Comment # 11 on bug 1222313 from Wenruo Qu

If you have some random IO workload, it's very possible that btrfs bookend
extents are causing the problem.

Furthermore, if you have truncated files (which is previously very large or
preallocated), it can also take tons of unexpected space.
Another point is, preallocation (falloc) is very btrfs unfriendly, if you have
something like VM images, you'd be much better disable the snapshot of that
subvolume, and set NOCOW flag for them.

But since you have snapshots, the normal way to solve the problem (defrag) is
not suitable as it would break the shared extents and cause extra space usage.

It's recommended to delete all the unnecessary snapshots, and try "btrfs fi
defrag" to see if it help (needs a sync after full defrag).
Although there is a limitation on defrag that truncated files may not be that
well defragged.

If regular defrag (after deleting all snapshots) is not helping, you may want
to try the following patches:

- kernel part to enhance defrag:
  https://lore.kernel.org/linux-btrfs/cover.1710213625.git.wqu@suse.com/

- btrfs-progs support
  https://lore.kernel.org/linux-btrfs/cover.1710214834.git.wqu@suse.com/