1 .. SPDX-License-Identifier: GPL-2.0
2
3 Block Group Descriptors
4 -----------------------
5
6 Each block group on the filesystem has one of
7 associated with it. As noted in the Layout sec
8 descriptors (if present) are the second item i
9 standard configuration is for each block group
10 the block group descriptor table unless the sp
11 is set.
12
13 Notice how the group descriptor records the lo
14 the inode table (i.e. they can float). This me
15 group, the only data structures with fixed loc
16 and the group descriptor table. The flex_bg me
17 property to group several block groups into a
18 of the groups' bitmaps and inode tables into o
19 group of the flex group.
20
21 If the meta_bg feature flag is set, then sever
22 grouped together into a meta group. Note that
23 however, the first and last two block groups w
24 group contain only group descriptors for the g
25 group.
26
27 flex_bg and meta_bg do not appear to be mutual
28
29 In ext2, ext3, and ext4 (when the 64bit featur
30 block group descriptor was only 32 bytes long
31 bg_checksum. On an ext4 filesystem with the 64
32 block group descriptor expands to at least the
33 the size is stored in the superblock.
34
35 If gdt_csum is set and metadata_csum is not se
36 checksum is the crc16 of the FS UUID, the grou
37 descriptor structure. If metadata_csum is set,
38 checksum is the lower 16 bits of the checksum
39 number, and the group descriptor structure. Bo
40 checksums are calculated against the FS UUID,
41 entire bitmap.
42
43 The block group descriptor is laid out in ``st
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 bitm
57 * - 0x4
58 - __le32
59 - bg_inode_bitmap_lo
60 - Lower 32-bits of location of inode bitm
61 * - 0x8
62 - __le32
63 - bg_inode_table_lo
64 - Lower 32-bits of location of inode tabl
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_ tab
81 * - 0x14
82 - __le32
83 - bg_exclude_bitmap_lo
84 - Lower 32-bits of location of snapshot e
85 * - 0x18
86 - __le16
87 - bg_block_bitmap_csum_lo
88 - Lower 16-bits of the block bitmap check
89 * - 0x1A
90 - __le16
91 - bg_inode_bitmap_csum_lo
92 - Lower 16-bits of the inode bitmap check
93 * - 0x1C
94 - __le16
95 - bg_itable_unused_lo
96 - Lower 16-bits of unused inode count. If
97 ``(sb.s_inodes_per_group - gdt.bg_itabl
98 inode table for this group.
99 * - 0x1E
100 - __le16
101 - bg_checksum
102 - Group descriptor checksum; crc16(sb_uui
103 RO_COMPAT_GDT_CSUM feature is set, or
104 crc32c(sb_uuid+group_num+bg_desc) & 0xF
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 * -
109 -
110 -
111 - These fields only exist if the 64bit fe
112 > 32.
113 * - 0x20
114 - __le32
115 - bg_block_bitmap_hi
116 - Upper 32-bits of location of block bitm
117 * - 0x24
118 - __le32
119 - bg_inode_bitmap_hi
120 - Upper 32-bits of location of inodes bit
121 * - 0x28
122 - __le32
123 - bg_inode_table_hi
124 - Upper 32-bits of location of inodes tab
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 e
145 * - 0x38
146 - __le16
147 - bg_block_bitmap_csum_hi
148 - Upper 16-bits of the block bitmap check
149 * - 0x3A
150 - __le16
151 - bg_inode_bitmap_csum_hi
152 - Upper 16-bits of the inode bitmap check
153 * - 0x3C
154 - __u32
155 - bg_reserved
156 - Padding to 64 bytes.
157
158 .. _bgflags:
159
160 Block group flags can be any combination of th
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 initiali
170 * - 0x2
171 - block bitmap is not initialized (EXT4_B
172 * - 0x4
173 - inode table is zeroed (EXT4_BG_INODE_ZE
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