[arch-general] can't balance full btrfs raid6 filesystem

[Guus Snijders]

Op 20 feb. 2016 02:38 schreef "Jameson Pugh" <imntreal at gmail.com>:
>
> I have a raid6 btrfs filesystem which I filled up, and added a disk to:
>
> [...]

> Despite having removed files, and added the new device, I still can't
balance the filesystem:
>
> >[...]

That's actually a known caveat with COW filesystems. Even removing files
means updating the meta data,  which takes space...
Also, the "removed" files may still be part of snapshots, still taking up
the same diskspace.
I'm surprised that adding a disk didn't resolve the problem, but the is
probably the same: no (or too little) space on the existing devices to
update the meta data.

One thing that *might* work, assuming you have backups is to remove (at
least one, possibly multiple) snapshots.

> Neither dmesg nor journalctl have anything useful. Am I doomed to rebuild
the filesystem?
>
> I'm actually at the point, now, where I'm considering ZFS  to avoid the
issues btrfs has with running out of space, free space reporting, and cache
disks. I really don't want to move to something that isn't GPL compatible,
but this is getting quite frustrating.

Afaik ZFS has the same problem. To avoid it, I can imagine 2 options:
- monitoring used space and not letting it grow too much (or adding disks
earlier)
- use quota to limit how much space can be used, though you'd still need to
monitor usage.

Just my 2 cents. Perhaps I'm overlooking something obvious.

Mvg, Guus