Allow tweaking max_over_subscription_ratio

Previously there was no way to configure max_over_subscription_ratio and
users had to use 20.0 as the default value in Cinder.

Closes-Bug: #2127021
Change-Id: I5182a0fc63a69dd1e5656739ecfd2b1d2dbfeaf3
Signed-off-by: Nobuto Murata <nobuto.murata@canonical.com>

src/config.yaml

@@ -3,6 +3,15 @@
     "type": "string"
     "default": "cinder_netapp"
     "description": "Service name to present to Cinder"
+  "max-over-subscription-ratio":
+    "type": "string"
+    "default": !!null ""
+    "description": |
+      Default ratio is 20.0, meaning provisioned capacity can be 20
+      times of the total physical capacity. This parameter also can be
+      set as "auto". When using auto, Cinder will automatically
+      calculate the max_over_subscription_ratio based on the provisioned
+      capacity and the used space.
   "netapp-storage-family":
     "type": "string"
     "default": "ontap_cluster"

src/lib/charm/openstack/cinder_netapp.py

@@ -79,6 +79,14 @@ class CinderNetAppCharm(
             driver_options_extension += [
                 ('netapp_lun_space_reservation', lun_space_reservation)]
 
+        if cget("max-over-subscription-ratio"):
+            driver_options_common += [
+                (
+                    "max_over_subscription_ratio",
+                    cget("max-over-subscription-ratio"),
+                )
+            ]
+
         return (driver_options_common + driver_transport +
                 driver_options_extension)