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Interesting one, that's doing 50% of the job, 
we just need to implement the missing imagebackend.py:Qcow2.flatten() method, checking it.


Matt Riedemann <mriedemos at gmail.com> wrote on ven. [2019-nov.-22 11:15:46 -0600]:
> On 11/22/2019 8:05 AM, Alexandre Arents wrote:
> >   C) Change create_image()/imagebackend driver behavior,
> >      to create a flatten qcow2 file in case of unshelving.
> >      flattening disk may be a solution because there will be no more "orphan backing file".
> >      (Basicly doing like "flat" backend driver except we need to stay in qcow2 instead of RAW)
> >          PROS:
> >           -we keep orignal unshelve behavior/assumption
> >          CONS:
> >           -It means that in your infra configured in COW some instances will be in "qcow2 flat",
> >            Flat qcow2 instance works great (livemigration/resize..). Would all installation ok with that ?
> >            Ok it seems a little odd to ask COW driver to not do COW in some case. Alternatilevy we can
> >            force using flat driver if unshelving, but we need to change flat driver to support also qcow2.
> >   D) During spawn() if unshelving we convert "qcow2 disk with backing file" to a "flatten qcow2 disk",
> >      just after self._create_image().
> >      It looks more like a workaround than a long term solution as it need to convert something created before,
> >      that do not meet the need(better to do C).
> 
> Does this already fix your problem?
> 
> https://review.opendev.org/#/q/If3c9d1de3ce0fe394405bd1e1f0fa08ce2baeda8
> 
> -- 
> 
> Thanks,
> 
> Matt
> 

-- 
Alexandre Arents