[openstack-dev] [Nova][Ironic] Question about scheduling two instances to same baremetal node

Sylvain Bauza sbauza at redhat.com
Fri Jan 9 09:17:30 UTC 2015

Le 09/01/2015 09:01, Alex Xu a écrit :
> Hi, All
> There is bug when running nova with ironic 
> https://bugs.launchpad.net/nova/+bug/1402658
> The case is simple: one baremetal node with 1024MB ram, then boot two 
> instances with 512MB ram flavor.
> Those two instances will be scheduling to same baremetal node.
> The problem is at scheduler side the IronicHostManager will consume 
> all the resources for that node whatever
> how much resource the instance used. But at compute node side, the 
> ResourceTracker won't consume resources
> like that, just consume like normal virtual instance. And 
> ResourceTracker will update the resource usage once the
> instance resource claimed, then scheduler will know there are some 
> free resource on that node, then will try to
> schedule other new instance to that node.
> I take look at that, there is NumInstanceFilter, it will limit how 
> many instance can schedule to one host. So can
> we just use this filter to finish the goal? The max instance is 
> configured by option 'max_instances_per_host', we
> can make the virt driver to report how many instances it supported. 
> The ironic driver can just report max_instances_per_host=1.
> And libvirt driver can report max_instance_per_host=-1, that means no 
> limit. And then we can just remove the
> IronicHostManager, then make the scheduler side is more simpler. Does 
> make sense? or there are more trap?
> Thanks in advance for any feedback and suggestion.

Mmm, I think I disagree with your proposal. Let me explain by the best I 
can why :

tl;dr: Any proposal unless claiming at the scheduler level tends to be wrong

The ResourceTracker should be only a module for providing stats about 
compute nodes to the Scheduler.
How the Scheduler is consuming these resources for making a decision 
should only be a Scheduler thing.

Here, the problem is that the decision making is also shared with the 
ResourceTracker because of the claiming system managed by the context 
manager when booting an instance. It means that we have 2 distinct 
decision makers for validating a resource.

Let's stop to be realistic for a moment and discuss about what could 
mean a decision for something else than a compute node. Ok, let say a 
Provided that *something* would report the volume statistics to the 
Scheduler, that would be the Scheduler which would manage if a volume 
manager could accept a volume request. There is no sense to validate the 
decision of the Scheduler on the volume manager, just maybe doing some 
error management.

We know that the current model is kinda racy with Ironic because there 
is a 2-stage validation (see [1]). I'm not in favor of complexifying the 
model, but rather put all the claiming logic in the scheduler, which is 
a longer path to win, but a safier one.


[1]  https://bugs.launchpad.net/nova/+bug/1341420

> Thanks
> Alex
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