From 8a98ec7c7b3901330a036af0f62f523c31d763da Mon Sep 17 00:00:00 2001 From: "Darrick J. Wong" Date: Fri, 5 Oct 2018 19:20:08 -0400 Subject: docs: promote the ext4 data structures book to top level Move the ext4 data structures book to Documentation/filesystems/ext4/ since the administrative information moved elsewhere. Signed-off-by: Darrick J. Wong Signed-off-by: Theodore Ts'o --- Documentation/filesystems/ext4/about.rst | 44 ++++++++++++++++++++++++++++++++ 1 file changed, 44 insertions(+) create mode 100644 Documentation/filesystems/ext4/about.rst (limited to 'Documentation/filesystems/ext4/about.rst') diff --git a/Documentation/filesystems/ext4/about.rst b/Documentation/filesystems/ext4/about.rst new file mode 100644 index 000000000000..0aadba052264 --- /dev/null +++ b/Documentation/filesystems/ext4/about.rst @@ -0,0 +1,44 @@ +.. 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 http://www.nongnu.org/ext2-doc/ for quite a collection of +information about ext2/3. Here's another old reference: +http://wiki.osdev.org/Ext2 -- cgit v1.2.1