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Filipe Manana authored
Currently send does not do the best decisions when it comes to decide between multiple clone sources, which results in clone operations for partial extent ranges, which has the following disadvantages: 1) We get less shared extents at the destination; 2) We have to read more data during the send operation and emit more write commands. Besides not being optimal behaviour, it also breaks user expectations and is often reported by users, with a recent example in the Link tag at the bottom of this change log. Part of the reason for this non-optimal behaviour is that the backref walking code does not provide information about the length of the file extent items that were found for each backref, so send is blind about which backref is the best to chose as a cloning source. The other existing reasons are just silliness, namely always prefering the inode with the lowest number when multiple are found for the same root and when we can clone from multiple roots, always prefer the send root over any of the other clone roots. This does not make any sense since any inode or root is fine and as good as any other inode/root. Fix this by making backref walking pass information about the number of bytes referenced by each file extent item and then have send's backref callback pick the inode with the highest number of bytes for each root. Finally select the root from which we can clone more bytes from. Example reproducer: $ cat test.sh #!/bin/bash DEV=/dev/sdi MNT=/mnt/sdi mkfs.btrfs -f $DEV mount $DEV $MNT xfs_io -f -c "pwrite -S 0xab -b 2M 0 2M" $MNT/foo cp --reflink=always $MNT/foo $MNT/bar cp --reflink=always $MNT/foo $MNT/baz sync # Overwrite the second half of file foo. xfs_io -c "pwrite -S 0xcd -b 1M 1M 1M" $MNT/foo sync echo echo "*** fiemap in the original filesystem ***" echo xfs_io -c "fiemap -v" $MNT/foo xfs_io -c "fiemap -v" $MNT/bar xfs_io -c "fiemap -v" $MNT/baz echo btrfs filesystem du $MNT btrfs subvolume snapshot -r $MNT $MNT/snap btrfs send -f /tmp/send_stream $MNT/snap umount $MNT mkfs.btrfs -f $DEV &> /dev/null mount $DEV $MNT btrfs receive -f /tmp/send_stream $MNT echo echo "*** fiemap in the new filesystem ***" echo xfs_io -r -c "fiemap -v" $MNT/snap/foo xfs_io -r -c "fiemap -v" $MNT/snap/bar xfs_io -r -c "fiemap -v" $MNT/snap/baz echo btrfs filesystem du $MNT rm -f /tmp/send_stream rm -f /tmp/snap.fssum umount $MNT Before this change: $ ./test.sh (...) *** fiemap in the original filesystem *** /mnt/sdi/foo: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..2047]: 26624..28671 2048 0x2000 1: [2048..4095]: 30720..32767 2048 0x1 /mnt/sdi/bar: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..4095]: 26624..30719 4096 0x2001 /mnt/sdi/baz: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..4095]: 26624..30719 4096 0x2001 Total Exclusive Set shared Filename 2.00MiB 1.00MiB - /mnt/sdi/foo 2.00MiB 0.00B - /mnt/sdi/bar 2.00MiB 0.00B - /mnt/sdi/baz 6.00MiB 1.00MiB 2.00MiB /mnt/sdi Create a readonly snapshot of '/mnt/sdi' in '/mnt/sdi/snap' At subvol /mnt/sdi/snap At subvol snap *** fiemap in the new filesystem *** /mnt/sdi/snap/foo: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..4095]: 26624..30719 4096 0x2001 /mnt/sdi/snap/bar: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..2047]: 26624..28671 2048 0x2000 1: [2048..4095]: 30720..32767 2048 0x1 /mnt/sdi/snap/baz: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..2047]: 26624..28671 2048 0x2000 1: [2048..4095]: 32768..34815 2048 0x1 Total Exclusive Set shared Filename 2.00MiB 0.00B - /mnt/sdi/snap/foo 2.00MiB 1.00MiB - /mnt/sdi/snap/bar 2.00MiB 1.00MiB - /mnt/sdi/snap/baz 6.00MiB 2.00MiB - /mnt/sdi/snap 6.00MiB 2.00MiB 2.00MiB /mnt/sdi We end up with two 1M extents that are not shared for files bar and baz. After this change: $ ./test.sh (...) *** fiemap in the original filesystem *** /mnt/sdi/foo: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..2047]: 26624..28671 2048 0x2000 1: [2048..4095]: 30720..32767 2048 0x1 /mnt/sdi/bar: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..4095]: 26624..30719 4096 0x2001 /mnt/sdi/baz: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..4095]: 26624..30719 4096 0x2001 Total Exclusive Set shared Filename 2.00MiB 1.00MiB - /mnt/sdi/foo 2.00MiB 0.00B - /mnt/sdi/bar 2.00MiB 0.00B - /mnt/sdi/baz 6.00MiB 1.00MiB 2.00MiB /mnt/sdi Create a readonly snapshot of '/mnt/sdi' in '/mnt/sdi/snap' At subvol /mnt/sdi/snap At subvol snap *** fiemap in the new filesystem *** /mnt/sdi/snap/foo: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..4095]: 26624..30719 4096 0x2001 /mnt/sdi/snap/bar: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..2047]: 26624..28671 2048 0x2000 1: [2048..4095]: 30720..32767 2048 0x2001 /mnt/sdi/snap/baz: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS 0: [0..2047]: 26624..28671 2048 0x2000 1: [2048..4095]: 30720..32767 2048 0x2001 Total Exclusive Set shared Filename 2.00MiB 0.00B - /mnt/sdi/snap/foo 2.00MiB 0.00B - /mnt/sdi/snap/bar 2.00MiB 0.00B - /mnt/sdi/snap/baz 6.00MiB 0.00B - /mnt/sdi/snap 6.00MiB 0.00B 3.00MiB /mnt/sdi Now there's a much better sharing, files bar and baz share 1M of the extent of file foo and the second extent of files bar and baz is shared between themselves. This will later be turned into a test case for fstests. Link: https://lore.kernel.org/linux-btrfs/20221008005704.795b44b0@crass-HP-ZBook-15-G2/Signed-off-by:Filipe Manana <fdmanana@suse.com> Signed-off-by:
David Sterba <dsterba@suse.com>
c7499a64
