Add CrossCellWeigher

When performing a resize, we'll want to (by default) select
target hosts from the source cell to do a traditional resize
if possible before considering target hosts in another cell
which will be slower and more complicated. If the source cell
is disabled or target flavor is not available in the source cell,
then we'll have no choice but to select a host from another cell.
But all things being equal between hosts, we want to stay within
the source cell (by default). Therefore this change adds a new
CrossCellWeigher and related configuration option to prefer hosts
within the source cell when moving a server. The weigher is
completely noop unless a cross-cell move is permitted by
configuration, which will be provided in a future change.

Part of blueprint cross-cell-resize

Change-Id: Ib18752efa56cfeb860487fe6b26102bb4b1db038

1 files changed, 63 insertions, 0 deletions

diff --git a/nova/scheduler/weights/cross_cell.py b/nova/scheduler/weights/cross_cell.py
new file mode 100644
index 0000000000..5e7593f398
--- /dev/null
+++ b/nova/scheduler/weights/cross_cell.py
@@ -0,0 +1,63 @@
+#
+#    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.
+"""
+Cross-cell move weigher. Weighs hosts based on which cell they are in. "Local"
+cells are preferred when moving an instance. In other words, select a host
+from the source cell all other things being equal.
+"""
+
+from nova import conf
+from nova.scheduler import utils
+from nova.scheduler import weights
+
+CONF = conf.CONF
+
+
+class CrossCellWeigher(weights.BaseHostWeigher):
+
+    def weight_multiplier(self, host_state):
+        """How weighted this weigher should be."""
+        return utils.get_weight_multiplier(
+            host_state, 'cross_cell_move_weight_multiplier',
+            CONF.filter_scheduler.cross_cell_move_weight_multiplier)
+
+    def _weigh_object(self, host_state, weight_properties):
+        """Higher weights win. Hosts within the "preferred" cell are weighed
+        higher than hosts in other cells.
+
+        :param host_state: nova.scheduler.host_manager.HostState object
+            representing a ComputeNode in a cell
+        :param weight_properties: nova.objects.RequestSpec - this is inspected
+            to see if there is a preferred cell via the requested_destination
+            field and if so, is the request spec allowing cross-cell move
+        :returns: 1 if cross-cell move and host_state is within the preferred
+            cell, -1 if cross-cell move and host_state is *not* within the
+            preferred cell, 0 for all other cases
+        """
+        # RequestSpec.requested_destination.cell should only be set for
+        # move operations. The allow_cross_cell_move value will only be True if
+        # policy allows.
+        if ('requested_destination' in weight_properties and
+                weight_properties.requested_destination and
+                'cell' in weight_properties.requested_destination and
+                weight_properties.requested_destination.cell and
+                weight_properties.requested_destination.allow_cross_cell_move):
+            # Determine if the given host is in the "preferred" cell from
+            # the request spec. If it is, weigh it higher.
+            if (host_state.cell_uuid ==
+                    weight_properties.requested_destination.cell.uuid):
+                return 1
+            # The host is in another cell, so weigh it lower.
+            return -1
+        # We don't know or don't care what cell we're going to be in, so noop.
+        return 0