Generic Volume Migration

Generic Volume Migration

https://blueprints.launchpad.net/cinder/+spec/generic-volume-migration

The intent is to allow volume drivers that do not support iSCSI and use other means for data transport to also participate in volume migration operations. At the moment, the existing code uses create_export to create and attach a volume via iSCSI to perform I/O operations. By making this more generic, we can allow other drivers to take part in volume migration as well.

This change is necessary to support volume migration for the Ceph driver.

Problem description

When migrating a volume between two backends, the copy_volume_data routine in the source volume’s driver is executed to move the blocks from one volume to another. This routine assumes that both source and destination volumes can be attached locally via iSCSI and calls create_export to attach the volume to the local cinder-volume instance. This is technically not necessary for local volumes and also prevents drivers such as Ceph from participating in volume migration operations.

Use Cases

Proposed change

By using a file-like object abstraction similar to the backup volume driver approach, we can allow the volume driver to determine how best to attach its volume locally. For most drivers, this will continue to be iSCSI. And for Ceph, a RBD object that support file semantics is returned. In both cases, the copy_volume_data routine operates on these file objects as it does now, but without explicit knowledge of the underlying transport mechanism.

Specifically, I would like to add a routine to the volume driver (cinder/volume/driver.py):

::
open_volume() close_volume()

These routines are called when doing local volume attachment as in copy_volume_data(). I think these routines are necessary because local file I/O operations may require different transport than the standard create_export/initialize_connection approach. In the case of Ceph, create_export does nothing and initialize_connection returns the necessary RBD connection info for Nova to communicate with the Ceph volume directly. We want driver authors to retain this flexibility.

The open_volume routine will return a volume file object that supports file operations such as read, write, seek, etc. The close_volume routine will handle teardown and cleanup of the volume file object.

This will allow Ceph to return a file-like object that communicates directly with the volume. Other backends, such as LVM, will continue to create an iSCSI export, attach it, open the resulting block device, and return a handle to this file. Additionally, local LVM volumes could skip the iSCSI step and open the local block device directly. This change could be made later, this effort is to make the minimum amount of changes necessary to support the feature.

With these driver routines in place, copy_volume_data is modified to make use of these calls instead of the using _attach_volume directly. For non-Ceph drivers the result is exactly the same as it was. And the RBD driver can now implement these routines to add support for volume migration.

Alternatives

The most obvious alternative is to change the Ceph driver to implement the export routines and allow volume attachment via iSCSI. This would mean using the rbd kernel driver which is not ideal, as it introduces code into the running kernel on the cinder-volume node and lags a bit in features as compared to librbd. We would like to avoid this if at all possible.

Data model impact

None

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

If everything works correctly, volume migration between Ceph and some other backend should function as expected. In both directions.

Performance Impact

The additional abstraction only enables alternatives to iSCSI attachment, no impact on existing functionality is introduced.

Other deployer impact

None

Developer impact

None

Implementation

Assignee(s)

Primary assignee:
jbernard

Work Items

  1. Introduce the file-object abstraction and change existing code to use iSCSI for volume attachment - just as it does now. Ceph migration will continue to fail.
  2. Implement the Ceph-specific attach logic, that should allow volume migration to succeed between other backends.
  3. Add test(s) to tempest to verify code paths are executed correctly and yield a block-for-block identicaly migration result.

Dependencies

None

Testing

I don’t think volume migration is covered in the gate, but this could be tested in tempest. Ceph volume migration to a non-Ceph backend should be successful in both directions. If that test passes, this effort was a success.

Documentation Impact

I’m don’t think anything is necessary here.

References

None

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