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[nova][cinder] Volume-backed instance disks and some operations that do not support those yet


> With this in mind, where do things stand right now, should we consider
> writing an image backend, are there other options or plans?

I don't think you should, no. The image backend code is messy and
problematic for a lot of reasons, and building on what we have there is
a path to madness I think. Rewriting it is no small feat, and I think
that if we did we'd want to do so in such a way that makes use of cinder
for anything other than local disk. That's a really nice ideal, but it's
a huge amount of work to do (and review) and also unlikely to ever
actually happen.

We can do a lot better by reducing the feature gap with volume-backed
instances. Implementing the features that aren't supported, and
improving the ones that are *weird* when used on a volume-backed
instance. These would be much smaller changes, easier to review, easier
to gain acceptance for, etc. Personally, if you want to do some work in
this area, I'd recommend picking a weird behavior and trying to propose
an improvement to it.

--Dan