linux-stable/Documentation/filesystems/ext4/blocks.rst
Ayush Ranjan 219db95bbe ext4: documentation fixes
This commit aims to fix the following issues in ext4 documentation:
- Flexible block group docs said that the aim was to group block
  metadata together instead of block group metadata.
- The documentation consistly uses "location" instead of "block number".
  It is easy to confuse location to be an absolute offset on disk. Added
  a line to clarify all location values are in terms of block numbers.
- Dirent2 docs said that the rec_len field is shortened instead of the
  name_len field.
- Typo in bg_checksum description.
- Inode size is 160 bytes now, and hence i_extra_isize is now 32.
- Cluster size formula was incorrect, it did not include the +10 to
  s_log_cluster_size value.
- Typo: there were two s_wtime_hi in the superblock struct.
- Superblock struct was outdated, added the new fields which were part
  of s_reserved earlier.
- Multiple mount protection seems to be implemented in fs/ext4/mmp.c.

Signed-off-by: Ayush Ranjan <ayushr2@illinois.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
2019-08-22 23:18:33 -04:00

144 lines
3 KiB
ReStructuredText

.. SPDX-License-Identifier: GPL-2.0
Blocks
------
ext4 allocates storage space in units of “blocks”. A block is a group of
sectors between 1KiB and 64KiB, and the number of sectors must be an
integral power of 2. Blocks are in turn grouped into larger units called
block groups. Block size is specified at mkfs time and typically is
4KiB. You may experience mounting problems if block size is greater than
page size (i.e. 64KiB blocks on a i386 which only has 4KiB memory
pages). By default a filesystem can contain 2^32 blocks; if the '64bit'
feature is enabled, then a filesystem can have 2^64 blocks. The location
of structures is stored in terms of the block number the structure lives
in and not the absolute offset on disk.
For 32-bit filesystems, limits are as follows:
.. list-table::
:widths: 1 1 1 1 1
:header-rows: 1
* - Item
- 1KiB
- 2KiB
- 4KiB
- 64KiB
* - Blocks
- 2^32
- 2^32
- 2^32
- 2^32
* - Inodes
- 2^32
- 2^32
- 2^32
- 2^32
* - File System Size
- 4TiB
- 8TiB
- 16TiB
- 256PiB
* - Blocks Per Block Group
- 8,192
- 16,384
- 32,768
- 524,288
* - Inodes Per Block Group
- 8,192
- 16,384
- 32,768
- 524,288
* - Block Group Size
- 8MiB
- 32MiB
- 128MiB
- 32GiB
* - Blocks Per File, Extents
- 2^32
- 2^32
- 2^32
- 2^32
* - Blocks Per File, Block Maps
- 16,843,020
- 134,480,396
- 1,074,791,436
- 4,398,314,962,956 (really 2^32 due to field size limitations)
* - File Size, Extents
- 4TiB
- 8TiB
- 16TiB
- 256TiB
* - File Size, Block Maps
- 16GiB
- 256GiB
- 4TiB
- 256TiB
For 64-bit filesystems, limits are as follows:
.. list-table::
:widths: 1 1 1 1 1
:header-rows: 1
* - Item
- 1KiB
- 2KiB
- 4KiB
- 64KiB
* - Blocks
- 2^64
- 2^64
- 2^64
- 2^64
* - Inodes
- 2^32
- 2^32
- 2^32
- 2^32
* - File System Size
- 16ZiB
- 32ZiB
- 64ZiB
- 1YiB
* - Blocks Per Block Group
- 8,192
- 16,384
- 32,768
- 524,288
* - Inodes Per Block Group
- 8,192
- 16,384
- 32,768
- 524,288
* - Block Group Size
- 8MiB
- 32MiB
- 128MiB
- 32GiB
* - Blocks Per File, Extents
- 2^32
- 2^32
- 2^32
- 2^32
* - Blocks Per File, Block Maps
- 16,843,020
- 134,480,396
- 1,074,791,436
- 4,398,314,962,956 (really 2^32 due to field size limitations)
* - File Size, Extents
- 4TiB
- 8TiB
- 16TiB
- 256TiB
* - File Size, Block Maps
- 16GiB
- 256GiB
- 4TiB
- 256TiB
Note: Files not using extents (i.e. files using block maps) must be
placed within the first 2^32 blocks of a filesystem. Files with extents
must be placed within the first 2^48 blocks of a filesystem. It's not
clear what happens with larger filesystems.