Re: troubleshooting I/O performance bottleneck

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

On Sat, Apr 25, 2009 at 07:13:06AM +0200, Emmanuel Dreyfus wrote:
> Hello
> 
> I run into a disk I/O bottleneck problem on a i386 xen setup:
> The problem is exhibited by performuing writes:
> # time dd if=/dev/zero of=test bs=1024k count=100
> 
> On the dom0 (xen 3.1.4, netbsd-4 as 20070707), it is fair
>     2.05s real     0.00s user     0.57s system
> 
> On any domU (netbsd-4 or netbsd-5), it is weak:
>     5.94s real     0.00s user     0.73s system
> 
> running systat vm on the dom0 during the I/O from domU gives the ouput above
> 
>  Any idea of where I should look at?

the same output while running dd in the dom0 could help.
Could it be a RAM size issue (the dd output fits in ram in dom0 but not
in domU) or different mount options ?
It could also come from partition layouts; modern disks are known to be faster
when writting at the beggining of the drive than at the end.

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