Extend a volume while volume is attached to an instance


This spec covers the Cinder changes in the volume extend volume flow that will allow volume drivers that support extending for attached volumes to extend the attached volume.

Extending an attached volume requires Nova side changes. A Nova spec covers the Nova changes necessary to extend a volume that is attached to one or more instances.

Cinder will interact with Nova via the existing os-server-external-events extension to notify Nova that the volume has changed size and react to the change in size.

After Nova has processed the extend size API call, the instance administrator might need to take action to make use of the new space associated with the volume. Within the instance the appropriate actions must be taken for the OS the instance is running to discover the new size of the volume. The instance administrator can then make use of the new space in the volume.

Problem description

Cinder requires that a volume be in the available state. This requires an attached volume to be detached from an instance before the volume can be extended in size. This means taking the volume offline from the standpoint of the application using the volume.

Use Cases

An operator wants to increase the size of a volume that is currently attached to an instance without detaching that volume from the instance.

Proposed change

In the Cinder extend API, allow the volume extend operation to continue processing when the volume status is ‘in‐use’ for volume drivers that can support extending an ‘in‐use’ volume. A volume could be extended if the volume status is ‘available’ or ‘in-use’ if the volume driver supports in-use extend. All other volume status values besides ‘available’ and ‘in-use’ would raise an exception.

A volume driver wishing to support in-use extend must add a new capability (in-use-extend) that indicates the driver supports extending a volume while that volume is ‘in-use’. If a driver does not have the capability to extend its volumes while they are ‘in-use’, the API will fail the extend request with a message indicating that the volume driver does not support extending a volume that is in the ‘in-use’ state.

A new policy will be introduced so a deployer can disable the volume extend operation on attached volume if the deployment does not fully support the feature. For example, Nova virt driver not supporting online volume size extension.

If the volume driver capabilities indicate that the driver supports in-use extend then the volume status will be changed to ‘extending’ and then call the driver extend_volume. After changing the volume status to ‘in‐use’, notify Nova that the volume has changed size. Nova will react to the change in size for all the instances that have the volume attached.


One alternative is to create a new volume and attach that volume to the instance. Once the new volume is discovered on the instance, use LVM to extend the space to the application LV. This is the method being used today to allow more space to be allocated. This is undesirable in the long run as each time a volume is extended the extension results in a new volume to be allocated, discovered on the instance, and added to a volume group, then extending the LV to include the volume. Operators have expressed concerns about using this approach as it eventually encounters limitations in the number of volumes that can be attached to a specific OS. For the volume controllers that have the ability to grow a volume while it is in‐use, it is desirable to use that ability.

Data model impact


REST API impact

No changes to the API interface itself, as no change is required to the arguments to the API. The API documentation will change to reflect that an in-use volume may now be extended, if the volume driver capabilities allow in-use extend.

Security impact


Notifications impact


Other end user impact

End user will be able to extend their volumes without having to detach them first.

Performance Impact


Other deployer impact


Developer impact

Driver owners whose backend controllers support extending a volume that is in-use by an instance may want to enable in-use extend.



Primary assignee: * Mathieu Gagné

Other contributors: * Gerald McBrearty * Sam Matzek

Work Items

The Cinder extend volume API will check volume driver capability to see if the driver supports in-use-extend when the status of the volume is in-use.

The extend_volume rpcAPI will need to change the status of a volume back to ‘in-use’ if the volume is attached to an instance or ‘available’ if the volume is not attached to an instance, unless the driver raises an error which will cause the volume status to be changed ‘error-extending’.


  • Nova spec: Allow an attached volume to be extended [1] Adds the Nova external events support for volume-changed. It will call the os-brick extend_volume API to trigger the host kernel size information to be updated on the host where the volume has been discovered.


The following scenarios for in-use volumes will need to be covered in UT and FVT

  1. volume driver without in-use-extend capability.
  2. volume driver with in-use-extend capability with Nova not supporting the volume-changed external event.
  3. volume driver with in-use-extend capability with Nova supporting the volume-changed external event.

Documentation Impact

The extend documentation will need to be updated to indicate that a volume that is attached to an instance can be extended if the driver has the capability ‘in-use-extend’.