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btrfs: qgroup: Don't trace subtree if we're dropping reloc tree

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Reloc tree doesn't contribute to qgroup numbers, as we have accounted
them at balance time (see replace_path()).

Skipping the unneeded subtree tracing should reduce the overhead.

[[Benchmark]]
Hardware:
	VM 4G vRAM, 8 vCPUs,
	disk is using 'unsafe' cache mode,
	backing device is SAMSUNG 850 evo SSD.
	Host has 16G ram.

Mkfs parameter:
	--nodesize 4K (To bump up tree size)

Initial subvolume contents:
	4G data copied from /usr and /lib.
	(With enough regular small files)

Snapshots:
	16 snapshots of the original subvolume.
	each snapshot has 3 random files modified.

balance parameter:
	-m

So the content should be pretty similar to a real world root fs layout.

                     | v4.19-rc1    | w/ patchset    | diff (*)
---------------------------------------------------------------
relocated extents    | 22929        | 22900          | -0.1%
qgroup dirty extents | 227757       | 167139         | -26.6%
time (sys)           | 65.253s      | 50.123s        | -23.2%
time (real)          | 74.032s      | 52.551s        | -29.0%

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Qu Wenruo 2018-09-27 14:42:33 +08:00 committed by David Sterba
parent 5f527822be
commit 2cd86d309b
1 changed files with 7 additions and 1 deletions
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8
fs/btrfs/extent-tree.c
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@ -8683,7 +8683,13 @@ skip:
parent = 0;
}
if (need_account) {
/*
* Reloc tree doesn't contribute to qgroup numbers, and we have
* already accounted them at merge time (replace_path),
* thus we could skip expensive subtree trace here.
*/
if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID &&
need_account) {
ret = btrfs_qgroup_trace_subtree(trans, next,
generation, level - 1);
if (ret) {