On 03/26/2014 11:17 AM, Khanh-Toan Tran wrote:

I don't know why you need a
compute node that belongs to 2 different availability-zones. Maybe
I'm wrong but for me it's logical that availability-zones do not
share the same compute nodes. The "availability-zones" have the role
of partition your compute nodes into "zones" that are physically
separated (in large term it would require separation of physical
servers, networking equipments, power sources, etc). So that when
user deploys 2 VMs in 2 different zones, he knows that these VMs do
not fall into a same host and if some zone falls, the others continue
working, thus the client will not lose all of his VMs.

See Vish's email.

Even under the original meaning of availability zones you could realistically have multiple orthogonal availability zones based on "room", or "rack", or "network", or "dev" vs "production", or even "has_ssds" and a compute node could reasonably be part of several different zones because they're logically in different namespaces.

Then an end-user could boot an instance, specifying "networkA", "dev", and "has_ssds" and only hosts that are part of all three zones would match.

Even if they're not used for orthogonal purposes, multiple availability zones might make sense. Currently availability zones are the only way an end-user has to specify anything about the compute host he wants to run on. So it's not entirely surprising that people might want to overload them for purposes other than physical partitioning of machines.


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