On Fri, Feb 21, 2020 at 04:41:10PM -0500, Josef Bacik wrote: > I noticed while running my snapshot torture test that we were getting a > lot of metadata chunks allocated with very little actually used. > Digging into this we would commit the transaction, still not have enough > space, and then force a chunk allocation. > > I noticed that we were barely flushing any delalloc at all, despite the > fact that we had around 13gib of outstanding delalloc reservations. It > turns out this is because of our btrfs_calc_reclaim_metadata_size() > calculation. It _only_ takes into account the outstanding ticket sizes, > which isn't the whole story. In this particular workload we're slowly > filling up the disk, which means our overcommit space will suddenly > become a lot less, and our outstanding reservations will be well more > than what we can handle. However we are only flushing based on our > ticket size, which is much less than we need to actually reclaim. > > So fix btrfs_calc_reclaim_metadata_size() to take into account the > overage in the case that we've gotten less available space suddenly. > This makes it so we attempt to reclaim a lot more delalloc space, which > allows us to make our reservations and we no longer are allocating a > bunch of needless metadata chunks. This seems to be relevant for stable@ but does not apply due to some cleanups that are not even on 5.5 and for the reset the code has moved to other files so this would need manual backport. I'll add the CC: tag so we have that tracked but none of the patches will apply.
