Re: troubleshooting I/O performance bottleneck

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

On Tue, Apr 28, 2009 at 10:03:52AM +0000, Emmanuel Dreyfus wrote:
> On Tue, Apr 28, 2009 at 11:25:40AM +0200, Manuel Bouyer wrote:
> > 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.
> 
> Sorry, it fluctates a lot, I made my snapshot at the wrong time. Here
> is another dd from the dom0. The same operation from the domU won't show
> a throughput better than 16 MB/s

This is also what I get on my test box. iostat -x shows that the vnd0
device is getting write requests of 32k (this is expected as the
xbd interface can't do 64k transfers so MAXPHYS is set to 32k for domUs),
and writes are forwarded to the disk as a stream of 32k requests.
When the write is done from dom0 the disk gets a stream of 64k requests.
This would explain the slowdown.

There's no clean solution to this, as it's a limitation of the xbd interface.
A workaround could be to set MAXPHYS back to 64k in the domU kernel
and have the xbd driver split the requests if needed. we'll have then
to hope that the xbdback driver will be able to aggregate the 2 requests back
in a 64k block.

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