Interesting: ZFS scrub prefetch hurting sequential scrub performance?

[Borja Marcos]

> On 3 Jan 2019, at 11:34, Borja Marcos <borjam at sarenet.es> wrote:
> 
> 
> Hi,
> 
> I have noticed that my scrubs have become painfully slow. I am wondering wether I’ve just hit some worst case or maybe
> there is some interaction between the ZFS sequential scrub and scrub prefetch. I don’t recall seeing this behavior
> before the sequential scrub code was committed. 
> 
> Did I hit some worst case or should scrub prefetch be disabled with the new sequential scrub code?

I have done a test with the old scrub code (vfs.zfs.zfs_scan_legacy=1) and I see a very similar behavior, with the 
scrub stalling again.

Once more, disabling prefetch for the scrub (vfs.zfs.no_scrub_prefetch=1) solves the issue.

I suffered this problem on 11 at some point but I attributed it (wrongly!) to hardware problems at the time.

Not I’ve just found a talk about a new prefetch mechanism for the scrub by Tom Caputi. Could it be the problem?
https://www.youtube.com/watch?v=upn9tYh917s





Borja.