openstack team mailing list archive

[Vishvananda Ishaya <vishvananda@xxxxxxxxx>]

On May 10, 2012, at 1:56 AM, Daniel P. Berrange wrote:

> On Thu, May 10, 2012 at 09:06:58AM +0100, Daniel P. Berrange wrote:
> 
> 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.
> 

This is good info.  Sounds like we should switch to 64K

Vish