Add configuration option to enable/disable migration at checkout

This commit adds support for a configuration setting called
migration_limit that makes migration at checkout optional. Additionally,
logging is added to report a VM parent host when it is checked out. Without this
change vmpooler assumes that migration at checkout is always enabled.
If this setting is not present, or if the setting is 0, then migration
at checkout will be disabled. If the setting is greater than 0 then that
setting will be used to enforce a limit for the number of simultaneous
migrations that will be evaluated.

Documentation of this configuration option is added to the
vmpooler.yaml.example file.

lib/vmpooler/pool_manager.rb

@@ -468,20 +468,36 @@ def migrate_vm(vm, pool)
     def _migrate_vm(vm, pool)
       $redis.srem('vmpooler__migrating__' + pool, vm)
       vm_object = find_vsphere_pool_vm(pool, vm)
-      host = $vsphere[pool].find_least_used_compatible_host(vm_object)
       parent_host = vm_object.summary.runtime.host
       parent_host_name = parent_host.name
-      if host == parent_host
-        $logger.log('s', "[ ] [#{pool}] No migration required for '#{vm}'")
+      # Check if migration_limit is set
+      migration_limit = $config[:config]['migration_limit']
+      # Ensure migration is disabled when migration_limit is set to 0
+      migration_limit = nil unless migration_limit != 0
+
+      if not migration_limit
+        $logger.log('s', "[ ] [#{pool}] '#{vm}' is running on #{parent_host_name}")
       else
-        start = Time.now
-        $vsphere[pool].migrate_vm_host(vm_object, host)
-        finish = '%.2f' % (Time.now - start)
-        $metrics.timing("migrate.#{vm['template']}", finish)
-        checkout_to_migration = '%.2f' % (Time.now - Time.parse($redis.hget('vmpooler__vm__' + vm, 'checkout')))
-        $redis.hset('vmpooler__vm__' + vm, 'migration_time', finish)
-        $redis.hset('vmpooler__vm__' + vm, 'checkout_to_migration', checkout_to_migration)
-        $logger.log('s', "[>] [#{pool}] '#{vm}' migrated from #{parent_host_name} to #{host.name} in #{finish} seconds")
+        migration_count = $redis.smembers('vmpooler__migration').size
+        if migration_count >= migration_limit
+          $logger.log('s', "[ ] [#{pool}] '#{vm}' is running on #{parent_host_name}. No migration will be evaluated since the migration_limit has been reached")
+        else
+          $redis.sadd('vmpooler__migration', vm)
+          host = $vsphere[pool].find_least_used_compatible_host(vm_object)
+          if host == parent_host
+            $logger.log('s', "[ ] [#{pool}] No migration required for '#{vm}'")
+          else
+            start = Time.now
+            $vsphere[pool].migrate_vm_host(vm_object, host)
+            finish = '%.2f' % (Time.now - start)
+            $metrics.timing("migrate.#{vm['template']}", finish)
+            checkout_to_migration = '%.2f' % (Time.now - Time.parse($redis.hget('vmpooler__vm__' + vm, 'checkout')))
+            $redis.hset('vmpooler__vm__' + vm, 'migration_time', finish)
+            $redis.hset('vmpooler__vm__' + vm, 'checkout_to_migration', checkout_to_migration)
+            $logger.log('s', "[>] [#{pool}] '#{vm}' migrated from #{parent_host_name} to #{host.name} in #{finish} seconds")
+          end
+          $redis.srem('vmpooler__migration', vm)
+        end
       end
     end
 

vmpooler.yaml.example

@@ -225,6 +225,13 @@
 #     If set, prefixes all created VMs with this string.  This should include
 #     a separator.
 #     (optional; default: '')
+#
+#   - migration_limit
+#     When set to any value greater than 0 enable VM migration at checkout.
+#     When enabled this capability will evaluate a VM for migration when it is requested
+#     in an effort to maintain a more even distribution of load across compute resources.
+#     The migration_limit ensures that no more than n migrations will be evaluated at any one time
+#     and greatly reduces the possibilty of VMs ending up bunched together on a particular host.
 
 # Example: