1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 Block Group Descriptors 3 Block Group Descriptors
4 ----------------------- 4 -----------------------
5 5
6 Each block group on the filesystem has one of 6 Each block group on the filesystem has one of these descriptors
7 associated with it. As noted in the Layout sec 7 associated with it. As noted in the Layout section above, the group
8 descriptors (if present) are the second item i 8 descriptors (if present) are the second item in the block group. The
9 standard configuration is for each block group 9 standard configuration is for each block group to contain a full copy of
10 the block group descriptor table unless the sp 10 the block group descriptor table unless the sparse_super feature flag
11 is set. 11 is set.
12 12
13 Notice how the group descriptor records the lo 13 Notice how the group descriptor records the location of both bitmaps and
14 the inode table (i.e. they can float). This me 14 the inode table (i.e. they can float). This means that within a block
15 group, the only data structures with fixed loc 15 group, the only data structures with fixed locations are the superblock
16 and the group descriptor table. The flex_bg me 16 and the group descriptor table. The flex_bg mechanism uses this
17 property to group several block groups into a 17 property to group several block groups into a flex group and lay out all
18 of the groups' bitmaps and inode tables into o 18 of the groups' bitmaps and inode tables into one long run in the first
19 group of the flex group. 19 group of the flex group.
20 20
21 If the meta_bg feature flag is set, then sever 21 If the meta_bg feature flag is set, then several block groups are
22 grouped together into a meta group. Note that 22 grouped together into a meta group. Note that in the meta_bg case,
23 however, the first and last two block groups w 23 however, the first and last two block groups within the larger meta
24 group contain only group descriptors for the g 24 group contain only group descriptors for the groups inside the meta
25 group. 25 group.
26 26
27 flex_bg and meta_bg do not appear to be mutual 27 flex_bg and meta_bg do not appear to be mutually exclusive features.
28 28
29 In ext2, ext3, and ext4 (when the 64bit featur 29 In ext2, ext3, and ext4 (when the 64bit feature is not enabled), the
30 block group descriptor was only 32 bytes long 30 block group descriptor was only 32 bytes long and therefore ends at
31 bg_checksum. On an ext4 filesystem with the 64 31 bg_checksum. On an ext4 filesystem with the 64bit feature enabled, the
32 block group descriptor expands to at least the 32 block group descriptor expands to at least the 64 bytes described below;
33 the size is stored in the superblock. 33 the size is stored in the superblock.
34 34
35 If gdt_csum is set and metadata_csum is not se 35 If gdt_csum is set and metadata_csum is not set, the block group
36 checksum is the crc16 of the FS UUID, the grou 36 checksum is the crc16 of the FS UUID, the group number, and the group
37 descriptor structure. If metadata_csum is set, 37 descriptor structure. If metadata_csum is set, then the block group
38 checksum is the lower 16 bits of the checksum 38 checksum is the lower 16 bits of the checksum of the FS UUID, the group
39 number, and the group descriptor structure. Bo 39 number, and the group descriptor structure. Both block and inode bitmap
40 checksums are calculated against the FS UUID, 40 checksums are calculated against the FS UUID, the group number, and the
41 entire bitmap. 41 entire bitmap.
42 42
43 The block group descriptor is laid out in ``st 43 The block group descriptor is laid out in ``struct ext4_group_desc``.
44 44
45 .. list-table:: 45 .. list-table::
46 :widths: 8 8 24 40 46 :widths: 8 8 24 40
47 :header-rows: 1 47 :header-rows: 1
48 48
49 * - Offset 49 * - Offset
50 - Size 50 - Size
51 - Name 51 - Name
52 - Description 52 - Description
53 * - 0x0 53 * - 0x0
54 - __le32 54 - __le32
55 - bg_block_bitmap_lo 55 - bg_block_bitmap_lo
56 - Lower 32-bits of location of block bitm 56 - Lower 32-bits of location of block bitmap.
57 * - 0x4 57 * - 0x4
58 - __le32 58 - __le32
59 - bg_inode_bitmap_lo 59 - bg_inode_bitmap_lo
60 - Lower 32-bits of location of inode bitm 60 - Lower 32-bits of location of inode bitmap.
61 * - 0x8 61 * - 0x8
62 - __le32 62 - __le32
63 - bg_inode_table_lo 63 - bg_inode_table_lo
64 - Lower 32-bits of location of inode tabl 64 - Lower 32-bits of location of inode table.
65 * - 0xC 65 * - 0xC
66 - __le16 66 - __le16
67 - bg_free_blocks_count_lo 67 - bg_free_blocks_count_lo
68 - Lower 16-bits of free block count. 68 - Lower 16-bits of free block count.
69 * - 0xE 69 * - 0xE
70 - __le16 70 - __le16
71 - bg_free_inodes_count_lo 71 - bg_free_inodes_count_lo
72 - Lower 16-bits of free inode count. 72 - Lower 16-bits of free inode count.
73 * - 0x10 73 * - 0x10
74 - __le16 74 - __le16
75 - bg_used_dirs_count_lo 75 - bg_used_dirs_count_lo
76 - Lower 16-bits of directory count. 76 - Lower 16-bits of directory count.
77 * - 0x12 77 * - 0x12
78 - __le16 78 - __le16
79 - bg_flags 79 - bg_flags
80 - Block group flags. See the bgflags_ tab 80 - Block group flags. See the bgflags_ table below.
81 * - 0x14 81 * - 0x14
82 - __le32 82 - __le32
83 - bg_exclude_bitmap_lo 83 - bg_exclude_bitmap_lo
84 - Lower 32-bits of location of snapshot e 84 - Lower 32-bits of location of snapshot exclusion bitmap.
85 * - 0x18 85 * - 0x18
86 - __le16 86 - __le16
87 - bg_block_bitmap_csum_lo 87 - bg_block_bitmap_csum_lo
88 - Lower 16-bits of the block bitmap check 88 - Lower 16-bits of the block bitmap checksum.
89 * - 0x1A 89 * - 0x1A
90 - __le16 90 - __le16
91 - bg_inode_bitmap_csum_lo 91 - bg_inode_bitmap_csum_lo
92 - Lower 16-bits of the inode bitmap check 92 - Lower 16-bits of the inode bitmap checksum.
93 * - 0x1C 93 * - 0x1C
94 - __le16 94 - __le16
95 - bg_itable_unused_lo 95 - bg_itable_unused_lo
96 - Lower 16-bits of unused inode count. If 96 - Lower 16-bits of unused inode count. If set, we needn't scan past the
97 ``(sb.s_inodes_per_group - gdt.bg_itabl 97 ``(sb.s_inodes_per_group - gdt.bg_itable_unused)`` th entry in the
98 inode table for this group. 98 inode table for this group.
99 * - 0x1E 99 * - 0x1E
100 - __le16 100 - __le16
101 - bg_checksum 101 - bg_checksum
102 - Group descriptor checksum; crc16(sb_uui 102 - Group descriptor checksum; crc16(sb_uuid+group_num+bg_desc) if the
103 RO_COMPAT_GDT_CSUM feature is set, or 103 RO_COMPAT_GDT_CSUM feature is set, or
104 crc32c(sb_uuid+group_num+bg_desc) & 0xF 104 crc32c(sb_uuid+group_num+bg_desc) & 0xFFFF if the
105 RO_COMPAT_METADATA_CSUM feature is set. 105 RO_COMPAT_METADATA_CSUM feature is set. The bg_checksum
106 field in bg_desc is skipped when calcul 106 field in bg_desc is skipped when calculating crc16 checksum,
107 and set to zero if crc32c checksum is u 107 and set to zero if crc32c checksum is used.
108 * - 108 * -
109 - 109 -
110 - 110 -
111 - These fields only exist if the 64bit fe 111 - These fields only exist if the 64bit feature is enabled and s_desc_size
112 > 32. 112 > 32.
113 * - 0x20 113 * - 0x20
114 - __le32 114 - __le32
115 - bg_block_bitmap_hi 115 - bg_block_bitmap_hi
116 - Upper 32-bits of location of block bitm 116 - Upper 32-bits of location of block bitmap.
117 * - 0x24 117 * - 0x24
118 - __le32 118 - __le32
119 - bg_inode_bitmap_hi 119 - bg_inode_bitmap_hi
120 - Upper 32-bits of location of inodes bit 120 - Upper 32-bits of location of inodes bitmap.
121 * - 0x28 121 * - 0x28
122 - __le32 122 - __le32
123 - bg_inode_table_hi 123 - bg_inode_table_hi
124 - Upper 32-bits of location of inodes tab 124 - Upper 32-bits of location of inodes table.
125 * - 0x2C 125 * - 0x2C
126 - __le16 126 - __le16
127 - bg_free_blocks_count_hi 127 - bg_free_blocks_count_hi
128 - Upper 16-bits of free block count. 128 - Upper 16-bits of free block count.
129 * - 0x2E 129 * - 0x2E
130 - __le16 130 - __le16
131 - bg_free_inodes_count_hi 131 - bg_free_inodes_count_hi
132 - Upper 16-bits of free inode count. 132 - Upper 16-bits of free inode count.
133 * - 0x30 133 * - 0x30
134 - __le16 134 - __le16
135 - bg_used_dirs_count_hi 135 - bg_used_dirs_count_hi
136 - Upper 16-bits of directory count. 136 - Upper 16-bits of directory count.
137 * - 0x32 137 * - 0x32
138 - __le16 138 - __le16
139 - bg_itable_unused_hi 139 - bg_itable_unused_hi
140 - Upper 16-bits of unused inode count. 140 - Upper 16-bits of unused inode count.
141 * - 0x34 141 * - 0x34
142 - __le32 142 - __le32
143 - bg_exclude_bitmap_hi 143 - bg_exclude_bitmap_hi
144 - Upper 32-bits of location of snapshot e 144 - Upper 32-bits of location of snapshot exclusion bitmap.
145 * - 0x38 145 * - 0x38
146 - __le16 146 - __le16
147 - bg_block_bitmap_csum_hi 147 - bg_block_bitmap_csum_hi
148 - Upper 16-bits of the block bitmap check 148 - Upper 16-bits of the block bitmap checksum.
149 * - 0x3A 149 * - 0x3A
150 - __le16 150 - __le16
151 - bg_inode_bitmap_csum_hi 151 - bg_inode_bitmap_csum_hi
152 - Upper 16-bits of the inode bitmap check 152 - Upper 16-bits of the inode bitmap checksum.
153 * - 0x3C 153 * - 0x3C
154 - __u32 154 - __u32
155 - bg_reserved 155 - bg_reserved
156 - Padding to 64 bytes. 156 - Padding to 64 bytes.
157 157
158 .. _bgflags: 158 .. _bgflags:
159 159
160 Block group flags can be any combination of th 160 Block group flags can be any combination of the following:
161 161
162 .. list-table:: 162 .. list-table::
163 :widths: 16 64 163 :widths: 16 64
164 :header-rows: 1 164 :header-rows: 1
165 165
166 * - Value 166 * - Value
167 - Description 167 - Description
168 * - 0x1 168 * - 0x1
169 - inode table and bitmap are not initiali 169 - inode table and bitmap are not initialized (EXT4_BG_INODE_UNINIT).
170 * - 0x2 170 * - 0x2
171 - block bitmap is not initialized (EXT4_B 171 - block bitmap is not initialized (EXT4_BG_BLOCK_UNINIT).
172 * - 0x4 172 * - 0x4
173 - inode table is zeroed (EXT4_BG_INODE_ZE 173 - inode table is zeroed (EXT4_BG_INODE_ZEROED).
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