linux/Documentation/filesystems/ext4/blockgroup.rst
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   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