1.. SPDX-License-Identifier: GPL-2.0 2 3About this Book 4=============== 5 6This document attempts to describe the on-disk format for ext4 7filesystems. The same general ideas should apply to ext2/3 filesystems 8as well, though they do not support all the features that ext4 supports, 9and 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 13structure definitions should be current as of Linux 4.18 and 14e2fsprogs-1.44. All comments and corrections are welcome, since there is 15undoubtedly plenty of lore that might not be reflected in freshly 16created demonstration filesystems. 17 18License 19------- 20This book is licensed under the terms of the GNU Public License, v2. 21 22Terminology 23----------- 24 25ext4 divides a storage device into an array of logical blocks both to 26reduce bookkeeping overhead and to increase throughput by forcing larger 27transfer sizes. Generally, the block size will be 4KiB (the same size as 28pages on x86 and the block layer's default block size), though the 29actual size is calculated as 2 ^ (10 + ``sb.s_log_block_size``) bytes. 30Throughout this document, disk locations are given in terms of these 31logical blocks, not raw LBAs, and not 1024-byte blocks. For the sake of 32convenience, the logical block size will be referred to as 33``$block_size`` throughout the rest of the document. 34 35When referenced in ``preformatted text`` blocks, ``sb`` refers to fields 36in the super block, and ``inode`` refers to fields in an inode table 37entry. 38 39Other References 40---------------- 41 42Also see http://www.nongnu.org/ext2-doc/ for quite a collection of 43information about ext2/3. Here's another old reference: 44http://wiki.osdev.org/Ext2 45