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[Matt Riedemann <mriedem.os@xxxxxxxxx>]

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

** Changed in: nova/rocky
       Status: New => Confirmed

** Changed in: nova/rocky
   Importance: Undecided => Low

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https://bugs.launchpad.net/bugs/1785327

Title:
  schedule_and_build_instances looks up host az for every instance even
  if using the same host

Status in OpenStack Compute (nova):
  Fix Released
Status in OpenStack Compute (nova) pike series:
  Confirmed
Status in OpenStack Compute (nova) queens series:
  Confirmed
Status in OpenStack Compute (nova) rocky series:
  In Progress

Bug description:
  This is a simple performance optimization bug. When creating multiple
  servers, if they are in an affinity group they are going to be on the
  same host. Or if simply the scheduler configuration is such that
  instances are packed onto as few hosts as possible rather than spread,
  the scheduler could return several of the same hosts for the list of
  instances being scheduled. We iterate over the instances and their
  selected hosts and we lookup the az for each host in the loop which is
  a query to the aggregates table in the API DB. If we have >1 of the
  same host in the list, we could optimize this by simply caching the
  host=az mapping.

  https://github.com/openstack/nova/blob/4c37ff72e5446c835a48d569dd5a1416fcd36c71/nova/conductor/manager.py#L1263

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