Re: high load, no bottleneck

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Sat, Sep 21, 2013 at 09:58:30AM +0000, Michael van Elst wrote:
> ef%math.uni-bonn.de@localhost (Edgar =?iso-8859-1?B?RnXf?=) writes:
> 
> >I myself can't make sense out of the combination of
> >-- vfs.wapbl.flush_disk_cache=0 mitigating the problem
> >-- neither the RAID set nor its components showing busy in iostat
> >Maybe during a flush, the discs are not regarded busy?
> 
> busy means that the driver is executing I/O requests or waiting for
> results from such operartions. The cache flush operation doesn't
> count by itself, but it slows down parallel I/O operations, so the
> disk appears busy as long as there are such I/O operations.

At last for wd(4) (and I suspect it's true for sd(4) as well),
new I/Os while a flush cache is running are stalled in the buffer queue, and
so disk_busy() is only called once the flush cache has completed.
So, the disk is effectively counted as unbusy while a cache flush is
in progress.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--