Kubernetes 1.26: Retroactive Default StorageClass

Author: Roman Bednář (Red Hat)

The v1.25 release of Kubernetes introduced an alpha feature to change how a default StorageClass was assigned to a PersistentVolumeClaim (PVC). With the feature enabled, you no longer need to create a default StorageClass first and PVC second to assign the class. Additionally, any PVCs without a StorageClass assigned can be updated later. This feature was graduated to beta in Kubernetes 1.26.

You can read retroactive default StorageClass assignment in the Kubernetes documentation for more details about how to use that, or you can read on to learn about why the Kubernetes project is making this change.

Why did StorageClass assignment need improvements

Users might already be familiar with a similar feature that assigns default StorageClasses to new PVCs at the time of creation. This is currently handled by the admission controller.

But what if there wasn't a default StorageClass defined at the time of PVC creation? Users would end up with a PVC that would never be assigned a class. As a result, no storage would be provisioned, and the PVC would be somewhat "stuck" at this point. Generally, two main scenarios could result in "stuck" PVCs and cause problems later down the road. Let's take a closer look at each of them.

Changing default StorageClass

With the alpha feature enabled, there were two options admins had when they wanted to change the default StorageClass:

  1. Creating a new StorageClass as default before removing the old one associated with the PVC. This would result in having two defaults for a short period. At this point, if a user were to create a PersistentVolumeClaim with storageClassName set to null (implying default StorageClass), the newest default StorageClass would be chosen and assigned to this PVC.

  2. Removing the old default first and creating a new default StorageClass. This would result in having no default for a short time. Subsequently, if a user were to create a PersistentVolumeClaim with storageClassName set to null (implying default StorageClass), the PVC would be in Pending state forever. The user would have to fix this by deleting the PVC and recreating it once the default StorageClass was available.

Resource ordering during cluster installation

If a cluster installation tool needed to create resources that required storage, for example, an image registry, it was difficult to get the ordering right. This is because any Pods that required storage would rely on the presence of a default StorageClass and would fail to be created if it wasn't defined.

What changed

We've changed the PersistentVolume (PV) controller to assign a default StorageClass to any unbound PersistentVolumeClaim that has the storageClassName set to null. We've also modified the PersistentVolumeClaim admission within the API server to allow the change of values from an unset value to an actual StorageClass name.

Null storageClassName versus storageClassName: "" - does it matter?

Before this feature was introduced, those values were equal in terms of behavior. Any PersistentVolumeClaim with the storageClassName set to null or "" would bind to an existing PersistentVolume resource with storageClassName also set to null or "".

With this new feature enabled we wanted to maintain this behavior but also be able to update the StorageClass name. With these constraints in mind, the feature changes the semantics of null. If a default StorageClass is present, null would translate to "Give me a default" and "" would mean "Give me PersistentVolume that also has "" StorageClass name." In the absence of a StorageClass, the behavior would remain unchanged.

Summarizing the above, we've changed the semantics of null so that its behavior depends on the presence or absence of a definition of default StorageClass.

The tables below show all these cases to better describe when PVC binds and when its StorageClass gets updated.

PVC binding behavior with Retroactive default StorageClass
PVC storageClassName = "" PVC storageClassName = null
Without default class PV storageClassName = "" binds binds
PV without storageClassName binds binds
With default class PV storageClassName = "" binds class updates
PV without storageClassName binds class updates

How to use it

If you want to test the feature whilst it's alpha, you need to enable the relevant feature gate in the kube-controller-manager and the kube-apiserver. Use the --feature-gates command line argument:

--feature-gates="...,RetroactiveDefaultStorageClass=true"

Test drive

If you would like to see the feature in action and verify it works fine in your cluster here's what you can try:

  1. Define a basic PersistentVolumeClaim:

    apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-1
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi

  2. Create the PersistentVolumeClaim when there is no default StorageClass. The PVC won't provision or bind (unless there is an existing, suitable PV already present) and will remain in Pending state.

    $ kc get pvc
NAME      STATUS    VOLUME   CAPACITY   ACCESS MODES   STORAGECLASS   AGE
pvc-1     Pending

  3. Configure one StorageClass as default.

    $ kc patch sc -p '{"metadata":{"annotations":{"storageclass.kubernetes.io/is-default-class":"true"}}}'
storageclass.storage.k8s.io/my-storageclass patched

  4. Verify that PersistentVolumeClaims is now provisioned correctly and was updated retroactively with new default StorageClass.

    $ kc get pvc
NAME      STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS      AGE
pvc-1     Bound    pvc-06a964ca-f997-4780-8627-b5c3bf5a87d8   1Gi        RWO            my-storageclass   87m

New metrics

To help you see that the feature is working as expected we also introduced a new retroactive_storageclass_total metric to show how many times that the PV controller attempted to update PersistentVolumeClaim, and retroactive_storageclass_errors_total to show how many of those attempts failed.

Getting involved

We always welcome new contributors so if you would like to get involved you can join our Kubernetes Storage Special-Interest-Group (SIG).

If you would like to share feedback, you can do so on our public Slack channel.

Special thanks to all the contributors that provided great reviews, shared valuable insight and helped implement this feature (alphabetical order):