Use extend volume completion action

This blueprint proposes to use the os-extend_volume_completion volume action that has been proposed for Cinder in [3], to provide feedback on success or failure when handling volume-extended external server events.

Problem description

Many remotefs-based volume drivers in Cinder use the qemu-img resize command to extend volume files. However, when the volume is attached to a guest, QEMU will lock the file and qemu-img will be unable to resize it.

In this case, only the QEMU process holding the lock can resize the volume, which can be triggered through the QEMU monitor command block-resize.

There is currently no adequate way for Cinder to use this feature, so the NFS, NetApp NFS, Powerstore NFS, and Quobyte volume drivers all disable extending attached volumes.

Use Cases

As a user, I want to extend a NFS/NetApp NFS/Powerstore NFS/Quobyte volume while it is attached to an instance and I want the volume size and status to reflect the success or failure of the operation.

Proposed change

Nova’s libvirt driver uses the block-resize command when handling the volume-extended external server event, to inform QEMU that the size of an attached volume has changed. It is in principle also capable of extending a volume file, but is currently unable to provide feedback to Cinder on the success of the operation.

Currently, Cinder will send the volume-extended external server event to Nova only after it has finalized the extend operation and reset the volume status from extending back to in-use.

With [3], Cinder will allow volume drivers to hold off finalizing the extend operation and leave the volume status as extending, until after it has send the volume-extended event and received feedback from Nova in form of the os-extend_volume_completion volume action, with an error argument indicating whether to finalize or to roll back the operation.

This will currently affect only the volume drivers mentioned above, all of which did not previously support online extend. All other drivers will continue to send the volume-extended event after finalizing the operation and resetting to in-use status, and will not expect a os-extend_volume_completion volume action.

Compute Agent

Nova’s compute agent will use the volume status to differentiate between the two behaviors when handling volume-extended events:

  • If the volume status is extending, then it will attempt to read extend_new_size from the volume’s metadata and use this value as the new size of the volume, instead of the volume size field.

    After successfully extending the volume, it will call the extend volume completion action of the volume, with "error": false.

    If anything goes wrong, including extend_new_size being missing from the metadata, or being smaller than the current size of the volume, it will log the error and call the os-extend_volume_completion action with "error": true, so Cinder can roll back the operation.

  • For any other volume status, including in-use, the event will be handled as before.


Nova’s API will introduce a new microversion, so that Cinder can make sure the new behavior is available, before leaving an extend operation unfinished.

To handle older compute agents during a rolling upgrade, the API will also check the compute service version of the target agent when receiving a volume-extended event with the new microversion. If a target compute agent is too old to support the feature, the API will discard the event and call the os-extend_volume_completion action with "error": true.


  • A previous change tried to use the volume-extended external server event to support online extend for the NFS driver [1], but did not rely on feedback from Nova to Cinder at all. Instead, it would just set the new size of the volume, change the status back to in-use, notify Nova, and hope for the best.

    If anything went wrong on Nova’s side, this would still result in a volume state indicating that the operation was successful, which is not acceptable.

  • A previous version of this spec proposed a new synchronous API in Nova [2], that would directly call CompVirtAPI.extend_image of the nova-compute instance managing the guest that a volume was attached to. This API would provide a single mechanism to trigger the resize operation, communicate the new size to Nova, and get feedback on the success of the operation.

    The problem with a synchronous API is, that RPC and API timeouts limit the maximum time an extend operation can take. For QEMU, this seemed to be acceptable, because storage preallocation is hard disabled for the block-resize command, and because all currently plausible file systems support sparse file operations.

    However, this may not be true for other volume or virt drivers that might require this API in the future. It would also break with the established pattern of asynchronous coordination between Nova and Cinder, which includes the assisted snapshot and volume migration features.

  • Following this pattern, we could make the proposed API asynchronous and use a new callback in Cinder, similar to Nova’s os-assisted-volume-snapshots API, which uses the os-update_snapshot_status snapshot action to provide feedback to Cinder.

    The function of the new Nova API would then just be to trigger the operation and to communicate the new size. The question is then, whether that warrants adding a new API to Nova, since there are existing mechanisms that could be used for either.

  • The existing mechanism for triggering the extend operation in Nova is of course the volume-extended external server event. Using it for this purpose, as this spec proposes, requires the target size to be transferred separately, because external server events only have a single text field that is freely usable, which for volume-extended is already used for the volume ID.

    Besides storing it in the admin metadata, as [3] and this spec propose, there is also the option of updating the size field of the volume, as [1] was essentially doing.

    This would require the volume size field to be reset on a failure. If an error response from Nova was lost, the volume would just keep the new size. We would need to extend os-reset_status to allow a size reset, or something similar to clean up volumes like this. This would be possible, but updating the size field only after the volume was successfully extended seems like a cleaner solution.

  • We could also extend the external server event API to accept additional data for events, and use this to communicate the new size to Nova.

    This option was judged favorably by reviewers on the previous version of this spec, [2], but it would be a more complex change to the Nova API.

    However, if additional data fields become available in a future version of the external server event API, it would be a relatively minor change to use this instead of volume metadata.

Data model impact


REST API impact

The behavior of the external server event API will change.

  • If Nova receives a volume-extended event, and the referenced volume has status of extending, Nova will look for the extend_new_size key in the volume metadata, and use this instead of the volume size field as the target size to update the block device mapping and to pass to the virt driver’s extend_volume method.

    Nova will also attempt to call Cinder’s new os-extend_volume_completion volume action proposed in [3] to let Cinder know if the operation was successful or not.

  • Otherwise, the API will behave as before.

Security impact


Notifications impact


Other end user impact


Performance Impact


Other deployer impact


Developer impact


Upgrade impact

Checking the target compute service version allows the API to handle rolling upgrades gracefully.



Primary assignee:


Other contributors:


Feature Liaison

Feature liaison:

None yet

Work Items

  • Update the external server event API to check the target compute service version for volume-extended events.

  • Update the ComputeVirtAPI.extend_volume method to follow the behavior outlined in Compute Agent.

  • Add unit tests.

  • Adapt NFS job in the Nova gate to validate online extend.


  • The extend volume completion action [3]


We should test that the os-extend_volume_completion gets called correctly in all possible error or success condition if a volume has extending status.

We should test the case that the call to os-extend_volume_completion fails.

We also need to test that volume-extended continues to be handled correctly for volumes not in extending status.

Documentation Impact

The new behavior of the volume-extended event should be added to the documentation of the external server event API.




Release Name


2023.1 Antelope