diff --git a/nova/tests/unit/virt/libvirt/test_driver.py b/nova/tests/unit/virt/libvirt/test_driver.py
index bfbe32d04923..60c16c3d1177 100644
--- a/nova/tests/unit/virt/libvirt/test_driver.py
+++ b/nova/tests/unit/virt/libvirt/test_driver.py
@@ -13765,6 +13765,36 @@ class LibvirtConnTestCase(test.NoDBTestCase):
         self._test_get_guest_config_parallels_volume(vm_mode.EXE, 4)
         self._test_get_guest_config_parallels_volume(vm_mode.HVM, 6)
 
+    def test_get_guest_disk_config_rbd_older_config_drive_fall_back(self):
+        # New config drives are stored in rbd but existing instances have
+        # config drives in the old location under the instances path.
+        # Test that the driver falls back to 'flat' for config drive if it
+        # doesn't exist in rbd.
+        self.flags(images_type='rbd', group='libvirt')
+        drvr = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), False)
+        drvr.image_backend = mock.Mock()
+        mock_rbd_image = mock.Mock()
+        mock_flat_image = mock.Mock()
+        mock_flat_image.libvirt_info.return_value = mock.sentinel.diskconfig
+        drvr.image_backend.image.side_effect = [mock_rbd_image,
+                                                mock_flat_image]
+        mock_rbd_image.exists.return_value = False
+        instance = objects.Instance()
+        disk_mapping = {'disk.config': {'bus': 'ide',
+                                        'dev': 'hdd',
+                                        'type': 'file'}}
+        flavor = objects.Flavor(extra_specs={})
+
+        diskconfig = drvr._get_guest_disk_config(
+            instance, 'disk.config', disk_mapping, flavor,
+            drvr._get_disk_config_image_type())
+
+        self.assertEqual(2, drvr.image_backend.image.call_count)
+        call1 = mock.call(instance, 'disk.config', 'rbd')
+        call2 = mock.call(instance, 'disk.config', 'flat')
+        drvr.image_backend.image.assert_has_calls([call1, call2])
+        self.assertEqual(mock.sentinel.diskconfig, diskconfig)
+
     def _test_prepare_domain_for_snapshot(self, live_snapshot, state):
         drvr = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), False)
         instance_ref = objects.Instance(**self.test_instance)
diff --git a/nova/virt/libvirt/driver.py b/nova/virt/libvirt/driver.py
index fae159b325bf..e82206c4b0ba 100644
--- a/nova/virt/libvirt/driver.py
+++ b/nova/virt/libvirt/driver.py
@@ -3327,6 +3327,15 @@ class LibvirtDriver(driver.ComputeDriver):
         image = self.image_backend.image(instance,
                                          name,
                                          image_type)
+        if (name == 'disk.config' and image_type == 'rbd' and
+                not image.exists()):
+            # This is likely an older config drive that has not been migrated
+            # to rbd yet. Try to fall back on 'flat' image type.
+            # TODO(melwitt): Add online migration of some sort so we can
+            # remove this fall back once we know all config drives are in rbd.
+            image = self.image_backend.image(instance, name, 'flat')
+            LOG.debug('Config drive not found in RBD, falling back to the '
+                      'instance directory', instance=instance)
         disk_info = disk_mapping[name]
         return image.libvirt_info(disk_info['bus'],
                                   disk_info['dev'],