TWx Linux Repository
7093f15291
[BUG] With older kernels (before v5.16), btrfs will defrag preallocated extents. While with newer kernels (v5.16 and newer) btrfs will not defrag preallocated extents, but it will defrag the extent just before the preallocated extent, even it's just a single sector. This can be exposed by the following small script: mkfs.btrfs -f $dev > /dev/null mount $dev $mnt xfs_io -f -c "pwrite 0 4k" -c sync -c "falloc 4k 16K" $mnt/file xfs_io -c "fiemap -v" $mnt/file btrfs fi defrag $mnt/file sync xfs_io -c "fiemap -v" $mnt/file The output looks like this on older kernels: /mnt/btrfs/file: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..7]: 26624..26631 8 0x0 1: [8..39]: 26632..26663 32 0x801 /mnt/btrfs/file: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..39]: 26664..26703 40 0x1 Which defrags the single sector along with the preallocated extent, and replace them with an regular extent into a new location (caused by data COW). This wastes most of the data IO just for the preallocated range. On the other hand, v5.16 is slightly better: /mnt/btrfs/file: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..7]: 26624..26631 8 0x0 1: [8..39]: 26632..26663 32 0x801 /mnt/btrfs/file: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..7]: 26664..26671 8 0x0 1: [8..39]: 26632..26663 32 0x801 The preallocated range is not defragged, but the sector before it still gets defragged, which has no need for it. [CAUSE] One of the function reused by the old and new behavior is defrag_check_next_extent(), it will determine if we should defrag current extent by checking the next one. It only checks if the next extent is a hole or inlined, but it doesn't check if it's preallocated. On the other hand, out of the function, both old and new kernel will reject preallocated extents. Such inconsistent behavior causes above behavior. [FIX] - Also check if next extent is preallocated If so, don't defrag current extent. - Add comments for each branch why we reject the extent This will reduce the IO caused by defrag ioctl and autodefrag. CC: stable@vger.kernel.org # 5.16 Reviewed-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: Qu Wenruo <wqu@suse.com> Signed-off-by: David Sterba <dsterba@suse.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.