RAID5 scrub performance

[Jorge Bastos <jorge.mrbastos@gmail.com>]

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.

Thanks,
Jorge