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)

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 Mitaka, 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 ‘deleting_compute_complete’.

This allows Cinder to distinguish whether Nova is currently processing information or whether Cinder has control of that snapshot again.


Leave things as they are (not really desirable).

Data model impact


REST API impact


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


Other end user impact


Performance Impact


Other deployer impact


Developer impact




Primary assignee:


Other contributors:


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)



This change most directly impacts the Cinder GlusterFS, NFS, and SMBFS drivers for Mitaka. These will have CI running tempest for Mitaka, which will validate this work.

Documentation Impact