This patchset can be fetched from github: https://github.com/adam900710/linux/tree/qgroup_balance_skip_trees The base commit is v4.19-rc1 tag.
There are a lot of reports of system hang for balance on quota enabled fs. It's most obvious for large fs. The hang is caused by tons of unmodified extents marked as qgroup dirty. Such unmodified/unrelated sources include: 1) Unmodified subtree 2) Subtree drop for reloc tree (BTW, other sources includes unmodified file extent items) E.g. OO = Old tree blocks from file tree NN = New tree blocks from reloc tree file tree reloc tree OO (a) NN (a) / \ / \ (b) OO OO (c) (b) NN NN (c) / \ / \ / \ / \ OO OO OO OO OO OO OO NN (d) (e) (f) (g) (d) (e) (f) (g) In above case, balance will modify nodeptr in OO(a) to point NN(b) and NN(c), and modify NN(a) to point to OO(B) and OO(c). Before this patch, quota will mark the whole subtree from its parent down to the leaves as dirty. So btrfs quota need to trace all tree block from (a) to (g). However tree blocks (d) (e) (f) are shared between both trees, thus there is no need to trace those 3 tree blocks. This patchset will change how this work by only tracing modified tree blocks in reloc tree, and their counter parts in file tree. With this patch, we could skip tree blocks OO(d)~OO(f) in above example, thus reduce some some overhead caused by qgroup. The improvement is mostly related to metadata relocation. Also, for metadata relocation, we don't really need to trace data extents as they're not modified at all. [[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 | 22874 | 22856 | +0.1% qgroup dirty extents | 225251 | 140431 | -37.7% time (sys) | 40.161s | 20.574s | -48.7% time (real) | 42.163s | 25.173s | -40.3% *: (val_new - val_old) / val_old * 100% And the difference is becoming more and more obvious if more snapshots are created. If we increase the number of snapshots to 64 (4 times the number of references, and 64 snapshots is already not recommended to use with quota) | v4.19-rc1 | w/ patchset | diff (*) --------------------------------------------------------------- relocated extents | 22462 | 22467 | +0.0% qgroup dirty extents | 314380 | 140826 | -55.2% time (sys) | 158.033s | 74.292s | -53.0% time (real) | 197.395s | 90.529s | -67.6% For larger fs the saving should be even more obvious. Changelog: v2: Rename "tree reloc tree" to "reloc tree". Add patch "Don't trace subtree if we're dropping reloc tree" into the patchset. Fix wrong btrfs_bin_search() call, which leads to unexpected ENOENT error for btrfs_qgroup_trace_extent_swap(). Now use dst_path->slots[] directly. v3: Add new patch to avoid unnecessary data extents trace for metadata relocation. Better delayed ref time root owner detection to avoid unnecessary tree block tracing. Add benchmark result for the patchset. v4: Move part of the benchmark result from cover letter to real patches. - The result is *NEW* result, since host kernel get several updates. Spectre fixes seem to degrade a little memory performance. - Per-patch performance change on total balance time, compared to *previous* patch: 4th -5% 5th -25% 6th -15% 7th 0% - Cover letter still uses old test result. - In patch commit message, the result is still compared to v4.19-rc1 Add more robust level check for qgroup_trace_new_subtree_blocks(). Make btrfs_qgroup_trace_subtree_swap() to report wrong parameter order at runtime. (Since there is only one caller, report at runtime should be enough to cover development time error). Move the comment for btrfs_qgroup_trace_subtree_swap() to qgroup.c. (Other comment cleanup will be sent as a separate patch) Fix one uncaught "tree reloc tree" naming. Remove unrelated changes in patch 6. Qu Wenruo (7): btrfs: qgroup: Introduce trace event to analyse the number of dirty extents accounted btrfs: qgroup: Introduce function to trace two swaped extents btrfs: qgroup: Introduce function to find all new tree blocks of reloc tree btrfs: qgroup: Use generation aware subtree swap to mark dirty extents btrfs: qgroup: Don't trace subtree if we're dropping reloc tree btrfs: delayed-ref: Introduce new parameter for btrfs_add_delayed_tree_ref() to reduce unnecessary qgroup tracing btrfs: qgroup: Only trace data extents in leaves if we're relocating data block group fs/btrfs/delayed-ref.c | 3 +- fs/btrfs/delayed-ref.h | 1 + fs/btrfs/extent-tree.c | 16 +- fs/btrfs/qgroup.c | 377 +++++++++++++++++++++++++++++++++++ fs/btrfs/qgroup.h | 6 + fs/btrfs/relocation.c | 15 +- include/trace/events/btrfs.h | 21 ++ 7 files changed, 423 insertions(+), 16 deletions(-) -- 2.19.0