On Mon, 15 Aug 2011 16:05:17 -0600, Orion Poplawski wrote:
> I have a server that I recently migrated from CentOS 5.6 to SL6.1.
> The host install serves as a basic disk server and runs our nightly
> backups with amanda.  We also have several virtual machines running 
> on
> it, so of which are mostly idle and others which can be quite busy.
> Since making the move I'm seeing the disk and network
> response/performance go to hell if I have any non-idle VMs running.
> One barometer - I have a raid10 rebuild running at the moment.  With
> no VMs I can reach 200,000K/sec resync speed.  I'm getting 131,000K
> with two idle VMs and some other basic activity.  If I start a busy
> vm, it drops to a few hundred K/sec and all disk access is very slow.
> Response to keystrokes in a network shell can be very slow as well.
>
> As anyone else experienced problems like this?  I'm not seeing any
> error messages on the host or VMs.

Hi,

The default settings are bad.

Please tell us more about your settings.

Otherwise, I recommend the following points:

- controller: virtio
- kvm option: cache=none
- qcow2 disk format with metadata preallocation
- create your disk image with: qemu-img create -f qcow2 -o 
size=40000000000,preallocation=metadata vdisk.img

Here's a good link to the topic:
http://www.ilsistemista.net/index.php/virtualization/11-kvm-io-slowness-on-rhel-6.html

Best regards,

Morten