b2084855b7
Spec for blueprint volume-snapshot-improvements This couldn't be finished in Liberty, thus moving to Mitaka Previously-approved: liberty Change-Id: I9a8b78f366531a4ee333a586be7030a787f4c90c blueprint: volume-snapshot-improvements
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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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Volume snapshot improvements
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==========================================
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https://blueprints.launchpad.net/nova/+spec/volume-snapshot-improvements
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This spec covers a few updates to the volume snapshot create and delete
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operations in the libvirt volume driver. These are needed to fix up
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some issues in Nova/Cinder related to volume file format handling and
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API cleanup.
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Problem description
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===================
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Nova does not currently pass enough information back to Cinder when
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manipulating snapshot files. Cinder needs to keep track of the format
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of each file (raw, qcow2, vhd, etc.) to maintain security and data
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integrity. Currently it has to guess about the outcome of a snapshot
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operation for this information.
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Nova currently sends a hard-coded '90%' progress value to indicate a
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state transition at the end of a snapshot operation. This should be
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replaced with something more generic that does not overload the
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progress field.
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Use Cases
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----------
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Deployer: Currently if a user writes a qcow2 header into a volume on
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certain Cinder volume drives, the volume may be marked as unusable. This
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work will fix things so that Cinder can avoid having to invalidate
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volumes in this scenario.
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Deployer: Increased (theoretical) security since Cinder doesn't have
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to use heuristics for the above check.
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Developer: API between Cinder and Nova becomes more clear (no magic
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progress value)
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Proposed change
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===============
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File format tracking
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--------------------
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Each time a volume snapshot create or delete operation occurs,
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add file format information about the changed files to the status
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update sent back to Cinder. (This is currently only used in the
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libvirt volume driver for file-based volume drivers but nothing
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prevents it from being more general.)
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For the libvirt volume driver, this information can be obtained by
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querying the instance VM's disk backing chain information via
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the domain.XMLDesc() output.
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For snapshot_create, determine the format of the new file, return
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this from _volume_snapshot_create() and add a dict such as::
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'file_format': { 'volume-1234.snapshot-abcd': 'qcow2' }
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to the _volume_update_snapshot_status() call.
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For snapshot_delete, determine the format of merge_target_file,
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or if that is None, file_to_merge, after the snapshot delete,
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and add that information to the _volume_update_snapshot_status()
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call::
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'file_format': { 'volume-1234': 'qcow2' }
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There may be some cases with old versions of libvirt where this
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information isn't explicitly given in the domain information. We
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can make assumptions in these cases for what format to return based
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on knowing that performing a blockCommit results in the format of
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the file being committed to, and a blockRebase results in the format
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of the file being pulled to.
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Progress Updating
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-----------------
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For Mitaka, continue to send the 'progress': '90%' flag in
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update_snapshot_status for compatibility. (Can be removed in the future.)
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Send a new status of 'creating_compute_complete' to indicate that
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the compute service is done with its portion of the create process.
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Cinder will translate this to a relevant volume state transition
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on its side.
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Same as above for deleting, with 'deleting_compute_complete'.
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This allows Cinder to distinguish whether Nova is currently processing
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information or whether Cinder has control of that snapshot again.
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Alternatives
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------------
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Leave things as they are (not really desirable).
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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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There was a security issue in the Juno timeframe in this area which
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was patched up enough to make it safe. This completes that effort
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by making the system fully robust rather than just patched up.
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[ref OSSA 2014-033]
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This will bring Nova and Cinder to always track and use knowledge of
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the file format of each volume/snapshot file.
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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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None
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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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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 new file format querying and reporting to libvirt snapshot code
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* Add new statuses to libvirt snapshot create/delete operations
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* Test with Cinder (where most of this change really has an effect)
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Dependencies
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============
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* Cinder changes (format): https://review.openstack.org/#/c/165393/
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* Cinder changes (status): https://review.openstack.org/#/c/231463/
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Testing
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=======
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This change most directly impacts the Cinder GlusterFS, NFS, and SMBFS
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drivers for Mitaka. These will have CI running tempest for Mitaka, which
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will validate this work.
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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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* OSSA 2014-033 https://bugs.launchpad.net/cinder/+bug/1350504
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* Cinder changes (format): https://review.openstack.org/#/c/165393/
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* Cinder changes (status): https://review.openstack.org/#/c/231463/
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