	<li/>\r\n<p class=\"western\">Unlinked and deleted: These are\r\n	pseudo-directories that are not in the directory tree. Placing\r\n	objects in these directories gives them a status of being unlinked\r\n	or deleted.</p>\r\n</ul>\r\n<p class=\"western\" align=\"left\">Each object has a name that is used\r\nto look up the object within the directory, Thus, each name within a\r\ndirectory must be unique. This rule does not apply to objects in the\r\nunlinked and deleted pseudo-directories since we never use names to\r\nlook up unlinked/deleted files and there may be many files of the\r\nsame name that have been deleted.</p>\r\n<p class=\"western\">Name resolution is sped up by two mechanisms:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">short_names\r\n	are stored directly in the yaffs_obj so that they don\'t have to be\r\n	loaded from flash.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Each\r\n	object has a “name sum”. This is a simple hashing of the name\r\n	which speeds up the matching process because it is cheaper to\r\n	compare the hash than compare the whole name string.</font></p>\r\n</ul>\r\n<p class=\"western\"><img src=\"data:image/png;base64,[encoded data, 22776 characters]\" name=\"Object2\" align=\"left\" width=\"412\" height=\"355\">\r\n  <br clear=\"left\"/>\r\n</img>\r\n<br/>\r\n<br/>\r\n\r\n</p>\r\n\r\n<p class=\"western\">The above directory structure represents the\r\nfollowing directory tree</p>\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"119\">\r\n	<col width=\"468\">\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Root directory</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/a</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Directory containing c,d,e</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/b</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Empty directory</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/lost+found</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Empty directory</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/a/c</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Directory</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/a/d</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Data file</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"119\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font face=\"Courier, monospace\"><font size=\"3\" style=\"font-size: 12pt\">/a/e</font></font></p>\r\n		</td>\r\n		<td width=\"468\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Data file</font></p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n<p class=\"western\" align=\"left\" style=\"margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">If a Yaffs partition has been somehow corrupted\r\nthen it is possible for an object to be created during scanning but\r\nwith no object header (which contains the object name) being found.\r\nThe object would then not have a name. In these cases, Yaffs creates\r\na pseudonym from the object number of the form objnnnn so that the\r\nobject can be placed in the directory structure. \r\n</p>\r\n\r\n<h2 class=\"western\" style=\"page-break-before: always\">12.4 Hard links</h2>\r\n<p class=\"western\">In POSIX file systems, the objects (inodes in\r\nLinux terminology, vnode in BSD-speak) and the object names (dentry\r\nin Linux terminology)  are independent in that an object can have\r\nzero, one or more names.</p>\r\n<p class=\"western\">Having files with zero links is caused by\r\nconstructs such as:</p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">h\r\n= open(“foo”,O_CREAT | O_RDWR, S_IREAD | S_IWRITE); /* create\r\nfile “foo” */</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">unlink(“foo”);	/*\r\nunlink it </font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">to\r\nremove</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">\r\ndirectory </font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">entry\r\n</font></code><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">*/</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">/*\r\nwe still have a handle to the file so can still access it via the\r\nhandle */</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">read(h,buffer,100);	</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">write(h,buffer,100);</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<font face=\"Courier 10 Pitch\"><font size=\"1\" style=\"font-size: 8pt\"><code class=\"western\"><font size=\"2\" style=\"font-size: 10pt\">close(h);\r\n/* as soon as the handle is closed the file is deleted */</font></code></font></font></p>\r\n<p class=\"western\" align=\"left\" style=\"margin-left: 1.25cm; margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">Yaffs does not store separate object and name\r\nrecords because that would waste space and incur extra\r\nreading/writing. Instead, Yaffs is optimised for the most typical\r\ncase of one name per object. That is, Yaffs stores one name per\r\nyaffs_obj and uses “cheats” to achieve other behavior:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Objects that have zero names (ie unlinked\r\n	objects) are handled by renaming the object into a fake directory\r\n	that is used to designate unlinked objects.</p>\r\n	<li/>\r\n<p class=\"western\">Multiple names object are handled by\r\n	designating one of the objects the main object and having multiple\r\n	hard link pseudo-objects that refer to it.</p>\r\n	<li/>\r\n<p class=\"western\">The inode number (vnid in BSD-speak) is a\r\n	per-file system unique number that identifies the object. The Yaffs\r\n	obj_id of the main object is presented to the user. That is, if the\r\n	inode number is queried from a hardlink object then the obj_id of\r\n	the equivalent object is returned.</p>\r\n</ul>\r\n<p class=\"western\">The linkage between hard link pseudo-objects and\r\ntheir equivalent object is through two fields:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Each object has a hard_links doubly linked\r\n	list node that links the list of hard links to that object.</p>\r\n	<li/>\r\n<p class=\"western\">Each hard link has a equiv_obj pointer to\r\n	the “equivalent object”.</p>\r\n</ul>\r\n\r\n<p class=\"western\" style=\"page-break-before: always\">The usage of\r\nthis structure is straight forward except for one interesting case\r\nwhen hard links are being deleted. Consider the following sequence of\r\nevents:</p>\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"50\">\r\n	<col width=\"80\">\r\n	<col width=\"449\">\r\n	<tr valign=\"top\">\r\n		<td width=\"50\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">1</font></p>\r\n		</td>\r\n		<td width=\"80\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\"># touch a</font></p>\r\n		</td>\r\n		<td width=\"449\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Create file a</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"50\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">2</font></p>\r\n		</td>\r\n		<td width=\"80\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\"># ln a b</font></p>\r\n		</td>\r\n		<td width=\"449\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Create a hard link b with\r\n			equivalent object a.</font></p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"50\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">3</font></p>\r\n		</td>\r\n		<td width=\"80\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\"># rm a</font></p>\r\n		</td>\r\n		<td width=\"449\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\" align=\"left\" style=\"orphans: 0; widows: 0; page-break-before: auto\">\r\n			<font size=\"3\" style=\"font-size: 12pt\">Remove a. The object must\r\n			still exist under the name b</font></p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n<p class=\"western\" align=\"left\" style=\"margin-bottom: 0cm; orphans: 0; widows: 0; page-break-before: auto\">\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">In step 3, Yaffs cannot just delete the equivalent\r\nobject since that would leave a “hanging” hard link with no\r\nequivalent object. \r\n</p>\r\n<p class=\"western\">We also cannot just change the type of one of the\r\nhardlink objects because that would cause the object\'s obj_id to\r\nchange. This would be unacceptable since it would break any mechanism\r\nthat use the obj_id (of which there are many within and outside of\r\nYaffs).</p>\r\n<p class=\"western\">Thus, if an object is deleted and it has hard\r\nlinks to it then we do the following:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Select one of the object\'s hard links.</p>\r\n	<li/>\r\n<p class=\"western\">Rename the object to the name in the hard\r\n	link.</p>\r\n	<li/>\r\n<p class=\"western\">Delete the hard link pseudo-object.</p>\r\n</ul>\r\n<h2 class=\"western\">12.5 Symbolic links and special objects</h2>\r\n<p class=\"western\">These object types just store values that are\r\nneeded to provide a full POSIX file system.</p>\r\n<p class=\"western\">A symbolic link object saves a symbolic link\r\nstring that is used to provide the POSIX symbolic string mechanism.</p>\r\n<p class=\"western\">A special object is used to store entries for\r\ndevice numbers, pipes and other similar special objects.</p>\r\n<h2 class=\"western\">12.6 File objects</h2>\r\n<p class=\"western\">The yaffs_Objects have a type and a variant\r\nportion which is a union of different data required to hold the state\r\nof different object types.</p>\r\n<p class=\"western\">The file objects have type YAFFS_OBJECT_TYPE_FILE,\r\nand an associated file variant. This stores the following main\r\nvalues:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">file_size:\r\n	file size</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">top_level:\r\n	depth of the Tnode tree</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">top:\r\n	pointer to top of Tnode tree</font></p>\r\n</ul>\r\n<p class=\"western\">The meaning of the file size is quite obvious.\r\nThis is the current length of the file. Note that a file might have\r\nholes in it caused by seeking forward past the extents of the file.</p>\r\n<p class=\"western\">The top and top_level work together to control the\r\nTnode trees. \r\n</p>\r\n<h3 class=\"western\" style=\"page-break-before: always\">12.6.1 Tnode\r\ntree</h3>\r\n<p class=\"western\">Each file has a tnode tree to provide the mapping\r\nfrom file position to actual NAND chunk address. top is a pointer to\r\nthe top of the tnode tree</p>\r\n\r\n<p class=\"western\"><img src=\"data:image/png;base64,[encoded data, 20236 characters]\" name=\"Object1\" align=\"left\" width=\"385\" height=\"378\">\r\n  <br clear=\"left\"/>\r\n</img>\r\n<br/>\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">The tnode tree is made up of tnodes (tree nodes)\r\narranged in levels, each of which holds either:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">At levels greater than 0, a tnode has 8\r\n	pointers to other tnodes in the next level down.</p>\r\n	<li/>\r\n<p class=\"western\">At level 0, a tnode has 16 NAND chunk Ids\r\n	which identify the chunk\'s location in RAM.</p>\r\n</ul>\r\n<p class=\"western\">These numbers are powers of 2 which makes for\r\nsimple look-ups by just applying bitmasks. The code that traverses\r\nthis tree is in yaffs_find_tnode_0().</p>\r\n<p class=\"western\">As the file size grows, more tnodes are added and\r\nas the levels fill, more levels are added. If the file is truncated\r\nor resized downwards, then the tree is pruned. If a chunk in a file\r\nis replaced (eg. data is overwritten or copied by garbage collection)\r\nthen the tnode tree must be updated to reflect the change.</p>\r\n<p class=\"western\">Tnodes typically make up the bulk of the Yaffs\'\r\nRAM usage.</p>\r\n<h1 class=\"western\">13 How various mechanisms work</h1>\r\n<p class=\"western\">Having looked at the Yaffs data structures, we now\r\nconsider how some of the mechanisms in Yaffs operates. This is most\r\neasily done by walking through some operations that Yaffs performs.</p>\r\n<h2 class=\"western\">13.1 Block and chunk management</h2>\r\n<h3 class=\"western\">13.1.1 Block states</h3>\r\n<p class=\"western\">Yaffs tracks the state of each block and chunk in\r\nthe partition. This information is first built up during scanning (or\r\nrecovered from a checkpoint). A block may be in one of the following\r\nstates (see definition of  enum yaffs_block_state):</p>\r\n\r\n<table width=\"605\" cellpadding=\"4\" cellspacing=\"0\">\r\n	<col width=\"146\">\r\n	<col width=\"441\">\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: 1px solid #000000; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0.1cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">UNKNOWN</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border: 1px solid #000000; padding: 0.1cm\">\r\n			<p class=\"western\">The block state is not yet known.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">NEEDS_SCANNING</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">During pre-scanning it has been determined that\r\n			this block has something on it and needs scanning.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">SCANNING</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This block is currently being scanned.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">EMPTY</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This block has nothing in it (is erased) and\r\n			may be used for allocation. The block can get to this state by\r\n			being erased.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">ALLOCATING</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This is the block currently being used for\r\n			allocation by the chunk allocator.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">FULL</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">All the chunks in this block have been\r\n			allocated and at least one chunk contains useful information that\r\n			has not yet been deleted.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">DIRTY</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">All the chunks in this block have been\r\n			allocated and all have been deleted. The block might have been in\r\n			the FULL or COLLECTING state before this. This block can now be\r\n			erased and returned to EMPTY state.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">CHECKPOINT</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This block has checkpoint data in it. When the\r\n			checkpoint is invalidated this block will be erased and returned\r\n			to the EMPTY state.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">COLLECTING</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This block is being garbage collected. As soon\r\n			as all live data has been inspected this block becomes DIRTY.</p>\r\n		</td>\r\n	</tr>\r\n	<tr valign=\"top\">\r\n		<td width=\"146\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: none; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0cm\">\r\n			<p class=\"western\">DEAD</p>\r\n		</td>\r\n		<td width=\"441\" style=\"border-top: none; border-bottom: 1px solid #000000; border-left: 1px solid #000000; border-right: 1px solid #000000; padding-top: 0cm; padding-bottom: 0.1cm; padding-left: 0.1cm; padding-right: 0.1cm\">\r\n			<p class=\"western\">This block has been retired or was marked bad.\r\n			This is a terminal state.</p>\r\n		</td>\r\n	</tr>\r\n</table>\r\n<p class=\"western\"><img src=\"data:image/png;base64,[encoded data, 42088 characters]\" name=\"Object3\" align=\"left\" width=\"575\" height=\"510\">\r\n  <br clear=\"left\"/>\r\n</img>\r\n<br/>\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">The regular runtime states are; EMPTY, ALLOCATING,\r\nFULL, COLLECTING and DIRTY.</p>\r\n<p class=\"western\">The checkpoint state is used for checkpoint\r\nblocks.</p>\r\n<p class=\"western\">The DEAD state is the terminal state for blocks\r\nthat were retired because they were either marked bad or an error was\r\ndetected.</p>\r\n<h3 class=\"western\">13.1.2 Block and chunk allocation</h3>\r\n<p class=\"western\">An available chunk must be allocated before it can\r\nbe written. The chunk allocation mechanism is provided by\r\nyaffs_alloc_chunk() and is very simple. Each yaffs_dev has a current\r\nblock that it is allocating from. This is termed the <i><b>allocation\r\nblock </b></i><i><b>(alloc_block)</b></i>. Chunks are allocated\r\nsequentially from the allocation block. As the chunks are allocated,\r\nblock and chunk management information is updated, including the\r\nblock\'s pages_in_use count and the chunk use bitmap.</p>\r\n<p class=\"western\">When the block is fully allocated, another empty\r\nblock is selected to become the allocation block. The blocks are\r\nselected by searching upwards from the previous allocation block.</p>\r\n<p class=\"western\">In older versions of Yaffs, the Yaffs scanning\r\nwould identify the allocation block being used when the file system\r\nwas last shut down and would continue allocation from that point.\r\nThis had the benefit of continuing to use the same block and slightly\r\nreduce garbage creation. However, doing this could potentially cause\r\nproblems due to attempting to write a block that had been in the\r\nmiddle of a write when power was lost. Thus Yaffs now always starts\r\nallocation on a new block after a mount. This slightly increases the\r\namount of garbage (typically by an insignificant amount) but improves\r\nrobustness.</p>\r\n<h3 class=\"western\">13.1.3 A word about wear leveling</h3>\r\n<p class=\"western\">Since flash memory blocks can only endure a\r\nlimited number of writes and erases ( termed the flash endurance), a\r\nflash file system needs to ensure that the same few blocks are not\r\noverwritten continuously causing those blocks to wear out while the\r\nother blocks are hardly used at all.. Instead it is important that\r\nthe file system spreads out the wear.</p>\r\n<p class=\"western\">This is extremely important in a file system such\r\nas FAT which uses a limited set of logical blocks to store the file\r\nallocation table entries which get modified frequently. If FAT is\r\nused with a static logical to physical block mapping then the file\r\nallocation table blocks will wear out far before other blocks causing\r\npremature device failure. Thus, for FAT, it is very important that\r\nthe logical to physical mapping gets stirred up a bit to cause the\r\nfile allocation tables to be written to different physical blocks,\r\nthus leveling out the wear.</p>\r\n<p class=\"western\">Wear leveling is far less of a concern in a log\r\nstructured file system such as Yaffs since we always write to the end\r\nof the log and thus don\'t keep on overwriting the same blocks.</p>\r\n<p class=\"western\">There are basically two ways to achieve wear\r\nleveling:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Use a\r\n	specifically written set of functions that actively perform wear\r\n	leveling.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Allow\r\n	a degree of wear leveling to happen as a side effect of other\r\n	activity.</font></p>\r\n</ul>\r\n<p class=\"western\">Yaffs takes the second approach.</p>\r\n<p class=\"western\">Firstly, being log structured, Yaffs inherently\r\nspreads out the wear by performing all writes in sequence. That means\r\nthat we don\'t single out a few blocks for a specific purpose and\r\nwrite them more than others.</p>\r\n<p class=\"western\">Secondly, blocks are allocated serially from the\r\nerased blocks in the partition. Thus the process of erasing (after\r\ndeletion of garbage collection) and then allocating blocks does tend\r\nto spread the block use and provides a level of wear leveling.</p>\r\n<p class=\"western\">Thus, even though there is no code to specifically\r\nperform wear leveling it does happen as a side effect of other\r\nactivity.</p>\r\n<p class=\"western\">This strategy performs well and has survived many\r\naccelerated lifetime tests in both simulations and on real NAND-based\r\nproducts.</p>\r\n<h3 class=\"western\">13.1.4 Management of yaffs_tnode and yaffs_obj\r\nstructures</h3>\r\n<p class=\"western\">yaffs_tnodes and yaffs_objs are dynamically\r\ncreated and released as they are needed. \r\n</p>\r\n<p class=\"western\">The allocation and management could be done by\r\njust using the standard memory allocator (malloc, kmalloc etc), but\r\nthis could cause performance problems, particularly in embedded\r\nsystems which tend to have rather simplistic memory allocators. \r\n</p>\r\n<p class=\"western\">The allocation and deletion of many small\r\nstructures can cause memory fragmentation causing out-of-memory and\r\nslowdown problems.</p>\r\n<p class=\"western\">To prevent these problems, Yaffs manages these\r\nobjects internally. Yaffs creates many structures (100 or so) at a\r\ntime and then allocates them singly from a free list. When structures\r\nare released they are placed on the free list for reuse.</p>\r\n<h2 class=\"western\">13.2 Internal cache</h2>\r\n<p class=\"western\">This describes the standard Yaffs internal cache\r\nimplementation. Alternative caches have been devised for other\r\npurposes and at some future point Yaffs might be extended to support\r\npluggable cache policies.</p>\r\n<p class=\"western\">The main purpose of the internal cache is to\r\nreduce NAND access for  badly behaved applications which perform many\r\nsmall reads and writes. Believe it or not, but there are applications\r\nout there that read or write files a few bytes at a time:</p>\r\n<p class=\"code-western\"><code class=\"western\">while(...)<br/>\r\n	write(h,buf,1);</code></p>\r\n<p class=\"western\" style=\"margin-left: 1.25cm\"><br/>\r\n<br/>\r\n\r\n</p>\r\n<p class=\"western\">This sort of behavior would be terrible for\r\nperformance and flash memory life if each read or write operation\r\nresulted in a flash access. Yaffs provides an internal cache to\r\nmitigate against this sort of behaviour.</p>\r\n<p class=\"western\">The Yaffs internal cache is called a “Short Op\r\nCache” (short operation cache) and only comes in to play for\r\noperations which are not chunk aligned. The caching mechanism is very\r\nsimple and was primarily written for less sophisticated OSs that lack\r\ntheir own file caching layer. The cache is still beneficial in OSs\r\nlike Linux, which provide a caching layer, because it caches both the\r\nread and write path. Thus, while short reads will be serviced by the\r\nLinux page cache, the short op cache does address non-chunk aligned\r\nwrites.</p>\r\n<p class=\"western\">Page aligned reads/writes bypass the cache unless\r\nthat chunk is already in the cache, except when inband tags are used.\r\nInband tags always require the cache because the cache is used to\r\nbuild up the buffer where the tags are attached to the data chunk\r\nbeing written.</p>\r\n<p class=\"western\">The cache is stored in an array of cache entries.\r\nThe number of cache entries is set in the yaffs_dev parameters\r\nn_caches and the cache is configured during initialisation. n_caches\r\nof zero disables the cache. The cache management algorithms are\r\nsimplistic and probably don\'t scale well to very large numbers. This\r\nmeans that it is probably best to stick with around 5 to 20 cache\r\nentries.</p>\r\n<p class=\"western\">Each cache entry holds:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">obj_id, chunk id, nbytes (the tag info of\r\n	the chunk being cached)</p>\r\n	<li/>\r\n<p class=\"western\">The actual data bytes</p>\r\n	<li/>\r\n<p class=\"western\">Cache slot status (last used counter,\r\n	dirty,...)</p>\r\n</ul>\r\n<p class=\"western\">Finding an unused cache entry is a simple\r\noperation. We just iterate through the cache entries searching for\r\none that is not assigned. If all cache entries are busy then a cache\r\npush-out is performed to free up a cahce entry.</p>\r\n<p class=\"western\">Whenever a cache entry is accessed (read or\r\nwritten) the last-use counter is is set to an incrementing number.\r\nThis counter value is used to drive the least-recently-used (LRU)\r\nalgorithm.</p>\r\n<p class=\"western\">Cache entries which have been modified and not yet\r\nwritten back to flash are marked with a dirty flag. This is tells us\r\nwhether we need to write out the data during a flush or push-out.</p>\r\n<p class=\"western\">A push-out happens whenever the cache is full and\r\nwe need to push an entry out of the cache to make space to load a new\r\nentry. Yaffs uses the LRU algorithm to select the LRU cache entry,\r\nwrite it out if needed and then assign that for use.</p>\r\n<p class=\"western\">Whenever a file is flushed (file flush or close),\r\nthe file\'s dirty entries are written out. At certain times (eg.\r\nbefore an unmount or at sync()) the entire cache is flushed.</p>\r\n<p class=\"western\">This caching policy is gives a useful performance\r\nimprovement with very simple algorithms (ie good bang-for-bucks).</p>\r\n<p class=\"western\">Apart from its normal caching function, the short\r\nop cache also does much of the heavy lifting for inband tags support\r\nsince all the inband tags accesses are not chunk aligned and must\r\ntypically be re-aligned. The cache  makes this easy.</p>\r\n<h2 class=\"western\">13.3 Scanning</h2>\r\n<p class=\"western\">Scanning is the process whereby file system state\r\nis rebuilt from scratch. This happens on mounting a Yaffs partition\r\nif no checkpoint data is available or if the mounting was told to\r\nignore the checkpoint data.</p>\r\n<p class=\"western\">Scanning involves reading the tags from all the\r\nactive chunks in the system so can take quite a while, but probably\r\nless time than you\'d expect.</p>\r\n<p class=\"western\">The scanning process is different for Yaffs1 vs\r\nYaffs2 because of the differences mentioned before.</p>\r\n<h3 class=\"western\">13.3.1 Yaffs1 scanning</h3>\r\n<p class=\"western\">Yaffs1 scanning is rather straight forward because\r\nYaffs1 uses deletion markers.</p>\r\n<p class=\"western\">All we really need to do is read the tags off all\r\nthe chunks in the partition and determine if the chunk is active.\r\nOrder is not important and blocks could be read in any order. If the\r\ndeletion marker has not been set then it is active and we need to add\r\nthe chunk to the file system state as follows:</p>\r\n<p class=\"western\">If the chunk is a data chunk (chunk_id &gt; 0)\r\nthen the chunk must be added to the relevant file\'s tnode tree. If\r\nthe object does not exist, then we create it in lost+found, although\r\nit has no name and other file details yet.</p>\r\n<p class=\"western\">If the chunk is an object header (chunk_id == 0)\r\nthen the object must be created (or it might already exist in lost +\r\nfound and we need to fix up its details and slot it into the correct\r\npart of the directory tree.</p>\r\n<p class=\"western\">If at any time during scanning we find that we are\r\nreading a second chunk with the same obj_id:chunk_id as a previously\r\nencountered chunk then the 2-bit serial number is used to resolve the\r\ndispute.</p>\r\n<p class=\"western\">Once the scanning is complete we still need to fix\r\nup a few things: \r\n</p>\r\n<p class=\"western\">Hard links must be connected up properly (it is\r\nmore efficient to defer this until after the scanning has completed).</p>\r\n<p class=\"western\">Also, objects which were marked as unlinked must\r\nbe deleted since there are no longer handles open to these objects.</p>\r\n<h3 class=\"western\">13.3.2 Yaffs2 scanning</h3>\r\n<p class=\"western\">Yaffs2 has no deletion markers which makes the\r\nscanning significantly more complex.</p>\r\n<p class=\"western\">The first thing that needs to be done is to\r\npre-scan the blocks to determine the sequence number for the blocks.\r\nThe list of blocks and sequence numbers is then sorted to make a\r\nchronologically ordered list.</p>\r\n<p class=\"western\">Yaffs then scans <i><b>backwards</b></i> through\r\nthese blocks (ie in reverse chronological order). Thus, the first\r\nencountered  obj_id:chunk_id pair is the current one and subsequent\r\nchunks with those tags are out of date.</p>\r\n<p class=\"western\" style=\"margin-left: 1.25cm\">We read the tags for\r\neach chunk in each block. \r\n</p>\r\n<p class=\"western\" style=\"margin-left: 1.25cm\">If it is a data chunk\r\n( ie. chunk_id &gt; 0) then:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">If a chunk with the same obj_id:chunk_id\r\n	has already been found then this is not the current chunk. Delete\r\n	this one.</p>\r\n	<li/>\r\n<p class=\"western\">Else if the object is marked as deleted, or\r\n	corrupted, then delete it.</p>\r\n	<li/>\r\n<p class=\"western\">Else if the object does not yet exist then\r\n	this chunk was written since the last object header write for this\r\n	object so this chunk must be the last written chunk in the file and\r\n	must have happened just before an unclean shutdown with the file\r\n	still open. We can create the file and use the chunk_id and nbytes\r\n	to set the file extents.</p>\r\n	<li/>\r\n<p class=\"western\">Else if the object does exist and the chunk\r\n	lies beyond the object\'s file scan size (extents) then it is a\r\n	truncated chunk and can be deleted.</p>\r\n	<li/>\r\n<p class=\"western\">Else it is placed into the file\'s tnode\r\n	tree.</p>\r\n</ul>\r\n<p class=\"western\" style=\"margin-left: 1.25cm\">If it is an object\r\nheader (ie. chunk_id == 0) then:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">If the object is in the deleted directory\r\n	then mark the object as deleted and set the object\'s shrink size to\r\n	zero.</p>\r\n	<li/>\r\n<p class=\"western\">Else if an object header has not yet been\r\n	found for this object, then this is the most current object header\r\n	and holds the most current name etc.  If no data chunks have yet\r\n	been found for this chunk then the size in the object header is also\r\n	the file extents. The file size is used to set the file scan size\r\n	since at the time it was written the file did not extend past that\r\n	point and we can safely ignore further data chunks for that file\r\n	that extend past that point.</p>\r\n	<li/>\r\n<p class=\"western\">Else just use the file size as the scan\r\n	size if it is smaller than the current file scan size.</p>\r\n	<li/>\r\n<p class=\"western\">Else this an obsolete object header. It is\r\n	deleted.</p>\r\n</ul>\r\n<p class=\"western\">A bit more explanation is probably needed to\r\ndescribe the file scan size. The whole purpose of this is to\r\ndetermine data chunks that should be ignored due to the file being\r\ntruncated.</p>\r\n<h2 class=\"western\">13.4 Checkpoint</h2>\r\n<p class=\"western\">Mount scanning takes quite a lot of time and slows\r\nmounting, though perhaps by less than you\'d think. Checkpointing is a\r\nmechanism to speed the mounting by taking a snapshot of the Yaffs\r\nruntime state at unmount or sync() and then reconstituting the\r\nruntime state on re-mounting. The speed up is dramatic.</p>\r\n<p class=\"western\">The actual checkpoint mechanism is quite simple. A\r\n“stream” of data is written to a set of blocks which are marked\r\nas holding checkpoint data and the important runtime state is written\r\nto the stream.</p>\r\n<p class=\"western\">Not all state needs to be saved, only the state\r\nneeded to reconstitute the structure of the runtime data. For\r\nexample, file metadata need not be saved since it is very easy to\r\nload that through lazy-loading.</p>\r\n<p class=\"western\">The checkpoint stores data in the following order:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Start\r\n	marker, including checkpoint format id.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">yaffs_dev\r\n	information</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Block\r\n	information</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Chunk\r\n	flags</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Objects,\r\n	including file structure</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">End\r\n	marker</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Checksum</font></p>\r\n</ul>\r\n<p class=\"western\">The checkpoint validity is ensured by the\r\nfollowing mechanisms:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">The\r\n	data is stored using whatever ECC mechanisms are in place.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">The\r\n	start marker includes a checkpoint version so that an obsolete\r\n	checkpoint will not be read if the checkpoint code changes.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Data\r\n	is written to the stream as structured records, each with a type and\r\n	size.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">The\r\n	end marker must be read when expected.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">Two\r\n	checksums are maintained across the whole checkpoint data set.</font></p>\r\n</ul>\r\n<p class=\"western\">If any checks fail, the checkpoint is ignored and\r\nthe state is re-scanned.</p>\r\n<p class=\"western\">The astute reader might notice that block state\r\nchanges as we\'re writing checkpoint blocks, so how do we manage to\r\nre-constitute the block states correctly? This is achieved by the\r\nfollowing:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">As we\r\n	write the checkpoint, the checkpoint block allocation selects EMPTY\r\n	blocks. We don\'t change the state as we\'re writing the checkpoint so\r\n	their state is written to the checkpoint as EMPTY or CHECKPOINT, it\r\n	does not really matter which.</font></p>\r\n	<li/>\r\n<p class=\"western\"><font face=\"Liberation Serif, serif\">After\r\n	reading the checkpoint, we know which blocks have been used to store\r\n	checkpoint data. We update those blocks to reflect the CHECKPOINT\r\n	state.</font></p>\r\n</ul>\r\n<p class=\"western\">A checkpoint is invalidated by any modifications\r\n(writes, erases,...) thus any modification path also checks to see if\r\nthere is a checkpoint and erases it.</p>\r\n<p class=\"western\">The regular Yaffs block allocator and garbage\r\ncollection logic also needs to be aware of the approximate size of\r\nthe checkpoint so that these can ensure enough spare space is\r\nreserved to store a checkpoint. There is a calculation that updates\r\nthe value whenever the number of objects etc changes.</p>\r\n<h2 class=\"western\">13.5 Extended tags and packed tags</h2>\r\n<p class=\"western\">Extended tags is an abstraction of a way to store\r\nmore information in the tags, allowing for faster scan times. Packed\r\nTags is a physical implementation of the storage of extended tags.</p>\r\n<p class=\"western\">When we\'re scanning we really only need to build\r\nup the structure of the file system. In other words, rebuild the\r\ndirectory relationships, tnode trees and such. Much of the detail\r\nsuch as file names, permissions etc can be lazy loaded if and when\r\nneeded.</p>\r\n<p class=\"western\">This required storing extra information, such as\r\nthe parent directory and object type, in the tags of object headers.\r\nBut how do we do this without enlarging the tags size?</p>\r\n<p class=\"western\">Object headers always have a chunk_id of zero and\r\nn_bytes has no meaning because it is not a data chunk. By using a\r\nsingle bit to identify object headers we suddenly open up a whole lot\r\nof space that we can stuff full of useful data. By being clever as to\r\nwhat data we use, we can pack in the important data in most\r\ncircumstances.</p>\r\n<p class=\"western\">Sometimes, for example very large files, the\r\npacking will not work because there are insufficient bits to hold the\r\nfield so we just write regular tags instead. That\'s OK since there\r\ncan only be few such files in the system so the performance reduction\r\nonly impacts very few files and the system level impact is\r\nunnoticeable. A simple, cheap, strategy that gives speed up 99% of\r\nthe time is a very handy one.</p>\r\n<p class=\"western\">Other details are lazy-loaded on demand. This will\r\nhappen when the object is looked up (eg. by a file open or searching\r\nfor the file in the directory).</p>\r\n<h2 class=\"western\">13.6 Block summaries</h2>\r\n<p class=\"western\">Scanning requires that the tags for each chunk are\r\nread so that they can be inspected. Reading the tags from each chunk\r\ncan be time consuming. Thus, block summaries were added in 2011.</p>\r\n<p class=\"western\">Block summaries write all the tags data for chunks\r\nin the block into the last few (typically only one) chunk in a block.\r\nThis allows all the tags for chunks in that block to be read in one\r\nhit. Depending on the system throughput (CPU, RAM and flash speeds),\r\nthis can speed up mount time dramatically.</p>\r\n<p class=\"western\">Note that the block summary does not replace the\r\ntags attached to each chunk. Both are written. Thus, if the whole\r\nblock is not written (and the block summary is thus not written), the\r\nchunk tags can still be read and used.</p>\r\n<p class=\"western\">If a block summary is not available for a block,\r\nthen the old mechanism is still used to scan the tags on each chunk.</p>\r\n<h2 class=\"western\">13.7 Inband tags</h2>\r\n<p class=\"western\">Thus far we have considered chunks where there are\r\nsufficient unallocated bytes in the spare (or out of band) area to\r\nstore the tags. \r\n</p>\r\n<p class=\"western\">But what happens if we don\'t have enough space in\r\nthe spare area to store the tags? Inband tags to the rescue! \r\n</p>\r\n<p class=\"western\">With inband tags we store the tags in the flash\r\ndata area alongside the data. This gives us far more flexibility in\r\nthe types of storage that Yaffs can support.</p>\r\n<p class=\"western\">There are three major penalties with inband tags:</p>\r\n<ul>\r\n	<li/>\r\n<p class=\"western\">Due to the loss of chunk alignment, all\r\n	transactions must pass through the short op cache. This costs an\r\n	extra memcpy().</p>\r\n	<li/>\r\n<p class=\"western\"></p>\r\n	<li/>\r\n<p class=\"western\">Reading tags during scanning etc can\r\n	involve reading the whole page, making this slower. The impact is\r\n	minimised by using checkpointing or block summaries.</p>\r\n</ul>\r\n<p class=\"western\">Thus, it is better, performance wise, to only use\r\ninband tags when regular tags will not work.</p>\r\n<h2 class=\"western\">13.8 Soft deletion</h2>\r\n<p class=\"western\">Soft deletion is a mechanism that was added to\r\nspeed up file deletion in Yaffs1 mode.</p>\r\n<p class=\"western\">Looking back to the description of how files are\r\nstored in Yaffs1, you can see that deleting a file involves writing\r\nthe deletion marker of every data chunk in the file. This can take\r\nquite some time for large files.</p>\r\n<p class=\"western\">The soft deletion mechanism was added to avoid\r\nwriting all those deletion markers. Instead of writing the deletion\r\nmarkers, soft deletion instead tracks the number of chunks in each\r\nblock that belong to deleted files and leaves the clean up to the\r\ngarbage collector.</p>\r\n<p class=\"western\">When the garbage collector looks at a block it can\r\nsee what current chunks (ie chunks not yet deleted) belong to deleted\r\nfiles and does not copy those, thus effectively deleting the chunk.\r\nOnce all the chunks associated with a deleted file have cleaned up by\r\nthe garbage collector the file header object can also be deleted.</p>\r\n<p class=\"western\">Yaffs2 mode does not write deletion markers and\r\nthus does not use soft deletion.</p>\r\n','This document gives a reasonable explanation of most of the core mechanisms that make Yaffs work. As of the time of writing this, the Yaffs2 code base includes approximately 19,000 lines of code making a very detailed discussion impractical. This document should serve as a first port of call for understanding Yaffs. Further detailed questions and discussions are welcomed on the Yaffs discussion list.','raw_html'),('what_is_special_about_yaffs',0,414,415,'und',0,'<p>If you want to use Yaffs on an Open Source project, then go ahead and <a href=\"/downloads\">download</a> the GNU General Public Licence v.2 now. You can get it straight away from the link on the right of the page.</p>\r\n\r\n<p>Yaffs is also available under a Commercial Licence&nbsp;for projects that don\'t want to disclose their source code.&nbsp; This Licence does not have the obligations of GPL, but is perpetual, world-wide, non-exclusive and carries no continuing royalty.</p>\r\n\r\n<p>You can use the Open Source code to investigate and evaluate Yaffs, then before shipping your product to anyone outside of your organisation you buy a licence from Aleph One which does not have the GNU GPL obligation to release your Source Code.&nbsp; Contact&nbsp; sales@aleph1.co.uk&nbsp; for more information or have a look at our <a href=\"/downloads\">licensing information</a>.</p>\r\n','If you want to use Yaffs on an Open Source project, then go ahead and <a href=\"/downloads\">download</a> the GNU General Public Licence, version 2 form now. It is also available under a commercial Licence without the obligations entailed in the GNU GPL version.','basic_html'),('what_is_special_about_yaffs',0,415,416,'und',0,'<p>When shopping for a software component, it\'s always useful to be able to try it out before buying.</p>\r\n\r\n<p>For your proprietary projects, the GPL version of Yaffs lets you do just that - download it, examine the code and test it out. Check out our documentation and videos.</p>\r\n\r\n<p>It\'s perfectly acceptable and safe to use the GNU GPL code inside your company, providing you don\'t ship a binary to anyone beyond it.&nbsp;That\'s when code disclosure and licensing provisions of the GNU GPL kick in.&nbsp;So before you do that, you\'ll want to talk to us about acquiring a paid Licence.</p>\r\n','If you think that Yaffs might be what you need, you can test it without buying a licence.','basic_html'),('what_is_special_about_yaffs',0,416,417,'und',0,'<p>Having started in 2002 Yaffs embodies more than ten years of experience with Flash memory and its specialised needs, coupled with continued growth in the number of people and companies using Yaffs.</p>\r\n\r\n<p>Since its inception numerous features and refinements have been added. The code continues to evolve, growing in value to users whilst retaining backward compatibility. The community of users continues to grow as more and more people and companies opt to use Yaffs, providing a pool of knowledge which is accessible to new users.&nbsp;</p>\r\n\r\n<p>The availability of the GNU General Public Licence version means that there\'s always an exit route for open source projects, and since it\'s open-source, existing commercial projects will at the very least always have the source code and the licence to continue to use it, and likely much more. Plus commercial projects benefit from the open source experience in Yaffs - both with respect to developers and knowledge.&nbsp;</p>\r\n\r\n<p>Yaffs is a safe choice for any project needing a Flash file system: it has well over ten years of history, proven sustainability, a continuously improving codebase, a growing user base, and exit routes, since it&nbsp;will continue to be available regardless of Yaffs.net.</p>\r\n','Yaffs has a long history, coupled with continuing development. There are thousands of developers who have used it, and it\'s deployed in many millions of devices.','basic_html'),('what_is_special_about_yaffs',0,417,418,'und',0,'<p>Yaffs support is available through the Yaffs community - primarily via the mail list - with help coming from our key developers as well as other people who use Yaffs. For those times when you don\'t feel comfortable asking a question in a public forum, perhaps because the project is sensitive, we will normally answer a few questions off-list before we offer you the opportunity of consultancy and direct access to our developers. And of course we are always willing to discuss other forms of support for customers who buy licences.</p>\r\n','Yaffs support is available through the Yaffs community but for those problems that aren\'t suitable for a public forum, we offer consultancy with direct access to our developers.','basic_html'),('what_is_special_about_yaffs',0,418,419,'und',0,'<p>Yaffs provides bad block handling and ECC algorithms to handle deficiencies in NAND flash.</p>\r\n\r\n<p>It is a log-structured file system which makes it very robust to corruptions caused by power failures.</p>\r\n\r\n<p>We spend about 60% of development time on testing Yaffs, and our test harness is available for download so you can check the testing and perform some yourself.</p>\r\n\r\n<p>We\'ve performed hundreds of thousands of tests to make sure that Yaffs is as safe as we can make it, even in the even of a power failure during a firmware update.</p>\r\n\r\n<p>You can find full details of the Yaffs testing regime in our paper <a href=\"http://www.yaffs.net/documents/yaffs-robustness-and-testing\">http://www.yaffs.net/documents/yaffs-robustness-and-testing</a>&nbsp; including all the details on the power-fail stress tests, the Yaffs Direct API tests for commercial RTOSs, Linux testing, and fuzz testing where we deliberately corrupt Yaffs file systems to check that Yaffs can still mount.</p>\r\n','Yaffs is designed for data safety and rigorously tested to ensure it does the best possible to keep users\' data safe. It\'s been crash-tested it hundreds of thousands of times ...','basic_html'),