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