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Vishvananda Ishaya wrote:
>
> 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

Right.  I'll submit a patch for that shortly.

Regards,
Jim