| .. 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 |
