Merge "Add ability for Cinder backend to report discard/unmap/trim"

specs/liberty/cinder-backend-report-discard.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+===========================================================
+Add ability for Cinder backend to report discard/unmap/trim
+===========================================================
+
+https://blueprints.launchpad.net/cinder/+spec/cinder-backend-report-discard
+
+Currently, libvirt/qemu has support for a discard option when attaching a
+volume to an instance. With this feature, the unmap/trim command can be sent
+from guest to the physical storage device.
+
+A cinder back-end should report a connection capability that can be used to
+instruct Nova how to attach a volume.
+
+This spec aims to add the assist to support enabling discard for cinder
+volumes.
+
+
+Problem description
+===================
+
+Currently there is no way for Nova to know if a Cinder back end supports
+discard/trim/unmap functionality.  This aims to provide that info to Nova.
+
+Use Cases
+=========
+
+If a Cinder backed uses media that can make use of discard functionality
+there should be a way to do this.  This will improve long term performance
+of such back ends.
+
+Proposed change
+===============
+
+Any driver which wants to support this will just add an entry to the
+properties returned by intialize_connection.
+
+"discard": True,
+
+An example from the Pure Storage driver would be
+
+.. code:: Python
+
+        properties = {
+            "driver_volume_type": "iscsi",
+            "data": {
+                "target_iqn": target_port["iqn"],
+                "target_portal": target_port["portal"],
+                "target_lun": connection["lun"],
+                "target_discovered": True,
+                "access_mode": "rw",
+                "discard": True,
+            },
+        }
+
+Alternatives
+------------
+
+Alternatives include adding a setting to cinder.conf for each backend that
+supports this. This includes a manual step that would be more error prone.
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+Documentation on the response to 'initialize_connection' will need to be
+updated with notes explaining that this new info is provided.
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+There will be a performance gain for back ends that benefit from having
+discard functionality.
+
+See https://en.wikipedia.org/wiki/Trim_(computing) for more info.
+
+Other deployer impact
+---------------------
+
+Deployers will also have to add a setting to nova.conf to make this work as
+well.
+
+[libvirt]
+hw_disk_discard = unmap
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+
+*  Daniel Wilson
+
+Work Items
+----------
+
+Add the "discard": True line to the Pure Storage driver.
+
+Implement the funcionality in Nova as well.
+https://review.openstack.org/#/c/205726/1
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+Tempest tests will not be needed for this spec.
+
+
+Documentation Impact
+====================
+
+Documentation on the response to 'initialize_connection' will need to be
+updated with notes explaining that this new info is provided.
+
+
+References
+==========
+
+https://en.wikipedia.org/wiki/Trim_(computing)