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

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

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 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


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


Notifications impact


Other end user impact


Performance Impact

There will be a performance gain for back ends that benefit from having discard functionality.

See for more info.

Other deployer impact

Deployers will need to add some properties to Glance images to make this work. Once an instance is booted from the modified image the discard capability reported by the driver will be activated.

hw_scsi_model=virtio-scsi hw_disk_bus=scsi

glance image-update –property hw_scsi_model=virtio-scsi

–property hw_disk_bus=scsi <image-uuid>

This causes the correct controller to be created on the instance via code changes here Once the correct controller is in place the unmap functionality specified by the Cinder backend will be utilized.

A secondary volume attachment will be treated separately and will honor the discard settings provided at connection time.

Developer impact




Primary assignee:

  • Daniel Wilson

Work Items

Add the “discard”: True line to the Pure Storage driver.

Implement the funcionality in Nova as well.




Tempest tests will not be needed for this spec. Pure Storage CI will test this code path with existing tempest tests.

Documentation Impact

Documentation on the response to ‘initialize_connection’ will need to be updated with notes explaining that this new info is provided.