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ext4: add missing bigalloc documentation.

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There was a broken link for bigalloc.  The page
https://ext4.wiki.kernel.org/index.php/Bigalloc was not migrated into
the current documentation sources.  This patch adds the contents of
that missing page into the section for Bigalloc itself.

Signed-off-by: Ayush Ranjan <ayushr2@illinois.edu>
Link: https://lore.kernel.org/r/20190831154419.GA30357@fa19-cs241-404.cs.illinois.edu
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Ayush Ranjan 2019-09-07 11:56:47 -04:00 committed by Theodore Ts'o
parent 6456ca6520
commit e85526404c
1 changed files with 22 additions and 10 deletions
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32
Documentation/filesystems/ext4/bigalloc.rst
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@ -9,14 +9,26 @@ ext4 code is not prepared to handle the case where the block size
exceeds the page size. However, for a filesystem of mostly huge files,
it is desirable to be able to allocate disk blocks in units of multiple
blocks to reduce both fragmentation and metadata overhead. The
`bigalloc <Bigalloc>`__ feature provides exactly this ability. The
administrator can set a block cluster size at mkfs time (which is stored
in the s\_log\_cluster\_size field in the superblock); from then on, the
block bitmaps track clusters, not individual blocks. This means that
block groups can be several gigabytes in size (instead of just 128MiB);
however, the minimum allocation unit becomes a cluster, not a block,
even for directories. TaoBao had a patchset to extend the “use units of
clusters instead of blocks” to the extent tree, though it is not clear
where those patches went-- they eventually morphed into “extent tree v2”
but that code has not landed as of May 2015.
bigalloc feature provides exactly this ability.
The bigalloc feature (EXT4_FEATURE_RO_COMPAT_BIGALLOC) changes ext4 to
use clustered allocation, so that each bit in the ext4 block allocation
bitmap addresses a power of two number of blocks. For example, if the
file system is mainly going to be storing large files in the 4-32
megabyte range, it might make sense to set a cluster size of 1 megabyte.
This means that each bit in the block allocation bitmap now addresses
256 4k blocks. This shrinks the total size of the block allocation
bitmaps for a 2T file system from 64 megabytes to 256 kilobytes. It also
means that a block group addresses 32 gigabytes instead of 128 megabytes,
also shrinking the amount of file system overhead for metadata.
The administrator can set a block cluster size at mkfs time (which is
stored in the s\_log\_cluster\_size field in the superblock); from then
on, the block bitmaps track clusters, not individual blocks. This means
that block groups can be several gigabytes in size (instead of just
128MiB); however, the minimum allocation unit becomes a cluster, not a
block, even for directories. TaoBao had a patchset to extend the “use
units of clusters instead of blocks” to the extent tree, though it is
not clear where those patches went-- they eventually morphed into
“extent tree v2” but that code has not landed as of May 2015.