From c09f3bac6d8c820b4b4ff0ffcff47e3a68b04e54 Mon Sep 17 00:00:00 2001 From: "Darrick J. Wong" Date: Sun, 29 Jul 2018 15:38:00 -0400 Subject: ext4: import high level design chapter from wiki page Import the chapter about high level design from the on-disk format wiki page into the kernel documentation. Signed-off-by: Darrick J. Wong Signed-off-by: Theodore Ts'o --- Documentation/filesystems/ext4/ondisk/bigalloc.rst | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) create mode 100644 Documentation/filesystems/ext4/ondisk/bigalloc.rst (limited to 'Documentation/filesystems/ext4/ondisk/bigalloc.rst') diff --git a/Documentation/filesystems/ext4/ondisk/bigalloc.rst b/Documentation/filesystems/ext4/ondisk/bigalloc.rst new file mode 100644 index 000000000000..c6d88557553c --- /dev/null +++ b/Documentation/filesystems/ext4/ondisk/bigalloc.rst @@ -0,0 +1,22 @@ +.. 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 +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 v1.2.3