All About Cinder Drivers

General Considerations

Cinder allows you to integrate various storage solutions into your OpenStack cloud. It does this by providing a stable interface for hardware providers to write drivers that allow you to take advantage of the various features that their solutions offer.

“Supported” drivers

In order to make it easier for you to assess the stability and quality of a particular vendor’s driver, The Cinder team has introduced the concept of a supported driver. These are drivers that:

  • have an identifiable driver maintainer

  • are included in the Cinder source code repository

  • use the upstream Cinder bug tracking mechanism

  • support the Cinder Required Driver Functions

  • maintain a third-party Continuous Integration system that runs the OpenStack Tempest test suite against their storage devices

    • this must be done for every Cinder commit, and the results must be reported to the OpenStack Gerrit code review interface

    • for details, see Driver Testing

In summary, there are two important aspects to a driver being considered as supported:

  • the code meets the Cinder driver specifications (so you know it should integrate properly with Cinder)

  • the driver code is continually tested against changes to Cinder (so you know that the code actually does integrate properly with Cinder)

The second point is particularly important because changes to Cinder can impact the drivers in two ways:

  • A Cinder change may introduce a bug that only affects a particular driver or drivers (this could be because many drivers implement functionality well beyond the Required Driver Functions). With a properly running and reporting third-party CI system, such a bug can be detected at the code review stage.

  • A Cinder change may exercise a new code path that exposes a driver bug that had previously gone undetected. A properly running third-party CI system will detect this and alert the driver maintainer that there is a problem.

Driver Compliance

The current policy for CI compliance is:

  • CIs must report on every patch, whether the code change is in their own driver code or not

  • The CI comments must be properly formatted to show up in the CI summary in Gerrit

Non-compliant drivers will be tagged as unsupported if:

  • No CI success reporting occurs within a two week span

  • The CI is found to not be testing the expected driver (CI runs using the default LVM driver, etc.)

  • Other issues are found but failed to be addressed in a timely manner

CI results are reviewed on a regular basis and if found non-compliant, a driver patch is submitted flagging it as ‘unsupported’. This can occur at any time during the development cycle. A driver can be returned to ‘supported’ status as soon as the CI problem is corrected.

We do a final compliance check around the third milestone of each release. If a driver is marked as ‘unsupported’, vendors have until the time of the first Release Candidate tag (two weeks after the third milestone) to become compliant, in which case the patch flagging the driver as ‘unsupported’ can be reverted. Otherwise, the driver will be considered ‘unsupported’ in the release.

The CI results are currently posted here:

“Unsupported” drivers

A driver is marked as ‘unsupported’ when it is out of compliance.

Such a driver will log a warning message to be logged in the cinder-volume log stating that it is unsupported and deprecated for removal.

In order to use an unsupported driver, an operator must set the configuration option enable_unsupported_driver=True in the driver’s configuration section of cinder.conf or the Cinder service will fail to load.

If the issue is not corrected before the next release, the driver will be removed from the Cinder code repository per the standard OpenStack deprecation policy.

Current Cinder Drivers

The Cinder team maintains a page of the current drivers and what exactly they support in the Driver Support Matrix.

You may find more details about the current drivers on the Available Drivers page.

Additionally, the configuration reference for each driver provides even more information. See Volume drivers.