Num instances scheduler filter

Allow for scheduler filtering based on number of instances already assigned to
that host.

FLAGS.max_instances_per_host controls the maximum number of instances allowed
per host.

Change-Id: Id3e8973311ea5ac8db2d9e4e594a5288a9e3f94d

nova/scheduler/filters/num_instances_filter.py

@@ -0,0 +1,42 @@
+# Copyright (c) 2012 OpenStack, LLC.
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from nova import flags
+from nova.openstack.common import cfg
+from nova.openstack.common import log as logging
+from nova.scheduler import filters
+
+LOG = logging.getLogger(__name__)
+
+max_instances_per_host_opt = cfg.IntOpt("max_instances_per_host",
+        default=50,
+        help="Ignore hosts that have too many instances")
+
+FLAGS = flags.FLAGS
+FLAGS.register_opt(max_instances_per_host_opt)
+
+
+class NumInstancesFilter(filters.BaseHostFilter):
+    """Filter out hosts with too many instances"""
+
+    def host_passes(self, host_state, filter_properties):
+        num_instances = host_state.num_instances
+        max_instances = FLAGS.max_instances_per_host
+        passes = num_instances < max_instances
+        if not passes:
+            LOG.debug(_("%(host_state)s fails num_instances check: Max "
+                        "instances per host is set to %(max_instances)s"),
+                        locals())
+        return passes

nova/tests/scheduler/test_host_filters.py

@@ -1292,5 +1292,19 @@ class HostFiltersTestCase(test.TestCase):
         filt_cls = self.class_map['IoOpsFilter']()
         host = fakes.FakeHostState('host1', 'compute',
                                    {'num_io_ops': 8})
+
+    def test_filter_num_instances_passes(self):
+        self.flags(max_instances_per_host=5)
+        filt_cls = self.class_map['NumInstancesFilter']()
+        host = fakes.FakeHostState('host1', 'compute',
+                                   {'num_instances': 4})
+        filter_properties = {}
+        self.assertTrue(filt_cls.host_passes(host, filter_properties))
+
+    def test_filter_num_instances_fails(self):
+        self.flags(max_instances_per_host=5)
+        filt_cls = self.class_map['NumInstancesFilter']()
+        host = fakes.FakeHostState('host1', 'compute',
+                                   {'num_instances': 5})
         filter_properties = {}
         self.assertFalse(filt_cls.host_passes(host, filter_properties))