Re: Scrub: no spae left on device

[Lionel Bouton <lionel-subscription@bouton.name>]

Le 08/12/2015 16:37, Holger Hoffstätte a écrit :
> 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.

How long must I think about it until it makes sense? :-)

Sorry I'm not sure why metadata is rewritten if no error is detected.
I've several theories but lack information: is the fact that no error
has been detected stored somewhere? is scrub using some kind of internal
temporary snapshot(s) to avoid interfering with other operations? other
reason I didn't think about?

Lionel