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Openstack: Cinder scheduler uses stale statistics in Active-Active deployments

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openstack
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virt
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Applies to

  • Openstack (all releases)

Issue

Cinder scheduler does not make use of a reliable "single source of truth" such as an etcd database.  As such:

  • backend statistics that include total_capacity_gb, provisioned_capacity_gb, max_over_subscription_ratio all reside in memory on each Cinder scheduler.  
  • Volume nodes will periodically send backend statistic updates to the Cinder scheduler nodes
  • However a race condition is possible where:
    • Cinder volumes are created within a few seconds of each other
    • Cinder scheduler statistics are not updated before a new volume create request is received
    • This allows cinder volumes to be created on backing storage pools that should have been filtered by Cinder scheduler (provided that Cinder scheduler had up to data statistics)
      • This could include scenarios where max_over_subscription_ratio is ignored, allowing a backing storage pool to be over provisioned beyond the allowed limit set by max_over_subscription_ratio
      • This could also include a scenario where a nearly full storage pool is selected over a pool with more available space.  

This issue impacts both the generic and NetApp cinder drivers.

 

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