[openstack-dev] [nova][scheduler] Availability Zones and Host aggregates..

Belmiro Moreira moreira.belmiro.email.lists at gmail.com
Fri Mar 28 12:56:16 UTC 2014

+1 for Phil comments.
I agree that VMs should spread between different default avzs if user
doesn't define one at boot time.
There is a blueprint for that feature that unfortunately didn't make it for


On Fri, Mar 28, 2014 at 12:01 PM, Day, Phil <philip.day at hp.com> wrote:

> >> Personally, I feel it is a mistake to continue to use the Amazon concept
> >> of an availability zone in OpenStack, as it brings with it the
> >> connotation from AWS EC2 that each zone is an independent failure
> >> domain. This characteristic of EC2 availability zones is not enforced in
> >> OpenStack Nova or Cinder, and therefore creates a false expectation for
> >> Nova users.
> >I think this is backwards training, personally. I think azs as separate
> failure
> >domains were done like that for a reason by amazon, and make good sense.
> >What we've done is overload that with cells, aggregates etc which should
> >have a better interface and are a different concept. Redefining well
> understood
> >terms because they don't suite your current implementation is a slippery
> slope,
> >and overloading terms that already have a meaning in the industry in just
> annoying.
> +1
> I don't think there is anything wrong with identifying new use cases and
> working out how to cope with them:
>  - First we generalized Aggregates
> - Then we mapped AZs onto aggregates as a special mutually exclusive group
> - Now we're recognizing that maybe we need to make those changes to
> support AZs more generic so we can create additional groups of mutually
> exclusive aggregates
> That all feels like good evolution.
> But I don't see why that means we have to fit that in under the existing
> concept of AZs - why can't we keep AZs as they are and have a better thing
> called Zones that is just an OSAPI concept and is better that AZs ?
>  Arguments around not wanting to add new options to create server seem a
> bit weak to me - for sure we don't want to add them in an uncontrolled way,
> but if we have a new, richer, concept we should be able to express that
> separately.
> I'm still not personally convinced by the need use cases of racks having
> orthogonal power failure domains and switch failure domains - that seems to
> me from a practical perspective that it becomes really hard to work out
> where to separate VMs so that they don't share a failure mode.    Every
> physical DC design I've been involved with tries to get the different
> failure domains to align.   However if it the use case makes sense to
> someone then I'm not against extending aggregates to support multiple
> mutually exclusive groups.
> I think I see a Design Summit session emerging here
> Phil
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