Scrub incredibly slow with 13.0-RC3 (as well as RC1 & 2)

[Martin Simmons]

>>>>> On Tue, 23 Mar 2021 10:38:13 +0100, Mathieu Chouquet-Stringer said:
> 
> So it's better but still not great, it's been running for 9.5 hours now:
> 
>   scan: scrub in progress since Tue Mar 23 01:03:09 2021
>         3.90T scanned at 121M/s, 3.90T issued at 121M/s, 24.5T total
>         0B repaired, 15.93% done, 2 days 01:24:58 to go
> 
> For reference I had the following after 12 hours with the new scrub algorithm:
> 
>   scan: scrub in progress since Sun Mar 21 23:47:40 2021
>         2.82T scanned at 77.8M/s, 2.24T issued at 62.0M/s, 24.5T total
>         0B repaired, 9.17% done, 4 days 08:23:32 to go

FWIW, I think the new scrub algorithm was added in 2018.

You could try to profile it:

https://wiki.freebsd.org/DTrace/Tutorial#Lesson_9._Profile_On-CPU_Kernel_Stacks

__Martin