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Second that question, Using KVM at least, I couldn't find any significant differences between QCOW2 and RAW based images.
By "significant", I mean, enough to justify tossing the benefits of qcow2.

On May 8, 2014, at 8:57 AM, Tim Bell <Tim.Bell at cern.ch> wrote:

> 
> Robert,
> 
> The difference between RAW and QCOW2 is pretty significant... what hypervisor are you using ?
> 
> Have you seen scenarios where the two SSDs are failing at the same time ? Red Hat was recommending against mirroring SSDs as with the same write pattern, the failure points for the same batch of SSDs would be close.
> 
> Tim
> 
> -----Original Message-----
> From: Robert van Leeuwen [mailto:Robert.vanLeeuwen at spilgames.com] 
> Sent: 08 May 2014 15:03
> To: Arne Wiebalck; openstack-operators at lists.openstack.org
> Subject: Re: [Openstack-operators] Configuring local instance storage
> 
>> In our cloud we use the non-shared local fs of the compute for instance storage. 
>> As our cloud gets more busy, this is now more and more becoming a serious bottleneck.
>> 
>> When discussing the various options to set this up, we wondering how 
>> other clouds deal with the problem of compute disk contention in general and the integration of SSDs in particular.
>> 
>> So, any suggestions or experiences in this area you'd like to share would be very welcome!
> 
> Hi Arne,
> 
> We run all our compute nodes with SSDs for local storage.
> 
> We optimized for 2 different flavors.
> Based on the flavor the instance will end up on the correct hypervisor.
> * normal instances: The are hosted on a SSD raid 1, using QCOW2 disk format
> * fastio instances (e.g. for our database team): These are hosted on a bigger raid 10 volume of SSDs and use RAW disk format
> 
> We noticed a very big impact of QCOW2 vs RAW in our IOPS test: 
> About a factor 10 with random 16k writes.
> 
> Since we have mostly internal customers we were also able to optimize the images.
> We made sure they do not do any unnecessary IOs.
> E.G. no local logging, everything goes to our central log servers.
> 
> Cheers,
> Robert van Leeuwen
> 
> 
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