btrfs-clean may leave space unallocated?

[rpdelaney]

Need to look into this and find out if it's still an issue.

  causasui | BasketCase: do you have any opinion on using something like snapshots e.g. in brtfs as a backup solution?
BasketCase | it works great in zfs.  not so great in btrfs.
  causasui | what's missing in brtfs?
  causasui | zfs doesn't seem like it could work for someone like me on consumer-grade hardware
BasketCase | the biggest advantage in zfs is that it makes deleting old backups super fast.  btrfs otoh just triggers a kernel thread called btrfs-cleaner to very slowly
           | free up the space
BasketCase | the worst part is that since btrfs-cleaner is a kernel thread you can't control the priority and you can't unmount the filesystem until it finishes which can
           | take days
BasketCase | this is especially fun when you don't know about it and you do a reboot and it just stops at the unmounting phase
BasketCase | oh, and if you force it with a reset button you just lose the space it was freeing up
   specing | you lose the space permanently?
BasketCase | I was never able to get it back
  causasui | what do you mean "lose the space"? Lol
BasketCase | I mean it was never freed up nor were the files available
BasketCase | it was simply disk space allocated to nothing