Open Stack

Hi Matt,

On 2015-09-24 1:45 PM, Matt Riedemann wrote:
> 
> 
> On 9/24/2015 11:50 AM, Mathieu Gagné wrote:
>>
>> May I suggest the following solutions:
>>
>> 1) Add ability to disable this whole AZ concept in Cinder so it doesn't
>> fail to create volumes when Nova asks for a specific AZ. This could
>> result in the same behavior as cinder.cross_az_attach config.
> 
> That's essentially what this does:
> 
> https://review.openstack.org/#/c/217857/
> 
> It defaults to False though so you have to be aware and set it if you're
> hitting this problem.
> 
> The nova block_device code that tries to create the volume and passes
> the nova AZ should have probably been taking into account the
> cinder.cross_az_attach config option, because just blindly passing it
> was the reason why cinder added that option.  There is now a change up
> for review to consider cinder.cross_az_attach in block_device:
> 
> https://review.openstack.org/#/c/225119/
> 
> But that's still making the assumption that we should be passing the AZ
> on the volume create request and will still fail if the AZ isn't in
> cinder (and allow_availability_zone_fallback=False in cinder.conf).
> 
> In talking with Duncan this morning he's going to propose a spec for an
> attempt to clean some of this up and decouple nova from handling this
> logic.  Basically a new Cinder API where you give it an AZ and it tells
> you if that's OK.  We could then use this on the nova side before we
> ever get to the compute node and fail.

IMO, the confusion comes from what I consider a wrong usage of AZ. To
quote Sylvain Bauza from a recent review [1][2]:

"because Nova AZs and Cinder AZs are very different failure domains"

This is not the concept of AZ I learned to know from cloud providers
where an AZ is global to the region, not per-service.

Google Cloud Platform:
- Persistent disks are per-zone resources. [3]
- Resources that are specific to a zone or a region can only be used by
other resources in the same zone or region. For example, disks and
instances are both zonal resources. To attach a disk to an instance,
both resources must be in the same zone. [4]

Amazon Web Services:
- Instances and disks are per-zone resources. [5]

So now we are stuck with AZ not being consistent across services and
confusing people.


[1] https://review.openstack.org/#/c/225119/2
[2] https://review.openstack.org/#/c/225119/2/nova/virt/block_device.py
[3] https://cloud.google.com/compute/docs/disks/persistent-disks
[4] https://cloud.google.com/compute/docs/zones
[5] http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/resources.html

-- 
Mathieu