Open Stack

> From: Ben Swartzlander [mailto:ben at swartzlander.org]
> Sent: Thursday, August 27, 2015 8:11 PM
> To: OpenStack Development Mailing List (not for usage questions)
> 
> 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

I disagree. You may have failure domains done on a different level, like using Ceph mechanisms for that. In such case you want to provide the user with a single backend regardless of compute AZ partitioning. To address such needs you would need to set multiple AZ per backend to make this achievable.