Commit c6887cd1 authored Mar 25, 2016 by Josef Bacik Committed by Chris Mason Jun 22, 2016

Btrfs: don't do nocow check unless we have to



Before we write into prealloc/nocow space we have to make sure that there are no
references to the extents we are writing into, which means checking the extent
tree and csum tree in the case of nocow.  So we don't want to do the nocow dance
unless we can't reserve data space, since it's a serious drag on performance.
With the following sequence

fallocate -l10737418240 /mnt/btrfs-test/file
cp --reflink /mnt/btrfs-test/file /mnt/btrfs-test/link
fio --name=randwrite --rw=randwrite --bs=4k --filename=/mnt/btrfs-test/file \
	--end_fsync=1

we get the worst case scenario where we have to fall back on to doing the check
anyway.

Without this patch
lat (usec): min=5, max=111598, avg=27.65, stdev=124.51
write: io=10240MB, bw=126876KB/s, iops=31718, runt= 82646msec

With this patch
lat (usec): min=3, max=91210, avg=14.09, stdev=110.62
write: io=10240MB, bw=212753KB/s, iops=53188, runt= 49286msec

We get twice the throughput, half of the runtime, and half of the average
latency.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
[ PAGE_CACHE_ removal related fixups ]
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

parent 0f873eca

fs/btrfs/file.c

+22 −22

Original line number	Diff line number	Diff line
		@@ -1534,6 +1534,8 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
		reserve_bytes = round_up(write_bytes + sector_offset,
		root->sectorsize);

		ret = btrfs_check_data_free_space(inode, pos, write_bytes);
		if (ret < 0) {
		if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW \|
		BTRFS_INODE_PREALLOC)) &&
		check_can_nocow(inode, pos, &write_bytes) > 0) {
		@@ -1548,16 +1550,14 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
		*/
		num_pages = DIV_ROUND_UP(write_bytes + offset,
		PAGE_SIZE);
		reserve_bytes = round_up(write_bytes + sector_offset,
		reserve_bytes = round_up(write_bytes +
		sector_offset,
		root->sectorsize);
		goto reserve_metadata;
		}

		ret = btrfs_check_data_free_space(inode, pos, write_bytes);
		if (ret < 0)
		} else {
		break;
		}
		}

		reserve_metadata:
		ret = btrfs_delalloc_reserve_metadata(inode, reserve_bytes);
		if (ret) {
		if (!only_release_metadata)

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