Re: [patch] zfs livelock and thread priorities

On Apr 16, 2009, at 9:23 AM, Peter Schuller wrote:
>> BTW, now that you're tinkering with ZFS threads and priorities,  
>> whould
>> you by any chance have any idea why zfs scrub is so painfully slow on
>> -current?
>> When I start scrub on my -stable box, it pretty much runs full speed
>> -- I can see disks under load all the time.
>> However on -current scrub seems to run in small bursts. Disks get  
>> busy
>> for a second or so and then things get quiet for about five seconds  
>> or
>> so and this pattern repeats over and over.
>
> This is intentional. The newer ZFS code has, if I remember correctly,
> something like "spend at most 1/5 of the time doing scrub for each
> underlying vdev". I could be wrong on the details and I don't have
> source refs off-hand, by I looked into this when I wanted to see if I
> could tweak this (while I definitely like it rate limited, I would
> have liked to up the threshold a bit). My conclusion at the time was
> that there was no way to tweak it other than recompiling the kernel.

I find it odd that they would pause for up to 5 seconds, though.  I  
would expect the throttling to occur over a much smaller time span.   
Do you see similarly large pauses in disk I/O during a scrub?

- Ben