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[placement][nova][ptg] resource provider affinity


Spec: https://review.openstack.org/650476

>From the commit message:

     To support NUMA and similar concepts, this proposes the ability
     to request resources from different providers nested under a
     common subtree (below the root provider).

There's much in the feature described by the spec and the surrounding
context that is frequently a source of contention in the placement
group, so working through this spec is probably going to require
some robust discussion. Doing most of that before the PTG will help
make sure we're not going in circles in person.k

Some of the areas of potential contention:

* Adequate for limited but maybe not all use case solutions
* Strict trait constructionism
* Evolving the complexity of placement solely for the satisfaction
   of hardware representation in Nova
* Inventory-less resource providers
* Developing new features in placement before existing features are
   fully used in client services
* Others?

I list this not because they are deal breakers or the only thing
that matters, but because they have presented stumbling blocks in
the past and we may as well work to address them (or make an
agreement to punt them until later) otherwise there will be
lingering dread.

And, beyond all that squishy stuff, there is the necessary
discussion over the solution described in the spec. There are
several alternatives listed in the spec, and a few more in the
comments. We'd like to figure out the best solution that can
actually be done in a reasonable amount of time, not the best
solution in the absolute.

Discuss!

-- 
Chris Dent                       Ù©â??̯â??Û¶           https://anticdent.org/
freenode: cdent                                         tw: @anticdent