1.. SPDX-License-Identifier: GPL-2.0 2 3Layout 4------ 5 6The layout of a standard block group is approximately as follows (each 7of these fields is discussed in a separate section below): 8 9.. list-table:: 10 :widths: 1 1 1 1 1 1 1 1 11 :header-rows: 1 12 13 * - Group 0 Padding 14 - ext4 Super Block 15 - Group Descriptors 16 - Reserved GDT Blocks 17 - Data Block Bitmap 18 - inode Bitmap 19 - inode Table 20 - Data Blocks 21 * - 1024 bytes 22 - 1 block 23 - many blocks 24 - many blocks 25 - 1 block 26 - 1 block 27 - many blocks 28 - many more blocks 29 30For the special case of block group 0, the first 1024 bytes are unused, 31to allow for the installation of x86 boot sectors and other oddities. 32The superblock will start at offset 1024 bytes, whichever block that 33happens to be (usually 0). However, if for some reason the block size = 341024, then block 0 is marked in use and the superblock goes in block 1. 35For all other block groups, there is no padding. 36 37The ext4 driver primarily works with the superblock and the group 38descriptors that are found in block group 0. Redundant copies of the 39superblock and group descriptors are written to some of the block groups 40across the disk in case the beginning of the disk gets trashed, though 41not all block groups necessarily host a redundant copy (see following 42paragraph for more details). If the group does not have a redundant 43copy, the block group begins with the data block bitmap. Note also that 44when the filesystem is freshly formatted, mkfs will allocate “reserve 45GDT block” space after the block group descriptors and before the start 46of the block bitmaps to allow for future expansion of the filesystem. By 47default, a filesystem is allowed to increase in size by a factor of 481024x over the original filesystem size. 49 50The location of the inode table is given by ``grp.bg_inode_table_*``. It 51is continuous range of blocks large enough to contain 52``sb.s_inodes_per_group * sb.s_inode_size`` bytes. 53 54As for the ordering of items in a block group, it is generally 55established that the super block and the group descriptor table, if 56present, will be at the beginning of the block group. The bitmaps and 57the inode table can be anywhere, and it is quite possible for the 58bitmaps to come after the inode table, or for both to be in different 59groups (flex\_bg). Leftover space is used for file data blocks, indirect 60block maps, extent tree blocks, and extended attributes. 61 62Flexible Block Groups 63--------------------- 64 65Starting in ext4, there is a new feature called flexible block groups 66(flex\_bg). In a flex\_bg, several block groups are tied together as one 67logical block group; the bitmap spaces and the inode table space in the 68first block group of the flex\_bg are expanded to include the bitmaps 69and inode tables of all other block groups in the flex\_bg. For example, 70if the flex\_bg size is 4, then group 0 will contain (in order) the 71superblock, group descriptors, data block bitmaps for groups 0-3, inode 72bitmaps for groups 0-3, inode tables for groups 0-3, and the remaining 73space in group 0 is for file data. The effect of this is to group the 74block group metadata close together for faster loading, and to enable 75large files to be continuous on disk. Backup copies of the superblock 76and group descriptors are always at the beginning of block groups, even 77if flex\_bg is enabled. The number of block groups that make up a 78flex\_bg is given by 2 ^ ``sb.s_log_groups_per_flex``. 79 80Meta Block Groups 81----------------- 82 83Without the option META\_BG, for safety concerns, all block group 84descriptors copies are kept in the first block group. Given the default 85128MiB(2^27 bytes) block group size and 64-byte group descriptors, ext4 86can have at most 2^27/64 = 2^21 block groups. This limits the entire 87filesystem size to 2^21 * 2^27 = 2^48bytes or 256TiB. 88 89The solution to this problem is to use the metablock group feature 90(META\_BG), which is already in ext3 for all 2.6 releases. With the 91META\_BG feature, ext4 filesystems are partitioned into many metablock 92groups. Each metablock group is a cluster of block groups whose group 93descriptor structures can be stored in a single disk block. For ext4 94filesystems with 4 KB block size, a single metablock group partition 95includes 64 block groups, or 8 GiB of disk space. The metablock group 96feature moves the location of the group descriptors from the congested 97first block group of the whole filesystem into the first group of each 98metablock group itself. The backups are in the second and last group of 99each metablock group. This increases the 2^21 maximum block groups limit 100to the hard limit 2^32, allowing support for a 512PiB filesystem. 101 102The change in the filesystem format replaces the current scheme where 103the superblock is followed by a variable-length set of block group 104descriptors. Instead, the superblock and a single block group descriptor 105block is placed at the beginning of the first, second, and last block 106groups in a meta-block group. A meta-block group is a collection of 107block groups which can be described by a single block group descriptor 108block. Since the size of the block group descriptor structure is 32 109bytes, a meta-block group contains 32 block groups for filesystems with 110a 1KB block size, and 128 block groups for filesystems with a 4KB 111blocksize. Filesystems can either be created using this new block group 112descriptor layout, or existing filesystems can be resized on-line, and 113the field s\_first\_meta\_bg in the superblock will indicate the first 114block group using this new layout. 115 116Please see an important note about ``BLOCK_UNINIT`` in the section about 117block and inode bitmaps. 118 119Lazy Block Group Initialization 120------------------------------- 121 122A new feature for ext4 are three block group descriptor flags that 123enable mkfs to skip initializing other parts of the block group 124metadata. Specifically, the INODE\_UNINIT and BLOCK\_UNINIT flags mean 125that the inode and block bitmaps for that group can be calculated and 126therefore the on-disk bitmap blocks are not initialized. This is 127generally the case for an empty block group or a block group containing 128only fixed-location block group metadata. The INODE\_ZEROED flag means 129that the inode table has been initialized; mkfs will unset this flag and 130rely on the kernel to initialize the inode tables in the background. 131 132By not writing zeroes to the bitmaps and inode table, mkfs time is 133reduced considerably. Note the feature flag is RO\_COMPAT\_GDT\_CSUM, 134but the dumpe2fs output prints this as “uninit\_bg”. They are the same 135thing. 136