background fsck high load on 8.1

[Kirk McKusick]

> Date: Tue, 12 Apr 2011 15:14:42 +0200
> From: Sergi Seira <s.seira at cdmon.com>
> To: freebsd-current at freebsd.org
> Subject: background fsck high load on 8.1
> 
> Hello,
> 
> We've experienced that background fsck on 8.1 degrades server
> performance on a higher degree than in previous fbsd versions (6.3,
> 7.3; amd64).
> 
> We've noticed it after upgrading - same hardware - to a 8.1-RELEASE.
> Now, performance of other services (i.e. apache, mysql) during a
> background fsck falls miserably.
> 
> Is there any way to calm fsck down?, nice(1)?, some sysctl?
> 
> We have also gmirror, but we prevent to rebuild it if there is a
> fsck running in background.
> 
> Thanks for your help,
> regards,
> Sergi

This is the first report that I have heard of the higher level of
slowdown by background fsck on 8.X systems. My guess is that it
comes about from work done to make the I/O subsystem faster which
in turn allows fsck to have a higher impact.

For a period we were working on a kernel feature (associated with
nice) that would allow the system to throttle I/O activity for a
given process. It turns out to be a much harder problem than it at
first appears. We set this aside when we got journaled soft updates
working as they eliminate the need for background fsck. While
journaled soft updates are too big of a change to be merged back
into the 8.X tree, we do expect them to be used by default on 9.X
systems.

	Kirk McKusick