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[Matt Riedemann <mriedem.os@xxxxxxxxx>]

** Changed in: nova
       Status: New => Opinion

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https://bugs.launchpad.net/bugs/1754782

Title:
  we skip critical scheduler filters when forcing the host on instance
  boot

Status in OpenStack Compute (nova):
  Opinion

Bug description:
  When booting an instance it's possible to force it to be placed on a
  specific host using the  "--availability-zone nova:host" syntax.

  If you do this, the code at 
  https://github.com/openstack/nova/blob/master/nova/scheduler/host_manager.py#L581 will return early rather than call self.filter_handler.get_filtered_objects()

  Based on discussions at the PTG with Dan Smith, the simplest solution
  would be to create a flag similar to RUN_ON_REBUILD which would be
  applied to the various scheduler filters in a manner analogous to how
  rebuild is handled now.

  Presumably we'd want to call something like this during the instance
  boot code to ensure we hit the existing "if not check_type" at L581:

  request_spec.scheduler_hints['_nova_check_type'] = ['build']

  
  Then in the various critical filers (NUMATopologyFilter for example, and PciPassthroughFilter, and maybe some others like ComputeFilter) we could define something like "RUN_ON_BUILD = True" to ensure that they run even when forcing a host.

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