On Thu, Jan 14, 2021 at 03:34:30PM +0100, David Sterba wrote:
From: Josef Bacik josef@toxicpanda.com
commit e076ab2a2ca70a0270232067cd49f76cd92efe64 upstream.
Commit 38d715f494f2 ("btrfs: use btrfs_start_delalloc_roots in shrink_delalloc") cleaned up how we do delalloc shrinking by utilizing some infrastructure we have in place to flush inodes that we use for device replace and snapshot. However this introduced a pretty serious performance regression. To reproduce the user untarred the source tarball of Firefox (360MiB xz compressed/1.5GiB uncompressed), and would see it take anywhere from 5 to 20 times as long to untar in 5.10 compared to 5.9. This was observed on fast devices (SSD and better) and not on HDD.
The root cause is because before we would generally use the normal writeback path to reclaim delalloc space, and for this we would provide it with the number of pages we wanted to flush. The referenced commit changed this to flush that many inodes, which drastically increased the amount of space we were flushing in certain cases, which severely affected performance.
We cannot revert this patch unfortunately because of 3d45f221ce62 ("btrfs: fix deadlock when cloning inline extent and low on free metadata space") which requires the ability to skip flushing inodes that are being cloned in certain scenarios, which means we need to keep using our flushing infrastructure or risk re-introducing the deadlock.
But we don't have 3d45f221ce62 in 5.10, which in turn makes me wonder why, as it's tagged for stable.
Instead of the backport, I'm going to take:
e076ab2a2ca7 ("btrfs: shrink delalloc pages instead of full inodes") 3d45f221ce62 ("btrfs: fix deadlock when cloning inline extent and low on free metadata space") f2f121ab500d ("btrfs: skip unnecessary searches for xattrs when logging an inode")
which deals with that missing stable tagged commit too.
Thanks!