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+desc) if the
103 RO_COMPAT_GDT_CSUM feature is set, or !! 103 RO\_COMPAT\_GDT\_CSUM feature is set, or crc32c(sb\_uuid+group\_desc) &
104 crc32c(sb_uuid+group_num+bg_desc) & 0xF !! 104 0xFFFF if the RO\_COMPAT\_METADATA\_CSUM feature is set.
105 RO_COMPAT_METADATA_CSUM feature is set. <<
106 field in bg_desc is skipped when calcul <<
107 and set to zero if crc32c checksum is u <<
108 * - 105 * -
109 - 106 -
110 - 107 -
111 - These fields only exist if the 64bit fe 108 - These fields only exist if the 64bit feature is enabled and s_desc_size
112 > 32. 109 > 32.
113 * - 0x20 110 * - 0x20
114 - __le32 !! 111 - \_\_le32
115 - bg_block_bitmap_hi !! 112 - bg\_block\_bitmap\_hi
116 - Upper 32-bits of location of block bitm 113 - Upper 32-bits of location of block bitmap.
117 * - 0x24 114 * - 0x24
118 - __le32 !! 115 - \_\_le32
119 - bg_inode_bitmap_hi !! 116 - bg\_inode\_bitmap\_hi
120 - Upper 32-bits of location of inodes bit 117 - Upper 32-bits of location of inodes bitmap.
121 * - 0x28 118 * - 0x28
122 - __le32 !! 119 - \_\_le32
123 - bg_inode_table_hi !! 120 - bg\_inode\_table\_hi
124 - Upper 32-bits of location of inodes tab 121 - Upper 32-bits of location of inodes table.
125 * - 0x2C 122 * - 0x2C
126 - __le16 !! 123 - \_\_le16
127 - bg_free_blocks_count_hi !! 124 - bg\_free\_blocks\_count\_hi
128 - Upper 16-bits of free block count. 125 - Upper 16-bits of free block count.
129 * - 0x2E 126 * - 0x2E
130 - __le16 !! 127 - \_\_le16
131 - bg_free_inodes_count_hi !! 128 - bg\_free\_inodes\_count\_hi
132 - Upper 16-bits of free inode count. 129 - Upper 16-bits of free inode count.
133 * - 0x30 130 * - 0x30
134 - __le16 !! 131 - \_\_le16
135 - bg_used_dirs_count_hi !! 132 - bg\_used\_dirs\_count\_hi
136 - Upper 16-bits of directory count. 133 - Upper 16-bits of directory count.
137 * - 0x32 134 * - 0x32
138 - __le16 !! 135 - \_\_le16
139 - bg_itable_unused_hi !! 136 - bg\_itable\_unused\_hi
140 - Upper 16-bits of unused inode count. 137 - Upper 16-bits of unused inode count.
141 * - 0x34 138 * - 0x34
142 - __le32 !! 139 - \_\_le32
143 - bg_exclude_bitmap_hi !! 140 - bg\_exclude\_bitmap\_hi
144 - Upper 32-bits of location of snapshot e 141 - Upper 32-bits of location of snapshot exclusion bitmap.
145 * - 0x38 142 * - 0x38
146 - __le16 !! 143 - \_\_le16
147 - bg_block_bitmap_csum_hi !! 144 - bg\_block\_bitmap\_csum\_hi
148 - Upper 16-bits of the block bitmap check 145 - Upper 16-bits of the block bitmap checksum.
149 * - 0x3A 146 * - 0x3A
150 - __le16 !! 147 - \_\_le16
151 - bg_inode_bitmap_csum_hi !! 148 - bg\_inode\_bitmap\_csum\_hi
152 - Upper 16-bits of the inode bitmap check 149 - Upper 16-bits of the inode bitmap checksum.
153 * - 0x3C 150 * - 0x3C
154 - __u32 !! 151 - \_\_u32
155 - bg_reserved !! 152 - bg\_reserved
156 - Padding to 64 bytes. 153 - Padding to 64 bytes.
157 154
158 .. _bgflags: 155 .. _bgflags:
159 156
160 Block group flags can be any combination of th 157 Block group flags can be any combination of the following:
161 158
162 .. list-table:: 159 .. list-table::
163 :widths: 16 64 160 :widths: 16 64
164 :header-rows: 1 161 :header-rows: 1
165 162
166 * - Value 163 * - Value
167 - Description 164 - Description
168 * - 0x1 165 * - 0x1
169 - inode table and bitmap are not initiali !! 166 - inode table and bitmap are not initialized (EXT4\_BG\_INODE\_UNINIT).
170 * - 0x2 167 * - 0x2
171 - block bitmap is not initialized (EXT4_B !! 168 - block bitmap is not initialized (EXT4\_BG\_BLOCK\_UNINIT).
172 * - 0x4 169 * - 0x4
173 - inode table is zeroed (EXT4_BG_INODE_ZE !! 170 - inode table is zeroed (EXT4\_BG\_INODE\_ZEROED).
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