Driver reinitialization after failure

Driver reinitialization after failure

https://blueprints.launchpad.net/cinder/+spec/driver-initialization-after-fail

This spec proposes support for reintialization of volume drivers after it fails during startup.

Problem description

During Cinder initialization, for many reasons, the storage backend might not be ready and responding. In this case, the driver will not be loaded even if the array becomes available right after.

As there is no retry in Cinder volume service, even later the backend storage is ready, Cinder volume service can’t recover by itself. It needs users to restart the volume service manually.

Use Cases

When a Cinder volume service starts, sometimes its corresponding storage services are not ready. But later the storage services become ready. As a result the volume service can’t work properly and can’t recover by itself. But the administrators probably prefer Cinder to automatically recover from the temporary failures without manual intervention of restarting the service.

Proposed change

The proposal is to

  • Allow reinitialization of a volume driver when it failed to initialize.
  • Provide a configuration to set the maximum retry numbers.
  • The interval of retry will exponentially backoff. Every interval is the exponentiation of retry count. The first interval is 1s, second interval is 2s, third interval is 4s, and so on.
  • Retry will be handled in init_host.

For this, the following additional config option would be needed:

  • ‘reinit_driver_count’ (default: 3)
    Set the maximum times to reintialize the driver if volume initialization fails. Default number is 3.

Alternatives

  • We also can differentiate whether it should retry with an exception. Like import error, config error, it may not retry. But the benefit is not very impressive, and implementing the differentiation needs work in every driver. As drivers don’t differentiate such errors from backend storage errors.

Data model impact

None

REST API impact

None.

Cinder-client impact

None.

Security impact

None.

Notifications impact

None.

Other end user impact

Users don’t need to restart volume service when the initialization of drivers fail on recoverable errors.

Performance Impact

None.

Other deployer impact

None.

Developer impact

None

Implementation

Assignee(s)

Primary assignee:
Lisa Li (xiaoyan.li@intel.com)

Work Items

  • Add the option ‘reinit_driver_count’.
  • Retry to initialize volume drivers when it fails.
  • Add related unit test cases.

Dependencies

None

Testing

  • Add unit tests to cover this change.

Documentation Impact

  • Add administrator documentation to advertise the option of ‘reinit_driver_count’ for driver reinitialization and explain how this should be used.

References

  • None
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