219db95bbe
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 - Superblock struct was outdated, added the new fields which were part of s_reserved earlier.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>
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ReStructuredText
145 lines
3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Blocks
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------
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ext4 allocates storage space in units of “blocks”. A block is a group of
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sectors between 1KiB and 64KiB, and the number of sectors must be an
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integral power of 2. Blocks are in turn grouped into larger units called
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block groups. Block size is specified at mkfs time and typically is
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4KiB. You may experience mounting problems if block size is greater than
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page size (i.e. 64KiB blocks on a i386 which only has 4KiB memory
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pages). By default a filesystem can contain 2^32 blocks; if the '64bit'
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feature is enabled, then a filesystem can have 2^64 blocks. The location
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of structures is stored in terms of the block number the structure lives
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in and not the absolute offset on disk.
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For 32-bit filesystems, limits are as follows:
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.. list-table::
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:widths: 1 1 1 1 1
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:header-rows: 1
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* - Item
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- 1KiB
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- 2KiB
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- 4KiB
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- 64KiB
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* - Blocks
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- 2^32
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- 2^32
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- 2^32
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- 2^32
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* - Inodes
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- 2^32
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- 2^32
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- 2^32
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- 2^32
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* - File System Size
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- 4TiB
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- 8TiB
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- 16TiB
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- 256PiB
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* - Blocks Per Block Group
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- 8,192
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- 16,384
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- 32,768
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- 524,288
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* - Inodes Per Block Group
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- 8,192
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- 16,384
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- 32,768
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- 524,288
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* - Block Group Size
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- 8MiB
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- 32MiB
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- 128MiB
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- 32GiB
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* - Blocks Per File, Extents
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- 2^32
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- 2^32
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- 2^32
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- 2^32
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* - Blocks Per File, Block Maps
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- 16,843,020
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- 134,480,396
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- 1,074,791,436
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- 4,398,314,962,956 (really 2^32 due to field size limitations)
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* - File Size, Extents
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- 4TiB
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- 8TiB
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- 16TiB
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- 256TiB
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* - File Size, Block Maps
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- 16GiB
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- 256GiB
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- 4TiB
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- 256TiB
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For 64-bit filesystems, limits are as follows:
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.. list-table::
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:widths: 1 1 1 1 1
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:header-rows: 1
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* - Item
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- 1KiB
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- 2KiB
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- 4KiB
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- 64KiB
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* - Blocks
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- 2^64
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- 2^64
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- 2^64
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- 2^64
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* - Inodes
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- 2^32
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- 2^32
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- 2^32
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- 2^32
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* - File System Size
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- 16ZiB
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- 32ZiB
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- 64ZiB
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- 1YiB
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* - Blocks Per Block Group
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- 8,192
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- 16,384
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- 32,768
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- 524,288
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* - Inodes Per Block Group
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- 8,192
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- 16,384
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- 32,768
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- 524,288
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* - Block Group Size
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- 8MiB
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- 32MiB
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- 128MiB
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- 32GiB
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* - Blocks Per File, Extents
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- 2^32
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- 2^32
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- 2^32
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- 2^32
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* - Blocks Per File, Block Maps
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- 16,843,020
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- 134,480,396
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- 1,074,791,436
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- 4,398,314,962,956 (really 2^32 due to field size limitations)
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* - File Size, Extents
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- 4TiB
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- 8TiB
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- 16TiB
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- 256TiB
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* - File Size, Block Maps
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- 16GiB
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- 256GiB
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- 4TiB
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- 256TiB
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Note: Files not using extents (i.e. files using block maps) must be
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placed within the first 2^32 blocks of a filesystem. Files with extents
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must be placed within the first 2^48 blocks of a filesystem. It's not
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clear what happens with larger filesystems.
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