BDM: Avoiding saving if there were no changes

Since objects currently only check if the attribute was set not that the
actual value was changed (and it's not a straightforward task in the
general case), we can still make this better in the DriverBlockDevice
classes and avoid unnecessary DB traffic while being able to make our
saves on as granular level as possible.

This patch makes the code check for changes before actually setting them
on the underlying object.

Change-Id: Ib00ab1a275baabf7d2ea7ad988183b4727905a70

1 files changed, 7 insertions, 3 deletions

diff --git a/nova/virt/block_device.py b/nova/virt/block_device.py
index 78b1bd09bf..6146466cdd 100644
--- a/nova/virt/block_device.py
+++ b/nova/virt/block_device.py
@@ -133,7 +133,9 @@ class DriverBlockDevice(dict):
 
     def save(self):
         for attr_name, key_name in self._update_on_save.iteritems():
-            setattr(self._bdm_obj, attr_name, self[key_name or attr_name])
+            lookup_name = key_name or attr_name
+            if self[lookup_name] != getattr(self._bdm_obj, attr_name):
+                setattr(self._bdm_obj, attr_name, self[lookup_name])
         self._bdm_obj.save()
 
 
@@ -285,8 +287,10 @@ class DriverVolumeBlockDevice(DriverBlockDevice):
         # NOTE(ndipanov): we might want to generalize this by adding it to the
         # _update_on_save and adding a transformation function.
         try:
-            self._bdm_obj.connection_info = jsonutils.dumps(
-                    self.get('connection_info'))
+            connection_info_string = jsonutils.dumps(
+                self.get('connection_info'))
+            if connection_info_string != self._bdm_obj.connection_info:
+                self._bdm_obj.connection_info = connection_info_string
         except TypeError:
             pass
         super(DriverVolumeBlockDevice, self).save()