openstack team mailing list archive

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Thu, May 10, 2012 at 09:06:58AM +0100, Daniel P. Berrange wrote:
> On Wed, May 09, 2012 at 11:08:13PM -0600, Jim Fehlig wrote:
> > Hi,
> > 
> > I've been tinkering with improving Xen support in the libvirt driver and
> > wanted to discuss a few issues before submitting patches.
> > 
> > Even the latest upstream release of Xen (4.1.x) contains a rather old
> > qemu, version 0.10.2, which rejects qcow2 images with cluster size >
> > 64K.  The libvirt driver creates the COW image with cluster size of 2M. 
> > Is this for performance reasons?  Any objections to removing that option
> > and going with 'qemu-img create' default of 64K?
> 
> In general larger cluster size does improve the performance of
> qcow2. I'm not sure how much of a delta we get by going from
> 64k to 2M though. If there's any doubt then I guess it could be
> made into a configuration parameter.

I had a quick chat with Kevin Wolf who's the upstream QEMU qcow2 maintainer
and he said that 64k is the current recommended cluster size for qcow2.
Above this size, the cost of COW becomes higher causing an overall
drop in performance.

Looking at GIT history, Nova has used cluster_size=2M since Vish first
added qcow2 support, and there's no mention of why in the commit message.
So unless further info comes to light, I'd say we ought to just switch
to use qemu-img's default setting of 64K for both Xen and KVM.

Daniel
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