Workload spikes on KVM host when doing IO on a guest...

[Erik Brakkee <erik@brakkee.org>]

Hi,


I am seeing high workload spikes of approx. 15 when I do IO inside a KVM 
guest, for instance

   dd if=/dev/zero bs=1G count=1 of=hog

When I execute a similar command on the host to write a file on the same 
physical disk, the workload only goes to about 3.

I am using virtio on the guest with cache mode none. Also, I am using 
the noop IO scheduler on the guest and the deadline IO scheduler on the 
host.
The guest is allocated a logical volume from the host.

When I execute the dd command on the guest, it finishes almost 
instantaneously but when I execute it on the host I have to wait for 
approx 10 seconds. Specifically,
on the guest I see a transfer speed of approx. 600 MB/s and on the host 
I get 75.9MB/s. The figure for the host is most reliable as this is 
close to what the hard disks can handle (WD enterprise class SATA hard 
disks).

What appears to be happening is that somehow it forwards all IO from the 
guest immediately to the host, just as if write back caching was used.

When I look at the output of 'virsh dumpxm <vmname>' I get this as part 
of the output which indicates that cache="none" is actually used

<driver name='qemu' type='raw' cache='none'/>
<source dev='/dev/bootdisks/sparrow'/>
<target dev='sda' bus='virtio'/>
<alias name='virtio-disk0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
</disk>

The host is an opensuse 11.3 system (Linux falcon 2.6.34.10-0.6-default 
#1 SMP 2011-12-13 18:27:38 +0100 x86_64 x86_64 x86_64 GNU/Linux).
The kvm version is

falcon:~ # rpm -qa | grep kvm
kvm-0.12.5-1.8.1.x86_64

Is this some known issue in this version of KVM and should I simply 
upgrade (or replace the host with a centos 6.2 system). Or is there a 
simple configuration that can fix this?

Cheers
   Erik