Re: btrfs scrub: cancel + resume not resuming?

["Holger Hoffstätte" <holger@applied-asynchrony.com>]

On 1/9/20 11:52 AM, Graham Cobb wrote:
> 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.
> 

Well, yes. Reduced the wait times a bit and:

$cat test-scrub
#!/bin/sh
btrfs scrub start /mnt/backup
sleep 30
btrfs scrub status -R /mnt/backup
btrfs scrub cancel /mnt/backup
btrfs scrub resume /mnt/backup
sleep 10
btrfs scrub status -R /mnt/backup

$./test-scrub
scrub started on /mnt/backup, fsid d163af2f-6e03-4972-bfd6-30c68b6ed312 (pid=26390)
UUID:             d163af2f-6e03-4972-bfd6-30c68b6ed312
Scrub started:    Thu Jan  9 12:02:18 2020
Status:           running
Duration:         0:00:25
	data_extents_scrubbed: 65419
	tree_extents_scrubbed: 28
	data_bytes_scrubbed: 4117274624
	tree_bytes_scrubbed: 458752
	read_errors: 0
	csum_errors: 0
	verify_errors: 0
	no_csum: 0
	csum_discards: 0
	super_errors: 0
	malloc_errors: 0
	uncorrectable_errors: 0
	unverified_errors: 0
	corrected_errors: 0
	last_physical: 3591372800
         ^^^^^^^^^^^^^^^^^^^^^^^^^
scrub cancelled
scrub resumed on /mnt/backup, fsid d163af2f-6e03-4972-bfd6-30c68b6ed312 (pid=26399)
UUID:             d163af2f-6e03-4972-bfd6-30c68b6ed312
Scrub resumed:    Thu Jan  9 12:02:49 2020
Status:           running
Duration:         0:00:36
	data_extents_scrubbed: 12648
	tree_extents_scrubbed: 28
	data_bytes_scrubbed: 823394304
	tree_bytes_scrubbed: 458752
	read_errors: 0
	csum_errors: 0
	verify_errors: 0
	no_csum: 0
	csum_discards: 0
	super_errors: 0
	malloc_errors: 0
	uncorrectable_errors: 0
	unverified_errors: 0
	corrected_errors: 0
	last_physical: 923205632
         ^^^^^^^^^^^^^^^^^^^^^^^^

Not sure what I'm doing wrong ;)

-h