35 lines
1.8 KiB
ReStructuredText
35 lines
1.8 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
Bigalloc
|
|
--------
|
|
|
|
At the moment, the default size of a block is 4KiB, which is a commonly
|
|
supported page size on most MMU-capable hardware. This is fortunate, as
|
|
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 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.
|
|
|