[patch] zfs livelock and thread priorities

[Peter Schuller]

> 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.

-- 
/ Peter Schuller

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