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Hi Matt,

Just looking into this,

> On 1 Jun 2017, at 9:08 am, Matt Riedemann <mriedemos at gmail.com> wrote:
> 
> This is a request for any operators out there that configure nova to set:
> 
> [cinder]
> cross_az_attach=False
> 
> To check out these two bug fixes:
> 
> 1. https://review.openstack.org/#/c/366724/
> 
> This is a case where nova is creating the volume during boot from volume and providing an AZ to cinder during the volume create request. Today we just pass the instance.availability_zone which is None if the instance was created without an AZ set. It's unclear to me if that causes the volume creation to fail (someone in IRC was showing the volume going into ERROR state while Nova was waiting for it to be available), but I think it will cause the later attach to fail here [1] because the instance AZ (defaults to None) and volume AZ (defaults to nova) may not match. I'm still looking for more details on the actual failure in that one though.
> 
> The proposed fix in this case is pass the AZ associated with any host aggregate that the instance is in.

If cross_az_attach is false won’t it always result in the instance AZ being None as it won’t be on a host yet?
I haven’t traced back the code fully so not sure if an instance gets scheduled onto a host and then the volume create call happens  or they happen in parallel etc. (in the case for boot from volume) 


When cross_az_attach is false:
If a user does a boot from volume (create new volume) and specifies an AZ then I would expect the instance and the volume to be created in the specified AZ.
If the AZ doesn’t exist in cinder or nova I would expect it to fail.

If a user doesn’t specify an AZ I would expect that the instance and the volume are in the same AZ.
If there isn’t a common AZ between cinder and nova I would expect it to fail.



> 
> 2. https://review.openstack.org/#/c/469675/
> 
> This is similar, but rather than checking the AZ when we're on the compute and the instance has a host, we're in the API and doing a boot from volume where an existing volume is provided during server create. By default, the volume's AZ is going to be 'nova'. The code doing the check here is getting the AZ for the instance, and since the instance isn't on a host yet, it's not in any aggregate, so the only AZ we can get is from the server create request itself. If an AZ isn't provided during the server create request, then we're comparing instance.availability_zone (None) to volume['availability_zone'] ("nova") and that results in a 400.
> 
> My proposed fix is in the case of BFV checks from the API, we default the AZ if one wasn't requested when comparing against the volume. By default this is going to compare "nova" for nova and "nova" for cinder, since CONF.default_availability_zone is "nova" by default in both projects.
> 

Is this an alternative approach? Just trying to get my head around this all.

Thanks,
Sam


> --
> 
> I'm requesting help from any operators that are setting cross_az_attach=False because I have to imagine your users have run into this and you're patching around it somehow, so I'd like input on how you or your users are dealing with this.
> 
> I'm also trying to recreate these in upstream CI [2] which I was already able to do with the 2nd bug.
> 
> Having said all of this, I really hate cross_az_attach as it's config-driven API behavior which is not interoperable across clouds. Long-term I'd really love to deprecate this option but we need a replacement first, and I'm hoping placement with compute/volume resource providers in a shared aggregate can maybe make that happen.
> 
> [1] https://github.com/openstack/nova/blob/f278784ccb06e16ee12a42a585c5615abe65edfe/nova/virt/block_device.py#L368
> [2] https://review.openstack.org/#/c/467674/
> 
> -- 
> 
> Thanks,
> 
> Matt
> 
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