group_descr.rst (5442B)
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).