Volume snapshot improvements

This spec covers a few updates to the volume snapshot
create and delete operations in the libvirt volume driver.
These are needed to fix up some issues in Nova/Cinder
related to volume file format handling and API cleanup.

Spec for blueprint volume-snapshot-improvements

Change-Id: If9ab6976d7bd2cfbfeca7e2a731c50233a77beee

specs/liberty/approved/volume-snapshot-improvements.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
+
+==========================================
+Volume snapshot improvements
+==========================================
+
+https://blueprints.launchpad.net/nova/+spec/volume-snapshot-improvements
+
+This spec covers a few updates to the volume snapshot create and delete
+operations in the libvirt volume driver.  These are needed to fix up
+some issues in Nova/Cinder related to volume file format handling and
+API cleanup.
+
+
+Problem description
+===================
+
+Nova does not currently pass enough information back to Cinder when
+manipulating snapshot files.  Cinder needs to keep track of the format
+of each file (raw, qcow2, vhd, etc.) to maintain security and data
+integrity.  Currently it has to guess about the outcome of a snapshot
+operation for this information.
+
+Nova currently sends a hard-coded '90%' progress value to indicate a
+state transition at the end of a snapshot operation.  This should be
+replaced with something more generic that does not overload the
+progress field.
+
+
+Use Cases
+----------
+
+Deployer: Currently if a user writes a qcow2 header into a volume on
+certain Cinder volume drives, the volume may be marked as unusable.  This
+work will fix things so that Cinder can avoid having to invalidate
+volumes in this scenario.
+
+Deployer: Increased (theoretical) security since Cinder doesn't have
+to use heuristics for the above check.
+
+Developer: API between Cinder and Nova becomes more clear (no magic
+progress value)
+
+
+Project Priority
+-----------------
+
+None. This is primarily a maintainablity/reliability issue.
+
+Proposed change
+===============
+
+File format tracking
+--------------------
+
+Each time a volume snapshot create or delete operation occurs,
+add file format information about the changed files to the status
+update sent back to Cinder.  (This is currently only used in the
+libvirt volume driver for file-based volume drivers but nothing
+prevents it from being more general.)
+
+For the libvirt volume driver, this information can be obtained by
+querying the instance VM's disk backing chain information via
+the domain.XMLDesc() output.
+
+For snapshot_create, determine the format of the new file, return
+this from _volume_snapshot_create() and add a dict such as::
+'file_format': { 'volume-1234.snapshot-abcd': 'qcow2' }
+to the _volume_update_snapshot_status() call.
+
+For snapshot_delete, determine the format of merge_target_file,
+or if that is None, file_to_merge, after the snapshot delete,
+and add that information to the _volume_update_snapshot_status()
+call::
+'file_format': { 'volume-1234': 'qcow2' }
+
+There may be some cases with old versions of libvirt where this
+information isn't explicitly given in the domain information.  We
+can make assumptions in these cases for what format to return based
+on knowing that performing a blockCommit results in the format of
+the file being committed to, and a blockRebase results in the format
+of the file being pulled to.
+
+Progress Updating
+-----------------
+For Liberty, continue to send the 'progress': '90%' flag in
+update_snapshot_status for compatibility.  (Can be removed in the future.)
+
+Send a new status of 'creating_compute_complete' to indicate that
+the compute service is done with its portion of the create process.
+Cinder will translate this to a relevant volume state transition
+on its side.
+
+Same as above for deleting, with 'delete_compute_complete'.
+
+This allows Cinder to distinguish whether Nova is currently processing
+information or whether Cinder has control of that snapshot again.
+
+Alternatives
+------------
+
+Leave things as they are (not really desirable).
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+There was a security issue in the Juno timeframe in this area which
+was patched up enough to make it safe.  This completes that effort
+by making the system fully robust rather than just patched up.
+[ref OSSA 2014-033]
+
+This will bring Nova and Cinder to always track and use knowledge of
+the file format of each volume/snapshot file.
+
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  deepakcs
+
+Other contributors:
+  None
+
+Work Items
+----------
+
+* Add new file format querying and reporting to libvirt snapshot code
+* Add new statuses to libvirt snapshot create/delete operations
+* Test with Cinder (where most of this change really has an effect)
+
+
+Dependencies
+============
+
+* Cinder changes (format): https://review.openstack.org/#/c/165393/
+* Cinder changes (status): https://review.openstack.org/#/c/172373/
+
+Testing
+=======
+
+This change most directly impacts the Cinder GlusterFS, NFS, and SMBFS
+drivers for Liberty.  These will have CI running tempest for Liberty, which
+will validate this work.
+
+
+
+Documentation Impact
+====================
+
+None
+
+
+References
+==========
+
+* OSSA 2014-033 https://bugs.launchpad.net/cinder/+bug/1350504
+* Cinder changes (format): https://review.openstack.org/#/c/165393/
+* Cinder changes (status): https://review.openstack.org/#/c/172373/