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author | Darrick J. Wong | 2018-10-06 01:20:08 +0200 |
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committer | Theodore Ts'o | 2018-10-06 01:20:08 +0200 |
commit | 8a98ec7c7b3901330a036af0f62f523c31d763da (patch) | |
tree | 70194bc8bf562d3aa9053ffc41ceba61676f864e /Documentation/filesystems/ext4/bigalloc.rst | |
parent | docs: move ext4 administrative docs to admin-guide/ (diff) | |
download | kernel-qcow2-linux-8a98ec7c7b3901330a036af0f62f523c31d763da.tar.gz kernel-qcow2-linux-8a98ec7c7b3901330a036af0f62f523c31d763da.tar.xz kernel-qcow2-linux-8a98ec7c7b3901330a036af0f62f523c31d763da.zip |
docs: promote the ext4 data structures book to top level
Move the ext4 data structures book to Documentation/filesystems/ext4/
since the administrative information moved elsewhere.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Diffstat (limited to 'Documentation/filesystems/ext4/bigalloc.rst')
-rw-r--r-- | Documentation/filesystems/ext4/bigalloc.rst | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/Documentation/filesystems/ext4/bigalloc.rst b/Documentation/filesystems/ext4/bigalloc.rst new file mode 100644 index 000000000000..c6d88557553c --- /dev/null +++ b/Documentation/filesystems/ext4/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 <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. + |