[openstack-dev] [nova] Evacuate instance which in server group with affinity policy

Sylvain Bauza sbauza at redhat.com
Mon Dec 22 13:50:44 UTC 2014

Le 22/12/2014 13:37, Alex Xu a écrit :
> 2014-12-22 10:36 GMT+08:00 Lingxian Kong <anlin.kong at gmail.com 
> <mailto:anlin.kong at gmail.com>>:
>     2014-12-22 9:21 GMT+08:00 Alex Xu <soulxu at gmail.com
>     <mailto:soulxu at gmail.com>>:
>     >
>     >
>     > 2014-12-22 9:01 GMT+08:00 Lingxian Kong <anlin.kong at gmail.com
>     <mailto:anlin.kong at gmail.com>>:
>     >>
>     >>
>     >> but what if the compute node is back to normal? There will be
>     >> instances in the same server group with affinity policy, but
>     located
>     >> in different hosts.
>     >>
>     >
>     > If operator decide to evacuate the instance from the failed
>     host, we should
>     > fence the failed host first.
>     Yes, actually. I mean the recommandation or prerequisite should be
>     emphasized somewhere, e.g. the Operation Guide, otherwise it'll make
>     things more confused. But the issue you are working around is indeed a
>     problem we should solve.
> Yea, you are right, we should doc it if we think this make sense. Thanks!

As I said, I'm not in favor of adding more complexity in the instance 
group setup that is done in the conductor for basic race condition reasons.

If I understand correctly, the problem is when there is only one host 
for all the instances belonging to a group with affinity filter and this 
host is down, then the filter will deny any other host and consequently 
the request will fail while it should succeed.

Is this really a problem ? I mean, it appears to me that's a normal 
behaviour because a filter is by definition an *hard* policy.

So, provided you would like to implement *soft* policies, that sounds 
more likely a *weigher* that you would like to have : ie. make sure that 
hosts running existing instances in the group are weighted more than 
other ones so they'll be chosen every time, but in case they're down, 
allow the scheduler to pick other hosts.


>     --
>     Regards!
>     -----------------------------------
>     Lingxian Kong
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