Kubernetes 1.24: Volume Populators Graduate to Beta

The volume populators feature is now two releases old and entering beta! The AnyVolumeDataSource feature gate defaults to enabled in Kubernetes v1.24, which means that users can specify any custom resource as the data source of a PVC.

An earlier blog article detailed how the volume populators feature works. In short, a cluster administrator can install a CRD and associated populator controller in the cluster, and any user who can create instances of the CR can create pre-populated volumes by taking advantage of the populator.

Multiple populators can be installed side by side for different purposes. The SIG storage community is already seeing some implementations in public, and more prototypes should appear soon.

Cluster administrations are strongly encouraged to install the volume-data-source-validator controller and associated VolumePopulator CRD before installing any populators so that users can get feedback about invalid PVC data sources.

New Features

The lib-volume-populator library on which populators are built now includes metrics to help operators monitor and detect problems. This library is now beta and latest release is v1.0.1.

The volume data source validator controller also has metrics support added, and is in beta. The VolumePopulator CRD is beta and the latest release is v1.0.1.

Trying it out

To see how this works, you can install the sample "hello" populator and try it out.

First install the volume-data-source-validator controller.

kubectl apply -f https://raw.githubusercontent.com/kubernetes-csi/volume-data-source-validator/v1.0.1/client/config/crd/populator.storage.k8s.io_volumepopulators.yaml
kubectl apply -f https://raw.githubusercontent.com/kubernetes-csi/volume-data-source-validator/v1.0.1/deploy/kubernetes/rbac-data-source-validator.yaml
kubectl apply -f https://raw.githubusercontent.com/kubernetes-csi/volume-data-source-validator/v1.0.1/deploy/kubernetes/setup-data-source-validator.yaml

Next install the example populator.

kubectl apply -f https://raw.githubusercontent.com/kubernetes-csi/lib-volume-populator/v1.0.1/example/hello-populator/crd.yaml
kubectl apply -f https://raw.githubusercontent.com/kubernetes-csi/lib-volume-populator/87a47467b86052819e9ad13d15036d65b9a32fbb/example/hello-populator/deploy.yaml

Your cluster now has a new CustomResourceDefinition that provides a test API named Hello. Create an instance of the Hello custom resource, with some text:

apiVersion: hello.example.com/v1alpha1
kind: Hello
metadata:
  name: example-hello
spec:
  fileName: example.txt
  fileContents: Hello, world!

Create a PVC that refers to that CR as its data source.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: example-pvc
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 10Mi
  dataSourceRef:
    apiGroup: hello.example.com
    kind: Hello
    name: example-hello
  volumeMode: Filesystem

Next, run a Job that reads the file in the PVC.

apiVersion: batch/v1
kind: Job
metadata:
  name: example-job
spec:
  template:
    spec:
      containers:
        - name: example-container
          image: busybox:latest
          command:
            - cat
            - /mnt/example.txt
          volumeMounts:
            - name: vol
              mountPath: /mnt
      restartPolicy: Never
      volumes:
        - name: vol
          persistentVolumeClaim:
            claimName: example-pvc

Wait for the job to complete (including all of its dependencies).

kubectl wait --for=condition=Complete job/example-job

And last examine the log from the job.

kubectl logs job/example-job

The output should be:

Hello, world!

Note that the volume already contained a text file with the string contents from the CR. This is only the simplest example. Actual populators can set up the volume to contain arbitrary contents.

How to write your own volume populator

Developers interested in writing new poplators are encouraged to use the lib-volume-populator library and to only supply a small controller wrapper around the library, and a pod image capable of attaching to volumes and writing the appropriate data to the volume.

Individual populators can be extremely generic such that they work with every type of PVC, or they can do vendor specific things to rapidly fill a volume with data if the volume was provisioned by a specific CSI driver from the same vendor, for example, by communicating directly with the storage for that volume.

How can I learn more?

The enhancement proposal, Volume Populators, includes lots of detail about the history and technical implementation of this feature.

Volume populators and data sources, within the documentation topic about persistent volumes, explains how to use this feature in your cluster.

Please get involved by joining the Kubernetes storage SIG to help us enhance this feature. There are a lot of good ideas already and we'd be thrilled to have more!

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