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185 lines
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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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Assisted Snapshot Improvements
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==========================================
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https://blueprints.launchpad.net/cinder/+spec/assisted-snapshot-improvements
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This spec aims to improve the infrastructure used to coordinate
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between Cinder and Nova for volume snapshots.
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Problem description
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===================
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The update_snapshot_status API is used to update fields in the Cinder
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database which the driver performing a snapshot delete uses to determine
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when Nova's part is finished and the Cinder driver can take over again.
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This currently overloads the 'progress' field of the snapshot to
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carry information from the API layer to the driver. This is hacky and
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should be reworked as more drivers are interested in supporting
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assisted snapshots.
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This work will also assist in supporting proper transitions between
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different phases of the snapshot create/delete process.
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Use Cases
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=========
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NA.
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There is no user/deployer impact as this is a internal only change.
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Proposed change
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===============
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API<->Volume service interaction
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--------------------------------
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Establish a snapshot_admin_metadata table which is similar to the
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volume_admin_metadata table::
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id : Integer, primary key,
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key : string,
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value : string,
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volume_id : id ref volumes.id,
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volume : relationship joining to volumes table
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Setting a snapshot_status metadata value will allow the API layer
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to transfer this information to the volume service.
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Volume manager<->driver interaction
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-----------------------------------
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Split the assisted snapshot processing code out of the driver
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(currently RemoteFS-based) and into the volume manager.
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This will allow drivers that support assisted snapshots to have a
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call for each stage, rather than only a single delete_snapshot call.
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The driver will have a property indicating it supports assisted
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snapshots, which triggers use of the following methods in the manager
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instead of the manager calling the driver's create_snapshot::
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create_snapshot_assisted_begin(snapshot_ref)
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create_snapshot_assisted_complete(snapshot_ref)
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delete_snapshot_assisted_begin(snapshot_ref)
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delete_snapshot_assisted_complete(snapshot_ref)
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Each of these calls can return a dict of fields to be updated for the
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snapshot and/or snapshot_admin_metadata by the volume manager.
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This will move some database accesses currently done in the driver
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to the volume manager, as well as better allowing support for defined
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state transitions between the Nova and Cinder phases.
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Alternatives
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------------
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Leave things as they are today, which may lead to a less robust/maintainable
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infrastructure for assisted snapshots.
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Data model impact
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-----------------
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Create new snapshot_admin_metadata table as described above.
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REST API impact
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---------------
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update_snapshot_status() API will look for a new 'compute_snapshot_status'
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field which will be used to populate the snapshot_admin_metadata entry.
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If this field is not present, it will use the 'progress' field as before
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for compatibility.
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Security impact
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---------------
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None, interactions in and out of Cinder expose the same information
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and level of access as today.
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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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None
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Other deployer impact
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---------------------
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Cinder will remain backward compatible with Liberty Nova for the APIs
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being modified here.
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Developer impact
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----------------
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New interfaces for drivers for creating and deleting snapshots assisted
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by the compute service.
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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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deepakcs
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Other contributors:
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None
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Work Items
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----------
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* Add processing to update_snapshot_status API for new fields
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* Add new fields to the update_snapshot_status call made by Nova
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* Add processing in snapshot-tracking code for the new compute_progress field
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* Create new driver interfaces in the volume manager
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* Migrate RemoteFS snapshot infrastructure to the new interfaces for the
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RemoteFSSnapDriver class.
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Dependencies
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============
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* Have Nova send new fields for update_snapshot_status API calls
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https://review.openstack.org/#/c/134517/
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Testing
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=======
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This will be covered by CI for GlusterFS, the NFS driver (once snapshots
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are added to it in Mitaka), and CI for other RemoteFS drivers.
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Documentation Impact
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====================
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None
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References
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==========
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* Nova change: https://review.openstack.org/#/c/134517/
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