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[Matt Riedemann <mriedem.os@xxxxxxxxx>]

** Also affects: nova/pike
   Importance: Undecided
       Status: New

** Changed in: nova/pike
       Status: New => Triaged

** Changed in: nova/pike
   Importance: Undecided => High

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https://bugs.launchpad.net/bugs/1724589

Title:
  Unable to transition to Ironic Node Resource Classes in Pike

Status in OpenStack Compute (nova):
  In Progress
Status in OpenStack Compute (nova) pike series:
  Triaged

Bug description:
  So the scenario is:

  * upgraded to pike
  * have ironic with a multiple flavor
  * attempting to transition to resource class based scheduling, now pike is installed

  In Pike we ask people to:

  * Update Ironic Node with a Resource Class
  * Update flavors to request the new Resource Class (and not request VCPU, RAM, DISK), using the docs: https://docs.openstack.org/ironic/latest/install/configure-nova-flavors.html#scheduling-based-on-resource-classes

  Consider this case:

  * some old instances are running from before the updates
  * some new instances are created after the updates

  In placement:

  * all inventory is correct, new resource class and legacy resource classes are both present
  * old instance allocations: only request

  In nova db:

  * old instances and new instances correctly request the new resource class in their flavor
  * new instances also include the anti-request for VCPU, DISK and RAM

  Now this is the flow that shows the problem:

  * get list of candidate allocations
  * this includes nodes that already have instances on (they only claim part of the inventory, but the new instance is only requesting the bit of the inventory the old instance isn't using)
  * boom, scheduling new instances fails after you hit the retry count, unless you got lucky and found a free slot by accident

  Possible reason for this:

  * Pike no longer updated instance allocations, if we updated the
  allocations of old instances to request the new custom resource class
  allocations, we would fix the above issue.

  Possible work around:

  * in the new flavor, keep requesting VCPU, RAM and CPU resources for
  pike, fix that up in queens?

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