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[Joe Gordon <1413119@xxxxxxxxxxxxxxxxxx>]

It sounds like the deeper issue is how the memory  stat going to 0 for
the instance, and if it went to ERROR state or not.  This should never
happen. Marking this bug as invalid and we should instead open a
separate bug on how to prevent us from getting stuck in that state. If
memory goes to 0 the instance should be in ERROR state.

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

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https://bugs.launchpad.net/bugs/1413119

Title:
  Pre-migration memory check-  Invalid error message if memory value is
  0

Status in OpenStack Compute (Nova):
  Invalid

Bug description:
  There is possible correction in the logic and error handling in the pre-migration memory check in case when the memory value is zero '0'.
  This check is present in the source code class: nova/conductor/tasks/live_migrate.py

  
  Below is the current code snippet:
          if not mem_inst or avail <= mem_inst:
              instance_uuid = self.instance.uuid
              dest = self.destination
              reason = _("Unable to migrate %(instance_uuid)s to %(dest)s: "
                         "Lack of memory(host:%(avail)s <= "
                         "instance:%(mem_inst)s)")
              raise exception.MigrationPreCheckError(reason=reason % dict(
                      instance_uuid=instance_uuid, dest=dest, avail=avail,
                      mem_inst=mem_inst))
  					
  It can be seen that when mem_inst value is 0, the if condition is met and control enters the block resulting in 'lack of memory' error with a message 'avail memory < 0'. This is absurd.

  Sample error message when memory is zero: 
  2014-10-13 19:52:09.441 3907 INFO nova.api.openstack.wsgi [req-8430dd30-4f17-4094-bbec-ec9cf3593c79 502 eec6e74886804b79b78ac4fceed5b685] NV-4EB7C79 HTTP exception thrown: 
  NV-78D5611 Migration pre-check error: NV-37B7976 Unable to migrate 352122ae-1ca1-43b3-8ba6-709d93fd580c to 9117MMB_100DBCP: Lack of memory(host:65536 <= instance:0)

  The trailing end part of the error doesn't make much sense (host:65536
  <= instance:0).

  Possible correction:
  The check should handle the special case of zero memory value in a different way. If memory equals zero, then a different error of exception should be raised about invalid memory.

  Sample fix:
  if mem_inst <= 0:
     reason = _("Unable to migrate %(instance_uuid)s to %(dest)s: "
                "Invalid value for existing instance memory: %(mem_inst)s.")
     raise exception.MigrationPreCheckError(reason=reason % dict(
                 instance_uuid=instance_uuid, dest=dest, mem_inst=mem_inst))
  elif avail <= mem_inst: 
     reason = _("Unable to migrate %(instance_uuid)s to %(dest)s: "
                "Lack of memory available at destination.  (host:%(avail)s <= "
                "instance:%(mem_inst)s.")
     raise exception.MigrationPreCheckError(reason=reason % dict(
                 instance_uuid=instance_uuid, dest=dest, avail=avail, mem_inst=mem_inst))

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