Re: Scrub: no spae left on device

["Holger Hoffstätte" <holger.hoffstaette@googlemail.com>]

On 12/08/15 16:06, Marc MERLIN wrote:
> Howdy,
> 
> Why would scrub need space and why would it cancel if there isn't enough of
> it?
> (kernel 4.3)
> 
> /etc/cron.daily/btrfs-scrub:
> btrfs scrub start -Bd /dev/mapper/cryptroot
> scrub device /dev/mapper/cryptroot (id 1) done
> 	scrub started at Mon Dec  7 01:35:08 2015 and finished after 258 seconds
> 	total bytes scrubbed: 130.84GiB with 0 errors
> btrfs scrub start -Bd /dev/mapper/pool1
> ERROR: scrubbing /dev/mapper/pool1 failed for device id 1 (No space left on device)
> scrub device /dev/mapper/pool1 (id 1) canceled

Scrub rewrites metadata (apparently even in -r aka readonly mode), and that
can lead to temporary metadata expansion (stuff gets COWed around); it's
a bit surprising but makes sense if you think about it. The fact that you
ENOSPCed means that the fs was probably already fully allocated.

If it bothers you, a subsequent balance with -musage=10 should vacuum things
up. Alternatively just keep using the filesystem; eventually the empty metadata
chunks should be collected, on the next remount at the latest.

tl;dr: Never allocate all the chunks. Yes, this needs more graceful handling.

-h