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> I think Nova should never have to rely on Cinder's hosts/backends
> information to do migrations or any other operation.
>
> In this case even if Nova had that info, it wouldn't be the solution.
> Cinder would reject migrations if there's an incompatibility on the
> Volume Type (AZ, Referenced backend, capabilities...)

I think I'm missing a bunch of cinder knowledge required to fully grok
this situation and probably need to do some reading. Is there some
reason that a volume type can't exist in multiple backends or something?
I guess I think of volume type as flavor, and the same definition in two
places would be interchangeable -- is that not the case?

> I don't know anything about Nova cells, so I don't know the specifics of
> how we could do the mapping between them and Cinder backends, but
> considering the limited range of possibilities in Cinder I would say we
> only have Volume Types and AZs to work a solution.

I think the only mapping we need is affinity or distance. The point of
needing to migrate the volume would purely be because moving cells
likely means you moved physically farther away from where you were,
potentially with different storage connections and networking. It
doesn't *have* to mean that, but I think in reality it would. So the
question I think Matt is looking to answer here is "how do we move an
instance from a DC in building A to building C and make sure the
volume gets moved to some storage local in the new building so we're
not just transiting back to the original home for no reason?"

Does that explanation help or are you saying that's fundamentally hard
to do/orchestrate?

Fundamentally, the cells thing doesn't even need to be part of the
discussion, as the same rules would apply if we're just doing a normal
migration but need to make sure that storage remains affined to compute.

> I don't know how the Nova Placement works, but it could hold an
> equivalency mapping of volume types to cells as in:
>
>  Cell#1        Cell#2
>
> VolTypeA <--> VolTypeD
> VolTypeB <--> VolTypeE
> VolTypeC <--> VolTypeF
>
> Then it could do volume retypes (allowing migration) and that would
> properly move the volumes from one backend to another.

The only way I can think that we could do this in placement would be if
volume types were resource providers and we assigned them traits that
had special meaning to nova indicating equivalence. Several of the words
in that sentence are likely to freak out placement people, myself
included :)

So is the concern just that we need to know what volume types in one
backend map to those in another so that when we do the migration we know
what to ask for? Is "they are the same name" not enough? Going back to
the flavor analogy, you could kinda compare two flavor definitions and
have a good idea if they're equivalent or not...

--Dan