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

[Avi Kivity <avi@redhat.com>]

On 05/20/2012 03:55 AM, Erik Brakkee wrote:
> 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.

This is not surprising.  Each I/O request executes in a thread.

>
> 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.
>

With logical volumes, you can use -drive ...,aio=native to avoid the
threads.  The load will disappear.

> 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).

try dd oflag=direct to force the data to disk.  No idea why the host
doesn't finish instantaneously.

>
> 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.

Write back caching is indeed used, since you did not specify oflag=direct.

> 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?

Nothing is broken, so it doesn't need fixing.  The high load is not an
indication of anything.

-- 
error compiling committee.c: too many arguments to function