Re: troubleshooting I/O performance bottleneck

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

On Tue, Apr 28, 2009 at 05:31:01AM +0200, Emmanuel Dreyfus wrote:
> Manuel Bouyer <bouyer%antioche.eu.org@localhost> wrote:
> 
> > probably not that much, but it's noticeable.
> > Did you have something else doing I/O when you did your test, like
> > a parity rewrite ? systat shows raid0 almost 100% busy in both cases,
> > and for almost the same data transfer rate.
> 
> There is no background disk activity.
> 
> Running systat vm on both dom0 and domU at the same time while
> performing the dd in domU indeed shows identical throughput to disk,
> around 15-20 MB/s
> 
> When dd runs on dom0, I the throughput is around 30-35 MB/s, so the
> difference si significant.

but systat shows 13MB/s throughput, how can dd see 30-35 MB/s ?
The only explainantion is that part of the file is cached, and the speed
reported by dd is wrong.

-- 
Manuel Bouyer, LIP6, Universite Paris VI.           
Manuel.Bouyer%lip6.fr@localhost
     NetBSD: 26 ans d'experience feront toujours la difference
--