filesystems/ext4/blockgroup.rst

1 .. SPDX-License-Identifier: GPL-2.0
3 Layout  chapter
4 ------
6 The layout of a standard block group is approximately as follows (each
9 .. list-table::
11    :header-rows: 1
13    * - Group 0 Padding
14      - ext4 Super Block
15      - Group Descriptors
16      - Reserved GDT Blocks
17      - Data Block Bitmap
18      - inode Bitmap
19      - inode Table
20      - Data Blocks
21    * - 1024 bytes
22      - 1 block
23      - many blocks
24      - many blocks
25      - 1 block
26      - 1 block
27      - many blocks
28      - many more blocks
63 ---------------------
71 superblock, group descriptors, data block bitmaps for groups 0-3, inode
72 bitmaps for groups 0-3, inode tables for groups 0-3, and the remaining
81 -----------------
85 128MiB(2^27 bytes) block group size and 64-byte group descriptors, ext4
103 the superblock is followed by a variable-length set of block group
106 groups in a meta-block group. A meta-block group is a collection of
109 bytes, a meta-block group contains 16 block groups for filesystems with
112 descriptor layout, or existing filesystems can be resized on-line, and
114 block group using this new layout.
120 -------------------------------
126 therefore the on-disk bitmap blocks are not initialized. This is
128 only fixed-location block group metadata. The INODE_ZEROED flag means