Use IaaS-native disk resize only when growing persistent disk

Author: bgandon

src/bosh-director/lib/bosh/director/deployment_plan/persistent_disk_collection.rb

@@ -144,6 +144,15 @@ def size_diff_only?(other)
           cloud_properties == other.cloud_properties &&
             size != other.size && name == other.name
         end
+
+        def is_bigger_than?(other)
+          unless other.is_a? PersistentDisk
+            raise Exception, 'Cannot compare persistent disk size to anything
+                              that is not a persistent disk.'
+          end
+
+          size >= other.size
+        end
       end
 
       class NewPersistentDisk < PersistentDisk

src/bosh-director/lib/bosh/director/disk_manager.rb

@@ -32,7 +32,7 @@ def update_persistent_disk(instance_plan)
 
         @logger.info("CPI resize disk enabled: #{Config.enable_cpi_resize_disk}")
 
-        if use_cpi_resize_disk?(old_disk, new_disk)
+        if use_iaas_native_disk_resize?(old_disk, new_disk)
           resize_disk(instance_plan, new_disk, old_disk)
         else
           update_disk(instance_plan, new_disk, old_disk)
@@ -84,8 +84,12 @@ def step_report
       DeploymentPlan::Stages::Report.new
     end
 
-    def use_cpi_resize_disk?(old_disk, new_disk)
-      Config.enable_cpi_resize_disk && new_disk && new_disk.size_diff_only?(old_disk) && new_disk.managed?
+    def use_iaas_native_disk_resize?(old_disk, new_disk)
+      Config.enable_cpi_resize_disk &&
+        new_disk &&
+        new_disk.managed? &&
+        new_disk.size_diff_only?(old_disk) &&
+        new_disk.is_bigger_than?(old_disk)
     end
 
     def add_event(action, deployment_name, instance_name, object_name = nil, parent_id = nil, error = nil)