qcow2 version woes

[Kashyap Chamarthy]

On Thu, Mar 05, 2015 at 11:48:08AM -0700, Mike Ruckman wrote:
> Fellow cloudies,
> 
> The recent builds of our images are v3 qcow2 (also known as qcow3).
> This presents a problem for our testing, since the openstack instances
> we have access to don't support v3, only v2. The only difference
> between v2 and v3 are some performance enhancements -

Right, just some details on the said performance enhancements in QCOW2
v3 format, from my notes (about a year old) from an earlier discussion
w/ some QEMU developers, the two new features in v3 of QCOW2:

  (1) lazy_refcounts -- A performance optimization interesting only when
  using 'cache=writethrough'. Useful when guest crashes or powers off.
  
  (2) Zero clusters -- Useful for (disk) image streaming over slow
  networks.

> so the question is, should we produce v2 images instead of v3 for
> easier testing?
 
> This would require us to request a tweak from releng to get the
> correct format, but would stop us from having to use a workaround [0]
> to test (and I worry about testing a converted image - because then we
> rely on "well, it *should* be the same" for our test results). 
> 
> Thoughts?
> 
> [0] qemu-img amend -f qcow2 -o compat=0.10 <image.qcow2>

-- 
/kashyap