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