On 2019/11/27 下午11:11, Jorge Bastos wrote:
> I believe this is a known issue but wonder if there's something I can
> do do optimize raid5 scrub speed, or if anything is in the works to
> improve it.
> 
> kernel 5.3.8
> btrfs-progs 5.3.1
> 
> 
> Single disk filesystem is performing as expected:
> 
> UUID:             9c0ed213-d9c5-4e93-b9db-218b43533c15
> Scrub started:    Tue Nov 26 21:58:20 2019
> Status:           finished
> Duration:         2:24:32
> Total to scrub:   1.04TiB
> Rate:             125.17MiB/s
> Error summary:    no errors found
> 
> 
> 
> 4 disk raid5 (raid1 metadata) on the same server using the same model
> disks as above:
> 
> UUID:             b75ee8b5-ae1c-4395-aa39-bebf10993057
> Scrub started:    Wed Nov 27 07:32:46 2019
> Status:           running
> Duration:         7:34:50
> Time left:        1:52:37
> ETA:              Wed Nov 27 17:00:18 2019
> Total to scrub:   1.20TiB
> Bytes scrubbed:   982.05GiB
> Rate:             36.85MiB/s
> Error summary:    no errors found
> 
> 
> 
> 6 SSD raid5 (raid1 metadata) also on the same server, still slow for
> SSDs but at least scrub performance is acceptable:
> 
> UUID:             e072aa60-33e2-4756-8496-c58cd8ba6053
> Scrub started:    Wed Nov 27 15:08:31 2019
> Status:           running
> Duration:         0:01:40
> Time left:        1:40:11
> ETA:              Wed Nov 27 16:50:24 2019
> Total to scrub:   3.24TiB
> Bytes scrubbed:   54.37GiB
> Rate:             556.73MiB/s
> Error summary:    no errors found
> 
> I still have some reservations about btrfs raid5/6, so use mostly for
> smaller filesystems for now, but this slow scrub performance will
> result in multi-day scrubs for a large filesystem, which isn't very
> practical.

Btrfs uses a not-so-optimal way for multi-disks scrub:
Queuing scrub for each disk at the same time.

So it's common to cause a lot of race and even conflicting seek requests.

Have you tried to only scrub one disk for such case?

Thanks,
Qu

> 
> Thanks,
> Jorge
>