Open Stack

----- Original Message -----
> > -----Original Message-----
> > From: Chris Friesen [mailto:chris.friesen at windriver.com]
> > Sent: 09 April 2014 15:37
> > To: openstack-dev at lists.openstack.org
> > Subject: Re: [openstack-dev] [Nova] Hosts within two Availability Zones :
> > possible or not ?
> > 
> > On 04/09/2014 03:55 AM, Day, Phil wrote:
> > 
> > > I would guess that affinity is more likely to be a soft requirement
> > > that anti-affinity,  in that I can see some services just not meeting
> > > their HA goals without anti-affinity but I'm struggling to think of a
> > > use case why affinity is a must for the service.
> > 
> > Maybe something related to latency?  Put a database server and several
> > public-facing servers all on the same host and they can talk to each other
> > with less latency then if they had to go over the wire to another host?
> > 
> I can see that as a high-want, but would you actually rather not start the
> service if you couldn't get it ?  I suspect not, as there are many other
> factors that could affect performance.  On the other hand I could imagine a
> case where I declare its not worth having a second VM at all if I can't get
> it on a separate server.   Hence affinity feels more "soft" and
> anti-affinity "hard" in terms or requirments.

As the orchestrator if affinity is important to me and it turns out I can't place all of the VMs in the group with affinity, I would likely use the failure to place the second (or subsequent) instance as my cue to rollback and destroy the original VM(s) as well. I don't think either policy is naturally any more hard or soft - it depends on the user and their workloads - this is why I think a "soft" implementation of either filter should be in addition to rather than instead of the existing ones, though "soft" may make more sense for the defaults. 

Thanks,

Steve