annotate www/ext2.html @ 1057:242c5de2bb22 draft

Replace for_each_pid_with_name_in_array_perform_callback_function_upon_translated_value() with name_to_pid(), comparing absolute paths or just basename() consistently as spotted by Lukasz Skalski, and adjust callers.
author Rob Landley <rob@landley.net>
date Tue, 03 Sep 2013 18:43:32 -0500
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1 <title>Rob's ext2 documentation</title>
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2
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3 <p>This page focuses on the ext2 on-disk format. The Linux kernel's filesystem
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4 implementation (the code to read and write it) is documented in the kernel
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5 source, Documentation/filesystems/ext2.txt.</p>
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6
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7 <p>Note: for our purposes, ext3 and ext4 are just ext2 with some extra data
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8 fields.</p>
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9
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10 <h2>Overview</h2>
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11
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12 <h2>Blocks and Block Groups</h2>
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13
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14 <p>Every ext2 filesystem consists of blocks, which are divided into block
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15 groups. Blocks can be 1k, 2k, or 4k in length.<super><a href="#1">[1]</a></super>
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16 All ext2 disk layout is done in terms of these logical blocks, never in
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17 terms of 512-byte logical blocks.</p>
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18
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19 <p>Each block group contains as many blocks as one block can hold a
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20 bitmap for, so at a 1k block size a block group contains 8192 blocks (1024
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21 bytes * 8 bits), and at 4k block size a block group contains 32768 blocks.
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22 Groups are numbered starting at 0, and occur one after another on disk,
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23 in order, with no gaps between them.</p>
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24
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25 <p>Block groups contain the following structures, in order:</p>
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26
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27 <ul>
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28 <li>Superblock (sometimes)</li>
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29 <li>Group table (sometimes)</li>
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30 <li>Block bitmap</li>
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31 <li>Inode bitmap</li>
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32 <li>Inode table</li>
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33 <li>Data blocks</li>
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34 </ul>
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35
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36 <p>Not all block groups contain all structures. Specifically, the first two
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37 (superblock and group table) only occur in some groups, and other block
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38 groups start with the block bitmap and go from there. This frees up more
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39 data blocks to hold actual file and directory data, see the superblock
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40 description for details.</p>
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41
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42 <p>Each structure in this list is stored in its' own block (or blocks in the
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43 case of the group and inode tables), and doesn't share blocks with any other
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44 structure. This can involve padding the end of the block with zeroes, or
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45 extending tables with extra entries to fill up the rest of the block.</p>
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46
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47 <p>The linux/ext2_fs.h #include file defines struct ext2_super_block,
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48 struct ext2_group_desc, struct ext2_inode, struct ext2_dir_entry_2, and a lot
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49 of constants. Toybox doesn't use this file directly, instead it has an e2fs.h
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50 include of its own containting cleaned-up versions of the data it needs.</p>
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51
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52 <h2>Superblock</h2>
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53
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54 <p>The superblock contains a 1024 byte structure, which toybox calls
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55 "struct ext2_superblock". Where exactly this structure is to be found is
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56 a bit complicated for historical reasons.</p>
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57
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58 <p>For copies of the superblock stored in block groups after the first,
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59 the superblock structure starts at the beginning of the first block of the
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60 group, with zero padding afterwards if necessary (I.E. if the block size is
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61 larger than 1k). In modern "sparse superblock" filesystems (everything
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62 anyone still cares about), the superblock occurs in group 0 and in later groups
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63 that are powers of 3, 5, and 7. (So groups 0, 1, 3, 5, 7, 9, 25, 27, 49, 81,
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64 125, 243, 343...) Any block group starting with a superblock will also
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65 have a group descriptor table, and ones that don't won't.</p>
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66
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67 <p>The very first superblock is weird. This is because if you format an entire
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68 block device (rather than a partition), you stomp the very start of the disk
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69 which contains the boot sector and the partition table. Back when ext2 on
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70 floppies was common, this was a big deal.</p>
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71
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72 <p>So the very first 1024 bytes of the very first block are always left alone.
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73 When the block size is 1024 bytes, then that block is left alone and the
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74 superblock is stored in the second block instead<super><a href="#2">[2]</a>.
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75 When the block size is larger than 1024 bytes, the first superblock starts
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76 1024 bytes into the block, with the original data preserved by mke2fs and
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77 appropriate zero padding added to the end of the block (if necessary).</p>
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78
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79 <h2>Group descriptor table</h2>
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80 <h2>Block bitmap</h2>
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81 <h2>Inode bitmap</h2>
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82 <h2>Inode table</h2>
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83 <h2>Data blocks</h2>
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84
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85 <h2>Directories</h2>
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86
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87 <p>For performance reasons, directory entries are 4-byte aligned (rec_len is
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88 a multiple of 4), so up to 3 bytes of padding (zeroes) can be added at the end
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89 of each name. (This affects rec_len but not the name_len.)</p>
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90
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91 <p>The last directory entry in each block is padded up to block size. If there
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92 isn't enough space for another struct ext2_dentry the last </p>
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93
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94 <p>Question: is the length stored in the inode also padded up to block size?</p>
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95
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96 <hr />
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97 <p><a name="1" />Footnote 1: On some systems blocks can be larger than 4k, but
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98 for implementation reasons not larger than PAGE_SIZE. So the Alpha can have
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99 8k blocks but most other systems couldn't mount them, thus you don't see this
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100 out in the wild much anymore.</p>
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101
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102 <p><a name="2" />Footnote 2: In this case, the first_data_block field in the
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103 superblock structure will be set to 1. Otherwise it's always 0. How this
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104 could POSSIBLY be useful information is an open question, since A) you have to
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105 read the superblock before you can get this information, so you know where
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106 it came from, B) the first copy of the superblock always starts at offset 1024
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107 no matter what, and if your block size is 1024 you already know you skipped the
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108 first block.</p>