[CentOS-virt] disk i/o benchmarking inside vm

Sun Oct 23 02:14:01 UTC 2011
Eric Shubert <ejs at shubes.net>

On 10/22/2011 06:00 PM, Dennis Jacobfeuerborn wrote:
> Running iostat shows no I/O activity when running the tests for /dev/vdb
> which explains the insane numbers. The question is why I get such different
> results when both devices are defined exactly the same way?

I'm not very familiar with KVM yet (got my first real lesson today), but 
I notice you said:
"In the guest the drive are running using virtio with type=raw and 
cache=none." Are these KVM settings, or did you use kernel parameters on 
the guest machine?

Also, what about the elevator (i/o scheduler) in the guest? In a VMware 
Server2 host (on Centos, so I'm not far OT) it's best to use the 
elevator=noop parameter. I wouldn't expect the elevator to skew results 
quite as much as you're seeing, but what do I know? ;)

-Eric 'shubes'