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On 08/27/2015 10:43 AM, Ivan Kolodyazhny wrote:
> Hi,
>
> Looks like we need to be able to set AZ per backend. What do you think 
> about such option?

I dislike such an option.

The whole premise behind an AZ is that it's a failure domain. The node 
running the cinder services is in exactly one such failure domain. If 
you have 2 backends in 2 different AZs, then the cinder services 
managing those backends should be running on nodes that are also in 
those AZs. If you do it any other way then you create a situation where 
a failure in one AZ causes loss of services in a different AZ, which is 
exactly what the AZ feature is trying to avoid.

If you do the correct thing and run cinder services on nodes in the AZs 
that they're managing then you will never have a problem with the 
one-AZ-per-cinder.conf design we have today.

-Ben


>
> Regards,
> Ivan Kolodyazhny
>
> On Mon, Aug 10, 2015 at 7:07 PM, John Griffith 
> <john.griffith8 at gmail.com <mailto:john.griffith8 at gmail.com>> wrote:
>
>
>
>     On Mon, Aug 10, 2015 at 9:24 AM, Dulko, Michal
>     <michal.dulko at intel.com <mailto:michal.dulko at intel.com>> wrote:
>
>         Hi,
>
>         In Kilo cycle [1] was merged. It started passing AZ of a
>         booted VM to Cinder to make volumes appear in the same AZ as
>         VM. This is certainly a good approach, but I wonder how to
>         deal with an use case when administrator cares about AZ of a
>         compute node of the VM, but wants to ignore AZ of volume. Such
>         case would be when fault tolerance of storage is maintained on
>         another level - for example using Ceph replication and failure
>         domains.
>
>         Normally I would simply disable AvailabilityZoneFilter in
>         cinder.conf, but it turns out cinder-api validates if
>         availability zone is correct [2]. This means that if Cinder
>         has no AZs configured all requests from Nova will fail on an
>         API level.
>
>         Configuring fake AZs in Cinder is also problematic, because AZ
>         cannot be configured on a per-backend manner. I can only
>         configure it per c-vol node, so I would need N extra nodes
>         running c-vol,  where N is number of AZs to achieve that.
>
>         Is there any solution to satisfy such use case?
>
>         [1] https://review.openstack.org/#/c/157041
>         [2]
>         https://github.com/openstack/cinder/blob/master/cinder/volume/flows/api/create_volume.py#L279-L282
>
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>
>     ​Seems like we could introduce the capability in cinder to ignore
>     that if it's desired?  It would probably be worth looking on the
>     Cinder side at being able to configure multiple AZ's for a volume
>     (perhaps even an aggregate Zone just for Cinder).  That way we
>     still honor the setting but provide a way to get around it for
>     those that know what they're doing.
>
>     John
>
>
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