Merge "Add ability to support discard/unmap/trim for Cinder backend"
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specs/mitaka/approved/cinder-backend-report-discard.rst
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specs/mitaka/approved/cinder-backend-report-discard.rst
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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============================================================
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Add ability to support discard/unmap/trim for Cinder backend
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============================================================
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https://blueprints.launchpad.net/nova/+spec/cinder-backend-report-discard
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Currently, libvirt/qemu has support for a discard option when attaching a
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volume to an instance. With this feature, the unmap/trim command can be sent
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from guest to the physical storage device.
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A cinder back-end will report a connection capability that Nova will use
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in attaching a volume.
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Problem description
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===================
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Currently there is no way for Nova to know if a Cinder back end supports
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discard/trim/unmap functionality. Functionality is being added in Cinder
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to supply this information. The spec seeks to add the ability to consume
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that information.
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Use Cases
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---------
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If a Cinder backend uses media that can make use of discard functionality
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there should be a way to do this. This will improve long term performance
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of such back ends.
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Proposed change
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===============
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Code will be added to check for a 'discard' property returned to Nova from
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the Cinder attach API. When present and set to True we will modify the config
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returned by the libvirt volume driver to contain::
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driver_discard = "unmap"
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This will only give the desired support if the instance is configured with a
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interface and bus type that will support Trim/Unmap commands. In the case where
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it is possible to detect that discard will not actually work for the instance
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we will log a warning, but continue on with the attach anyway.
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Currently the virtio-blk backend does not support discard.
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There will be several ways to get an instance that will support discard, one
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example is to use the virtio-scsi storage interface with a scsi bus type. To
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create an instance with this support it must be booted from an image
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configured with ``hw_scsi_model=virtio-scsi`` and ``hw_disk_bus=scsi``.
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It is important to note that the nova.conf option hw_disk_discard is NOT read
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for this feature. We rely entirely on Cinder to specify whether or not discard
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should be used for the volume.
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Alternatives
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------------
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Alternatives include adding discard for all drives if the operator has set
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hw_disk_discard but it was decided this was not a good way to solve the
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problem as you could not mix different underlying volume providers easily.
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We could also hot-plug a SCSI controller that is capable of supporting discard
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when attaching Cinder volumes. This would allow for mixing a non-trim boot
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disk from an image and then attaching a Cinder volume that would get the
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benefits. The risk is that the instance may not be able to actually support
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doing UNMAP.
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Data model impact
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-----------------
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None
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REST API impact
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---------------
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None
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Security impact
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---------------
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None
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Notifications impact
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--------------------
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None
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Other end user impact
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---------------------
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None
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Performance Impact
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------------------
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There will be a performance gain for back ends that benefit from having
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discard functionality.
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See https://en.wikipedia.org/wiki/Trim_(computing) for more info.
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Other deployer impact
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---------------------
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Deployers wanting to use this feature with their Cinder backend will need to
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ensure the instances are configured with a SCSI model and bus that support
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discard. This includes IDE, AHCI, and Xen disks. virtio-blk is the only
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backend missing this support.
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A simple way to enable this is to modify Glance images to contain the
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following properties::
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hw_scsi_model=virtio-scsi
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hw_disk_bus=scsi
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In addition compute nodes will need to be using libvirt 1.0.6 or higher and
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QEMU 1.6.0 or higher.
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Developer impact
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----------------
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None
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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* Patrick East
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Work Items
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----------
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* Modify volume attach code in libvirt driver to check for the new Cinder
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connection property.
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* Add unit tests for new functionality, modify any existing as needed.
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* Configure Pure Storage 3rd party CI system to enable the feature and
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validate it as a Cinder CI. This configuration change will be made available
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to any other 3rd party CI maintainer to allow additional systems to test with
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this feature enabled.
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Dependencies
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============
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Cinder Blueprint (Completed and released in Liberty):
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https://blueprints.launchpad.net/cinder/+spec/cinder-backend-report-discard
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Testing
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=======
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Unit tests needs to include all permutations of the discard
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flag from Cinder.
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We could enable one of the jenkins jobs to be configured to enable this. A nice
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starting point would maybe be the Ceph jobs. Potentially a Tempest test could
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be added behind a config option to validate volume attachments do get the
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correct discard settings.
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Documentation Impact
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====================
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We may want to add documentation to the Cloud Administrator Guide on how to
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utilize this feature.
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References
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==========
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Cinder Blueprint:
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https://blueprints.launchpad.net/cinder/+spec/cinder-backend-report-discard
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Cinder Spec:
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http://specs.openstack.org/openstack/cinder-specs/specs/liberty/cinder-backend-report-discard.html
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History
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=======
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.. list-table:: Revisions
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:header-rows: 1
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* - Release Name
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- Description
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* - Mitaka
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- Introduced
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