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[placement][nova][ptg] Shared resource providers for shared disk on compute hosts


This topic is from the cross project etherpad [1]

There's a spec in progress for this: https://review.openstack.org/#/c/650188/

It does a pretty good job of explaining some of the corner cases. We
need to make sure we have a good awareness of all the corners
because when we try to do (or even think about) this feature we
discover a problem that derails the work.

Which is unfortunate because supporting shared disk is one of the
principles on which placement was sold. All the necessary
functionality is already there in placement, so making it work is
mostly on the nova side, a lot of it in nova-compute. Much of the
challenge is associated with dealing with move operations.

A question I posed on the spec is whether it is possible to make
this happen without having to rely on additional config. It seems
like something where it would be super nice (but perhaps not
possible) for a virtdriver to be able to turn on and manage itself.

So the discussion surrounding this topic is perhaps around:

* teasing out the dark corners as much as possible so we don't
   stumble again

* nailing the issues with moves

* exploring the options for making it as hands-free as possible


[1] https://etherpad.openstack.org/p/ptg-train-xproj-nova-placement

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