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
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 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 https://review.openstack.org/#/c/70263/. 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¶
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. 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.
