Inspection Mechanism For Capacity Limited Host

Cinder now has a scheduler filter called CapacityFilter which allow users to create volume under capacity limit. But some APIs which don’t go through cinder-scheduler will break the destination host’s capacity.

Problem description

Some backends now support the thin provisioning capability. But only Cinder gets and checks this capability in cinder-scheduler’s filter: CapacityFilter. It means that some APIs which go to the backend directly without cinder-scheduler doesn’t get and check the reserved_percentage and the max_over_subscription_ratio of the backend. Then it may break the backend’s capacity. Such as “extend volume”, “create volume from snapshot”, “copy volume” and so on.

Use Cases

Usually, the capacity is a hard limit for the specified backend when Cinder works with the CapacityFilter (It is one of the default filters in Cinder). It means that every action to this kind of backend should keep its capacity. We should not break the capability of the backend which support the thin provisioning.

Proposed change

  • Add some common functions to check the backend’s capacity at the cinder-volume layer. The patch[1] is a POC to solve volume creation problem.

  • The APIs which 1) change the backend’s capacity and, 2) don’t go through cinder-scheduler, should be directly checked. It includes:

    • POST /v3/{project_id}/volumes/{volume_id}/action (os-extend) (SOLVED)[2]

    • POST /v3/{project_id}/volumes (create from snapshot, volume or replica)

    • POST /v3/{project_id}/groups/action (create_from_src)

    • POST /v3/{project_id}/snapshots


  • Create volume requests for a snapshot, volume, or replica will change the backend’s capacity, and will therefore cause the volume’s status to switch to “error”.

  • The cinder admin isn’t required to use the CapacityFilter, since the volume capacity limit doesn’t need to be checked at the driver layer.


For the APIs which will change the backend’s capacity and not go through cinder-scheduler, we could route them through the scheduler instead of jumping right to a volume-service.

The problem for the extend API is solved by a bug fix[2] recently. In the patch, it passes a “volume” parameter which includes the host name to cinder-scheduler to indicate that the destination host is specified. Then cinder-scheduler will only check the specified host with backend_passes_filters function. If no error raises, it means that the POST action is allowed.


  • We need to bump the RPC call’s version between cinder-api and cinder-scheduler to keep backward compatibility for each related API.

  • The create actions will always call through the cinder-scheduler, even if the destination host is specified. If the cinder-scheduler can’t match the new RPC version, the old behavior is kept.

Data model impact


REST API impact


Security impact


Notifications impact

Instead of sending notifications, we’d better send user messages if the actions fail.

Other end user impact

The create requests which will exceed the backend’s capacity will make the volume’s status to “error”.

Performance Impact


Other deployer impact


Developer impact




Primary assignee:

wangxiyuan <> Huang Zhiteng <> Erlon R. Cruz <> tommylikehu<>

Work Items

  • Add some common check functions and call them in:

    • create volume from volume, snapshot or replica

    • create cg from source

    • create group from source

    • create snapshot

  • Add and update the unit test.




Unit tests will be added and updated.

Documentation Impact



[1] Huang Zhiteng: [2] Erlon R.Cruz: