[openstack-dev] [Scheduler] New scheduler feature - VM Ensembles

Gary Kotton gkotton at redhat.com
Mon Jan 28 09:42:54 UTC 2013

On 01/24/2013 12:14 PM, Gary Kotton wrote:
> On 01/22/2013 05:03 PM, Russell Bryant wrote:
>> On 01/08/2013 08:27 AM, Gary Kotton wrote:
>>> Hi,
>>> The link
>>> https://docs.google.com/document/d/1bAMtkaIFn4ZSMqqsXjs_riXofuRvApa--qo4UTwsmhw/edit 
>>> <https://docs.google.com/document/d/1bAMtkaIFn4ZSMqqsXjs_riXofuRvApa--qo4UTwsmhw/edit> 
>>> introduces the concept of a VM ensemble or VM group into Nova. An
>>> ensemble will provide the tenant the ability to group together VMs that
>>> provide a certain service or part of the same application. More
>>> specifically it enables configuring scheduling policies per group. This
>>> will in turn allow for a more robust and resilient service.
>>> Specifically, it will allow a tenant to deploy a multi-VM application
>>> that is designed for VM fault tolerance in a way that application
>>> availability is actually resilient to physical host failure.
>>> Any inputs and comments will be greatly appreciated.
>> I just read over the blueprint and document for this.
>> My gut reaction is that this seems to be bleeding some of what Heat does
>> into Nova, which I don't like.
> No. This does not take any of Heats functionality. It actually 
> provides Heat with tools that can provide better placements.
>> The document provides an example using the existing different_host
>> scheduler hint.  It shows how it might schedule things in a way that
>> won't work, but only if the compute cluster is basically running out of
>> capacity.  I'm wondering how realistic of a problem this example is
>> demonstrating.
> This may be a rare case if you are looking at a 40%-50% overall 
> resource utilization. However, once you are pushing utilization to the 
> 70%-80% and (where most cloud operators would expect it to be), then 
> resource contention would be a very common phenomena that cannot be 
> ignored. 70-80% utilization is not an edge case.
> We also foresee that at the target utilization levels of 75%+ we will 
> also need to deal with resource fragmentation.  Such that, 
> de-fragmentation of resources will need to automatically triggered 
> when needed. Therefore, VM groups scheduling policies need to be 
> stored and used for future re-scheduling of VM group members.
>> The document mentions that when requesting N instances, specifying that
>> anti-affinity is required for the hosts is not possible.  Perhaps that's
>> something that could be added.
> This will not work - the best example is to look at that in the 
> document. In order to provide a complete solution the scheduler would 
> be required to see the whole group in order to select the best 
> placement strategy. The scheduling hints do not provide a solution 
> when the scheduler need to see the whole picture.
> This is certainly a topic that we certainly need to speak about at up 
> and coming summit. I think that it would also be very beneficial if we 
> can have this in the G version.

I did not get a response to the answers to the questions. I have added 
additional implementations - this was following the review comments. 
What do we still need to do in order to get approval for the blueprint. 
I think that there is consensus that this is a very valuable feature and 
that OpenStack can benefit from it.


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