v1.24 blog post: Volume populators beta
Ben Swartzlander
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layout: blog
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title: "Kubernetes 1.24: Volume Populators Graduate to Beta"
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date: 2022-05-16
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slug: volume-populators-beta
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---
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**Author:**
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Ben Swartzlander (NetApp)
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The volume populators feature is now two releases old and entering beta! The `AnyVolumeDataSouce` feature
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gate defaults to enabled in Kubernetes v1.24, which means that users can specify any custom resource
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as the data source of a PVC.
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An [earlier blog article](/blog/2021/08/30-volume-populators-redesigned/) detailed how the
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volume populators feature works. In short, a cluster administrator can install a CRD and
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associated populator controller in the cluster, and any user who can create instances of
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the CR can create pre-populated volumes by taking advantage of the populator.
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Multiple populators can be installed side by side for different purposes. The SIG storage
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community is already seeing some implementations in public, and more prototypes should
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appear soon.
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Cluster administrations are **strongly encouraged** to install the
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volume-data-source-validator controller and associated `VolumePopulator` CRD before installing
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any populators so that users can get feedback about invalid PVC data sources.
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## New Features
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The [lib-volume-populator](https://github.com/kubernetes-csi/lib-volume-populator) library
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on which populators are built now includes metrics to help operators monitor and detect
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problems. This library is now beta and latest release is v1.0.1.
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The [volume data source validator](https://github.com/kubernetes-csi/volume-data-source-validator)
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controller also has metrics support added, and is in beta. The `VolumePopulator` CRD is
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beta and the latest release is v1.0.1.
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## Trying it out
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To see how this works, you can install the sample "hello" populator and try it
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out.
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First install the volume-data-source-validator controller.
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```shell
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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
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kubectl apply -f https://raw.githubusercontent.com/kubernetes-csi/volume-data-source-validator/v1.0.1/deploy/kubernetes/rbac-data-source-validator.yaml
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kubectl apply -f https://raw.githubusercontent.com/kubernetes-csi/volume-data-source-validator/v1.0.1/deploy/kubernetes/setup-data-source-validator.yaml
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```
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Next install the example populator.
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```shell
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kubectl apply -f https://raw.githubusercontent.com/kubernetes-csi/lib-volume-populator/v1.0.1/example/hello-populator/crd.yaml
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kubectl apply -f https://raw.githubusercontent.com/kubernetes-csi/lib-volume-populator/87a47467b86052819e9ad13d15036d65b9a32fbb/example/hello-populator/deploy.yaml
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```
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Your cluster now has a new CustomResourceDefinition that provides a test API named Hello.
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Create an instance of the `Hello` custom resource, with some text:
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```yaml
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apiVersion: hello.example.com/v1alpha1
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kind: Hello
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metadata:
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name: example-hello
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spec:
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fileName: example.txt
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fileContents: Hello, world!
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```
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Create a PVC that refers to that CR as its data source.
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```yaml
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apiVersion: v1
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kind: PersistentVolumeClaim
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metadata:
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name: example-pvc
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spec:
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accessModes:
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- ReadWriteOnce
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resources:
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requests:
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storage: 10Mi
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dataSourceRef:
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apiGroup: hello.example.com
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kind: Hello
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name: example-hello
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volumeMode: Filesystem
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```
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Next, run a Job that reads the file in the PVC.
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```yaml
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apiVersion: batch/v1
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kind: Job
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metadata:
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name: example-job
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spec:
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template:
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spec:
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containers:
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- name: example-container
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image: busybox:latest
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command:
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- cat
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- /mnt/example.txt
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volumeMounts:
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- name: vol
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mountPath: /mnt
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restartPolicy: Never
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volumes:
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- name: vol
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persistentVolumeClaim:
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claimName: example-pvc
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```
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Wait for the job to complete (including all of its dependencies).
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```shell
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kubectl wait --for=condition=Complete job/example-job
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```
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And last examine the log from the job.
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```shell
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kubectl logs job/example-job
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```
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The output should be:
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```terminal
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Hello, world!
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```
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Note that the volume already contained a text file with the string contents from
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the CR. This is only the simplest example. Actual populators can set up the volume
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to contain arbitrary contents.
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## How to write your own volume populator
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Developers interested in writing new poplators are encouraged to use the
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[lib-volume-populator](https://github.com/kubernetes-csi/lib-volume-populator) library
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and to only supply a small controller wrapper around the library, and a pod image
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capable of attaching to volumes and writing the appropriate data to the volume.
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Individual populators can be extremely generic such that they work with every type
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of PVC, or they can do vendor specific things to rapidly fill a volume with data
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if the volume was provisioned by a specific CSI driver from the same vendor, for
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example, by communicating directly with the storage for that volume.
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## How can I learn more?
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The enhancement proposal,
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[Volume Populators](https://github.com/kubernetes/enhancements/tree/master/keps/sig-storage/1495-volume-populators), includes lots of detail about the history and technical implementation
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of this feature.
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[Volume populators and data sources](/docs/concepts/storage/persistent-volumes/#volume-populators-and-data-sources), within the documentation topic about persistent volumes,
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explains how to use this feature in your cluster.
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Please get involved by joining the Kubernetes storage SIG to help us enhance this
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feature. There are a lot of good ideas already and we'd be thrilled to have more!
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