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

[Matt Churchyard]

-----Original Message-----
From: owner-freebsd-fs at freebsd.org <owner-freebsd-fs at freebsd.org> On Behalf Of Mathieu Chouquet-Stringer
Sent: 24 March 2021 12:59
To: Martin Simmons <martin at lispworks.com>
Cc: freebsd-fs at freebsd.org
Subject: Re: Scrub incredibly slow with 13.0-RC3 (as well as RC1 & 2)

On Wed, Mar 24, 2021 at 12:30:52PM +0000, Martin Simmons wrote:
> FWIW, I think the new scrub algorithm was added in 2018.

You're right, git says Wed Nov 15 20:27:01 2017 -0500

> You could try to profile it:
> 
> https://wiki.freebsd.org/DTrace/Tutorial#Lesson_9._Profile_On-CPU_Kernel_Stacks

> dtrace, hello old friend... :-)

> That's the next step...

Just an a aside, I did post a message a few weeks ago with a similar problem on 13 (as well as snapshot issues). Scrub seemed ok for a short while, but then ground to a halt. It would take 10+ minutes to go 0.01%, with everything appearing fairly idle. I finally gave up and stopped it after about 20 hours. Moving to 12.2 and rebuilding the pool, the system scrubbed the same data in an hour, and I've just scrubbed the same system after a month of use with about 4 times the data in 3 hours 20. As far as I'm aware, both should be using effectively the same "new" scrub code.

Will be interesting if you find a cause as I didn't get any response to what for me was a complete showstopper for moving to 13.

Matt

-- 
Mathieu Chouquet-Stringer
            The sun itself sees not till heaven clears.
	             -- William Shakespeare --
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