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I'm reading through mgagne's spec to support attached volume size 
extension callbacks in Nova [1] and the question that comes up is what 
happens when the backend compute does not support this, either because 
it's too old (Ocata) or the virt driver does not support the event?

The spec is targeted at libvirt to use os-brick, but the hyper-v driver 
is also using os-brick since Ocata, and the Windows connector support 
the extend_volume operation, so that should work.

I don't know about powervm, vmware or xen though.

This is not discoverable at the moment, for the end user or cinder, so 
I'm trying to figure out what the failure mode looks like.

This all starts on the cinder side to extend the size of the attached 
volume. Cinder is going to have to see if Nova is new enough to handle 
this (via the available API versions) before accepting the request and 
resizing the volume. Then Cinder sends the event to Nova. This is where 
it gets interesting.

On the Nova side, if all of the computes aren't new enough, we could 
just fail the request outright with a 409. What does Cinder do then? 
Rollback the volume resize?

But let's say the computes are new enough, but the instance is on a 
compute that does not support the operation. Then what? Do we register 
an instance fault and put the instance into ERROR state? Then the admin 
would need to intervene.

Are there other ideas? Until we have capabilities (info) exposed out of 
the API we're stuck with questions like this.

[1] https://review.openstack.org/#/c/453272/

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Thanks,

Matt