Open Stack

> -----Original Message-----
> From: Jay Pipes [mailto:jaypipes at gmail.com]
> Sent: 08 April 2014 14:25
> To: openstack-dev at lists.openstack.org
> Subject: Re: [openstack-dev] [Nova] Hosts within two Availability Zones :
> possible or not ?
> 
> On Tue, 2014-04-08 at 10:49 +0000, Day, Phil wrote:
> > On a large cloud you’re protect against this to some extent if the
> > number of servers is >> number of instances in the quota.
> >
> > However it does feel that there are a couple of things missing to
> > really provide some better protection:
> >
> > -         A quota value on the maximum size of a server group
> > -         A policy setting so that the ability to use service-groups
> > can be controlled on a per project basis
> 
> Alternately, we could just have the affinity filters serve as weighting filters
> instead of returning NoValidHosts.
> 
> That way, a request containing an affinity hint would cause the scheduler to
> prefer placing the new VM near (or not-near) other instances in the server
> group, but if no hosts exist that meet that criteria, the filter simply finds a
> host with the most (or fewest, in case of anti-affinity) instances that meet
> the affinity criteria.
> 

I agree that  "hint" would be more consistent with weighting that filtering ("constraint" would be a better word for that) - but how does the user get feedback on whether the hint has been honoured or not ?

In the case of anti-affinity they would need to:
	- Create a VM
	- Check the host hash value is different
	- if they really care delete the VM and try again

... which is pretty much the same cycle they can do without the hint (the filter/weighter just gives it a better chance of working first time a small system) 

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.