btrfs: send: avoid unnecessary backref lookups when finding clone source
At find_extent_clone(), unless we are given an inline extent, a file extent item that represents hole or an extent that starts beyond the i_size, we always do backref walking to look for clone sources, unless if we have more than SEND_MAX_EXTENT_REFS (64) known references on the extent. However if we know we only have one reference in the extent item and only one clone source (the send root), then it's pointless to do the backref walking to search for clone sources, as we can't clone from any other root. So skip the backref walking in that case. The following test was run on a non-debug kernel (Debian's default kernel config): $ cat test.sh #!/bin/bash DEV=/dev/sdi MNT=/mnt/sdi mkfs.btrfs -f $DEV mount $DEV $MNT # Create an extent tree that's not too small and none of the # extents is shared. for ((i = 1; i <= 50000; i++)); do xfs_io -f -c "pwrite 0 4K" $MNT/file_$i > /dev/null echo -ne "\r$i files created..." done echo btrfs subvolume snapshot -r $MNT $MNT/snap start=$(date +%s%N) btrfs send $MNT/snap > /dev/null end=$(date +%s%N) dur=$(( (end - start) / 1000000 )) echo -e "\nsend took $dur milliseconds" umount $MNT Before this change: send took 5389 milliseconds After this change: send took 4519 milliseconds (-16.1%) Signed-off-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
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