MY FAULT. Re: Interesting: ZFS scrub prefetch hurting sequential scrub performance?

[Borja Marcos]

> On 4 Jan 2019, at 11:52, Borja Marcos <borjam at sarenet.es> wrote:
> 
> 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

My apologies for the false alarm!

My fault was to use a l2arc in the first place. 

It seems that the latest updates have made l2arc more detrimental in relatively low RAM situations. And yes, in those cases in which l2arc is not recommended in the first place, scrub prefetch can make the situation worse.

But the blame should go to the improper usage of a l2arc, not to the scrub prefetch instead. 

Sorry for the confusion and false alarm, although I still think that this lesson can be included in the guidelines for NOT using a l2arc for she sake of it.




Borja (jumping into a barrel full of tarr and feathers)