Re: btrfs scrub: cancel + resume not resuming?

[Graham Cobb <g.btrfs@cobb.uk.net>]

On 09/01/2020 10:34, Holger Hoffstätte wrote:
> On 1/9/20 11:03 AM, Sebastian Döring wrote:
>> Maybe I'm doing it entirely wrong, but I can't seem to get 'btrfs
>> scrub resume' to work properly. During a running scrub the resume
>> information (like data_bytes_scrubbed:1081454592) gets written to a
>> file in /var/lib/btrfs, but as soon as the scrub is cancelled all
>> relevant fields are zeroed. 'btrfs scrub resume' then seems to
>> re-start from the very beginning.
>>
>> This is on linux-5.5-rc5 and btrfs-progs 5.4, but I've been seeing
>> this for a while now.
>>
>> Is this intended/expected behavior? Am I using the btrfs-progs wrong?
>> How can I interrupt and resume a scrub?
> 
> Using 5.4.9+ (all of btrfs-5.5) and btrfs-progs 5.4 I just tried and
> it still works for me (and always has):
> 
> $btrfs scrub start /mnt/backup
> scrub started on /mnt/backup, fsid d163af2f-6e03-4972-bfd6-30c68b6ed312
> (pid=25633)
> 
> $btrfs scrub cancel /mnt/backup
> scrub cancelled
> 
> $btrfs scrub resume /mnt/backup
> scrub resumed on /mnt/backup, fsid d163af2f-6e03-4972-bfd6-30c68b6ed312
> (pid=25704)
> 
> ..and it keeps munching away as expected.

Can you check that the resume has really started from where the scrub
was cancelled? What I (and, I think, Sebastian) are seeing is that the
resume "works" but actually restarts from the beginning.

For example, something like:

btrfs scrub start /mnt/backup
sleep 300
btrfs scrub status -R /mnt/backup
btrfs scrub cancel /mnt/backup
btrfs scrub resume /mnt/backup
sleep 100
btrfs scrub status -R /mnt/backup

and check the last_physical in the second status is higher than the one
in the first status.