From f616f5cd9da7fceb7d884812da380b26040cd083 Mon Sep 17 00:00:00 2001 From: Qu Wenruo Date: Wed, 23 Jan 2019 15:15:17 +0800 Subject: btrfs: qgroup: Use delayed subtree rescan for balance Before this patch, qgroup code traces the whole subtree of subvolume and reloc trees unconditionally. This makes qgroup numbers consistent, but it could cause tons of unnecessary extent tracing, which causes a lot of overhead. However for subtree swap of balance, just swap both subtrees because they contain the same contents and tree structure, so qgroup numbers won't change. It's the race window between subtree swap and transaction commit could cause qgroup number change. This patch will delay the qgroup subtree scan until COW happens for the subtree root. So if there is no other operations for the fs, balance won't cause extra qgroup overhead. (best case scenario) Depending on the workload, most of the subtree scan can still be avoided. Only for worst case scenario, it will fall back to old subtree swap overhead. (scan all swapped subtrees) [[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. And after file system population, there is no other activity, so it should be the best case scenario. | v4.20-rc1 | w/ patchset | diff ----------------------------------------------------------------------- relocated extents | 22615 | 22457 | -0.1% qgroup dirty extents | 163457 | 121606 | -25.6% time (sys) | 22.884s | 18.842s | -17.6% time (real) | 27.724s | 22.884s | -17.5% Signed-off-by: Qu Wenruo Signed-off-by: David Sterba --- fs/btrfs/qgroup.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'fs/btrfs/qgroup.h') diff --git a/fs/btrfs/qgroup.h b/fs/btrfs/qgroup.h index 8dc17020e5be..3f5e0b413312 100644 --- a/fs/btrfs/qgroup.h +++ b/fs/btrfs/qgroup.h @@ -416,5 +416,7 @@ int btrfs_qgroup_add_swapped_blocks(struct btrfs_trans_handle *trans, struct extent_buffer *subvol_parent, int subvol_slot, struct extent_buffer *reloc_parent, int reloc_slot, u64 last_snapshot); +int btrfs_qgroup_trace_subtree_after_cow(struct btrfs_trans_handle *trans, + struct btrfs_root *root, struct extent_buffer *eb); #endif -- cgit v1.2.3