linux/Documentation/filesystems/ext4/about.rst
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   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 https://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