From e85526404ca70fa49f3d13b3898b067392522b24 Mon Sep 17 00:00:00 2001 From: Ayush Ranjan Date: Sat, 7 Sep 2019 11:56:47 -0400 Subject: ext4: add missing bigalloc documentation. 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 Link: https://lore.kernel.org/r/20190831154419.GA30357@fa19-cs241-404.cs.illinois.edu Signed-off-by: Theodore Ts'o --- Documentation/filesystems/ext4/bigalloc.rst | 32 ++++++++++++++++++++--------- 1 file changed, 22 insertions(+), 10 deletions(-) (limited to 'Documentation/filesystems') diff --git a/Documentation/filesystems/ext4/bigalloc.rst b/Documentation/filesystems/ext4/bigalloc.rst index c6d88557553c..72075aa608e4 100644 --- a/Documentation/filesystems/ext4/bigalloc.rst +++ b/Documentation/filesystems/ext4/bigalloc.rst @@ -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 `__ 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. -- cgit