Stock zones follows a fill-first methodology—the current zone is filled with instances before other zones are considered. This adds a flag to nova to select a spread-first methodology. The implementation is simply adding a random.shuffle() prior to sorting the list of potential compute hosts by weights.

nova/scheduler/abstract_scheduler.py

@@ -20,8 +20,8 @@ customize the behavior: filter_hosts() and weigh_hosts(). The default
 behavior is to simply select all hosts and weight them the same.
 """
 
-import operator
 import json
+import operator
 
 import M2Crypto
 

nova/scheduler/base_scheduler.py

@@ -27,6 +27,8 @@ from nova.scheduler import abstract_scheduler
 from nova.scheduler import host_filter
 
 FLAGS = flags.FLAGS
+flags.DEFINE_boolean('spread_first', False,
+                     'Use a spread-first zone scheduler strategy')
 LOG = logging.getLogger('nova.scheduler.base_scheduler')
 
 
@@ -68,4 +70,9 @@ class BaseScheduler(abstract_scheduler.AbstractScheduler):
         if num_instances > 0:
             instances.extend(hosts[:num_instances])
 
+        # Adjust the weights for a spread-first strategy
+        if FLAGS.spread_first:
+            for i, host in enumerate(hosts):
+                host['weight'] = i + 1
+
         return instances