1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 About this Book 3 About this Book
4 =============== 4 ===============
5 5
6 This document attempts to describe the on-disk 6 This document attempts to describe the on-disk format for ext4
7 filesystems. The same general ideas should app 7 filesystems. The same general ideas should apply to ext2/3 filesystems
8 as well, though they do not support all the fe 8 as well, though they do not support all the features that ext4 supports,
9 and the fields will be shorter. 9 and the fields will be shorter.
10 10
11 **NOTE**: This is a work in progress, based on 11 **NOTE**: This is a work in progress, based on notes that the author
12 (djwong) made while picking apart a filesystem 12 (djwong) made while picking apart a filesystem by hand. The data
13 structure definitions should be current as of 13 structure definitions should be current as of Linux 4.18 and
14 e2fsprogs-1.44. All comments and corrections a 14 e2fsprogs-1.44. All comments and corrections are welcome, since there is
15 undoubtedly plenty of lore that might not be r 15 undoubtedly plenty of lore that might not be reflected in freshly
16 created demonstration filesystems. 16 created demonstration filesystems.
17 17
18 License 18 License
19 ------- 19 -------
20 This book is licensed under the terms of the G 20 This book is licensed under the terms of the GNU Public License, v2.
21 21
22 Terminology 22 Terminology
23 ----------- 23 -----------
24 24
25 ext4 divides a storage device into an array of 25 ext4 divides a storage device into an array of logical blocks both to
26 reduce bookkeeping overhead and to increase th 26 reduce bookkeeping overhead and to increase throughput by forcing larger
27 transfer sizes. Generally, the block size will 27 transfer sizes. Generally, the block size will be 4KiB (the same size as
28 pages on x86 and the block layer's default blo 28 pages on x86 and the block layer's default block size), though the
29 actual size is calculated as 2 ^ (10 + ``sb.s_ 29 actual size is calculated as 2 ^ (10 + ``sb.s_log_block_size``) bytes.
30 Throughout this document, disk locations are g 30 Throughout this document, disk locations are given in terms of these
31 logical blocks, not raw LBAs, and not 1024-byt 31 logical blocks, not raw LBAs, and not 1024-byte blocks. For the sake of
32 convenience, the logical block size will be re 32 convenience, the logical block size will be referred to as
33 ``$block_size`` throughout the rest of the doc 33 ``$block_size`` throughout the rest of the document.
34 34
35 When referenced in ``preformatted text`` block 35 When referenced in ``preformatted text`` blocks, ``sb`` refers to fields
36 in the super block, and ``inode`` refers to fi 36 in the super block, and ``inode`` refers to fields in an inode table
37 entry. 37 entry.
38 38
39 Other References 39 Other References
40 ---------------- 40 ----------------
41 41
42 Also see https://www.nongnu.org/ext2-doc/ for 42 Also see https://www.nongnu.org/ext2-doc/ for quite a collection of
43 information about ext2/3. Here's another old r 43 information about ext2/3. Here's another old reference:
44 http://wiki.osdev.org/Ext2 44 http://wiki.osdev.org/Ext2
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