xref: /linux/Documentation/filesystems/ext4/group_descr.rst (revision 4f2c0a4acffbec01079c28f839422e64ddeff004) 
1.. SPDX-License-Identifier: GPL-2.0
2
3Block Group Descriptors
4-----------------------
5
6Each block group on the filesystem has one of these descriptors
7associated with it. As noted in the Layout section above, the group
8descriptors (if present) are the second item in the block group. The
9standard configuration is for each block group to contain a full copy of
10the block group descriptor table unless the sparse_super feature flag
11is set.
12
13Notice how the group descriptor records the location of both bitmaps and
14the inode table (i.e. they can float). This means that within a block
15group, the only data structures with fixed locations are the superblock
16and the group descriptor table. The flex_bg mechanism uses this
17property to group several block groups into a flex group and lay out all
18of the groups' bitmaps and inode tables into one long run in the first
19group of the flex group.
20
21If the meta_bg feature flag is set, then several block groups are
22grouped together into a meta group. Note that in the meta_bg case,
23however, the first and last two block groups within the larger meta
24group contain only group descriptors for the groups inside the meta
25group.
26
27flex_bg and meta_bg do not appear to be mutually exclusive features.
28
29In ext2, ext3, and ext4 (when the 64bit feature is not enabled), the
30block group descriptor was only 32 bytes long and therefore ends at
31bg_checksum. On an ext4 filesystem with the 64bit feature enabled, the
32block group descriptor expands to at least the 64 bytes described below;
33the size is stored in the superblock.
34
35If gdt_csum is set and metadata_csum is not set, the block group
36checksum is the crc16 of the FS UUID, the group number, and the group
37descriptor structure. If metadata_csum is set, then the block group
38checksum is the lower 16 bits of the checksum of the FS UUID, the group
39number, and the group descriptor structure. Both block and inode bitmap
40checksums are calculated against the FS UUID, the group number, and the
41entire bitmap.
42
43The block group descriptor is laid out in ``struct ext4_group_desc``.
44
45.. list-table::
46   :widths: 8 8 24 40
47   :header-rows: 1
48
49   * - Offset
50     - Size
51     - Name
52     - Description
53   * - 0x0
54     - __le32
55     - bg_block_bitmap_lo
56     - Lower 32-bits of location of block bitmap.
57   * - 0x4
58     - __le32
59     - bg_inode_bitmap_lo
60     - Lower 32-bits of location of inode bitmap.
61   * - 0x8
62     - __le32
63     - bg_inode_table_lo
64     - Lower 32-bits of location of inode table.
65   * - 0xC
66     - __le16
67     - bg_free_blocks_count_lo
68     - Lower 16-bits of free block count.
69   * - 0xE
70     - __le16
71     - bg_free_inodes_count_lo
72     - Lower 16-bits of free inode count.
73   * - 0x10
74     - __le16
75     - bg_used_dirs_count_lo
76     - Lower 16-bits of directory count.
77   * - 0x12
78     - __le16
79     - bg_flags
80     - Block group flags. See the bgflags_ table below.
81   * - 0x14
82     - __le32
83     - bg_exclude_bitmap_lo
84     - Lower 32-bits of location of snapshot exclusion bitmap.
85   * - 0x18
86     - __le16
87     - bg_block_bitmap_csum_lo
88     - Lower 16-bits of the block bitmap checksum.
89   * - 0x1A
90     - __le16
91     - bg_inode_bitmap_csum_lo
92     - Lower 16-bits of the inode bitmap checksum.
93   * - 0x1C
94     - __le16
95     - bg_itable_unused_lo
96     - Lower 16-bits of unused inode count. If set, we needn't scan past the
97       ``(sb.s_inodes_per_group - gdt.bg_itable_unused)`` th entry in the
98       inode table for this group.
99   * - 0x1E
100     - __le16
101     - bg_checksum
102     - Group descriptor checksum; crc16(sb_uuid+group_num+bg_desc) if the
103       RO_COMPAT_GDT_CSUM feature is set, or
104       crc32c(sb_uuid+group_num+bg_desc) & 0xFFFF if the
105       RO_COMPAT_METADATA_CSUM feature is set.  The bg_checksum
106       field in bg_desc is skipped when calculating crc16 checksum,
107       and set to zero if crc32c checksum is used.
108   * -
109     -
110     -
111     - These fields only exist if the 64bit feature is enabled and s_desc_size
112       > 32.
113   * - 0x20
114     - __le32
115     - bg_block_bitmap_hi
116     - Upper 32-bits of location of block bitmap.
117   * - 0x24
118     - __le32
119     - bg_inode_bitmap_hi
120     - Upper 32-bits of location of inodes bitmap.
121   * - 0x28
122     - __le32
123     - bg_inode_table_hi
124     - Upper 32-bits of location of inodes table.
125   * - 0x2C
126     - __le16
127     - bg_free_blocks_count_hi
128     - Upper 16-bits of free block count.
129   * - 0x2E
130     - __le16
131     - bg_free_inodes_count_hi
132     - Upper 16-bits of free inode count.
133   * - 0x30
134     - __le16
135     - bg_used_dirs_count_hi
136     - Upper 16-bits of directory count.
137   * - 0x32
138     - __le16
139     - bg_itable_unused_hi
140     - Upper 16-bits of unused inode count.
141   * - 0x34
142     - __le32
143     - bg_exclude_bitmap_hi
144     - Upper 32-bits of location of snapshot exclusion bitmap.
145   * - 0x38
146     - __le16
147     - bg_block_bitmap_csum_hi
148     - Upper 16-bits of the block bitmap checksum.
149   * - 0x3A
150     - __le16
151     - bg_inode_bitmap_csum_hi
152     - Upper 16-bits of the inode bitmap checksum.
153   * - 0x3C
154     - __u32
155     - bg_reserved
156     - Padding to 64 bytes.
157
158.. _bgflags:
159
160Block group flags can be any combination of the following:
161
162.. list-table::
163   :widths: 16 64
164   :header-rows: 1
165
166   * - Value
167     - Description
168   * - 0x1
169     - inode table and bitmap are not initialized (EXT4_BG_INODE_UNINIT).
170   * - 0x2
171     - block bitmap is not initialized (EXT4_BG_BLOCK_UNINIT).
172   * - 0x4
173     - inode table is zeroed (EXT4_BG_INODE_ZEROED).
174