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When using the NO_HOLES feature, during an incremental send we often issue
write operations for holes when we should not, because that range is already
a hole in the destination snapshot. While that does not change the contents
of the file at the receiver, it avoids preservation of file holes, leading
to wasted disk space and extra IO during send/receive.
A couple examples where the holes are not preserved follows.
$ mkfs.btrfs -O no-holes -f /dev/sdb
$ mount /dev/sdb /mnt
$ xfs_io -f -c "pwrite -S 0xaa 0 4K" /mnt/foo
$ xfs_io -f -c "pwrite -S 0xaa 0 4K" -c "pwrite -S 0xbb 1028K 4K" /mnt/bar
$ btrfs subvolume snapshot -r /mnt /mnt/snap1
# Now add one new extent to our first test file, increasing its size and
# leaving a 1Mb hole between the first extent and this new extent.
$ xfs_io -c "pwrite -S 0xbb 1028K 4K" /mnt/foo
# Now overwrite the last extent of our second test file.
$ xfs_io -c "pwrite -S 0xcc 1028K 4K" /mnt/bar
$ btrfs subvolume snapshot -r /mnt /mnt/snap2
$ xfs_io -r -c "fiemap -v" /mnt/snap2/foo
/mnt/snap2/foo:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..7]: 25088..25095 8 0x2000
1: [8..2055]: hole 2048
2: [2056..2063]: 24576..24583 8 0x2001
$ xfs_io -r -c "fiemap -v" /mnt/snap2/bar
/mnt/snap2/bar:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..7]: 25096..25103 8 0x2000
1: [8..2055]: hole 2048
2: [2056..2063]: 24584..24591 8 0x2001
$ btrfs send /mnt/snap1 -f /tmp/1.snap
$ btrfs send -p /mnt/snap1 /mnt/snap2 -f /tmp/2.snap
$ umount /mnt
# It's not relevant to enable no-holes in the new filesystem.
$ mkfs.btrfs -O no-holes -f /dev/sdc
$ mount /dev/sdc /mnt
$ btrfs receive /mnt -f /tmp/1.snap
$ btrfs receive /mnt -f /tmp/2.snap
$ xfs_io -r -c "fiemap -v" /mnt/snap2/foo
/mnt/snap2/foo:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..7]: 24576..24583 8 0x2000
1: [8..2063]: 25624..27679 2056 0x1
$ xfs_io -r -c "fiemap -v" /mnt/snap2/bar
/mnt/snap2/bar:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..7]: 24584..24591 8 0x2000
1: [8..2063]: 27680..29735 2056 0x1
The holes do not exist in the second filesystem and they were replaced
with extents filled with the byte 0x00, making each file take 1032Kb of
space instead of 8Kb.
So fix this by not issuing the write operations consisting of buffers
filled with the byte 0x00 when the destination snapshot already has a
hole for the respective range.
A test case for fstests will follow soon.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.