Generic ephemeral volumes | Storage

Overview of generic ephemeral volumes
Lifecycle and persistent volume claims
Security
Persistent volume claim naming
Creating generic ephemeral volumes

Overview of generic ephemeral volumes

To manage scratch data by using standard storage drivers, use generic ephemeral volumes. These volumes provide per-pod directories similar to emptyDir volumes but work with any driver that supports persistent volumes and dynamic provisioning, so that you can leverage existing storage infrastructure for temporary needs.

Generic ephemeral volumes are specified inline in the pod specification and follow the lifecycle of a pod. They are created and deleted along with the pod.

Generic ephemeral volumes have the following features:

Storage can be local or network-attached.
Volumes can have a fixed size that pods cannot exceed.
Volumes might have some initial data, depending on the driver and parameters.
Typical operations on volumes are supported, assuming that the driver supports them, including snapshotting, cloning, resizing, and storage capacity tracking.

Generic ephemeral volumes do not support offline snapshotting and resizing.

Due to this limitation, the following Container Storage Interface (CSI) drivers do not support the following features for generic ephemeral volumes:

Azure Disk CSI driver does not support resize.
Cinder CSI driver does not support snapshot.

Lifecycle and persistent volume claims

To bind the lifecycle of storage resources to a specific pod, configure persistent volume claim (PVC) parameters within the volume source of the pod. This setup ensures that the ephemeral volume controller creates the PVC in the same namespace upon pod creation and automatically deletes the PVC when the pod is removed.

Labels, annotations, and the whole set of fields for persistent volume claims (PVCs) are supported.

The ephemeral volume controller creates a PVC object from the template shown in the Creating generic ephemeral volumes procedure.

Volume binding and provisioning can be triggered in one of two ways:

Either immediately, if the storage class uses immediate volume binding.

With immediate binding, the scheduler is forced to select a node that has access to the volume after it is available.
When the pod is tentatively scheduled onto a node (WaitForFirstConsumer volume binding mode).

This volume binding option is recommended for generic ephemeral volumes because then the scheduler can choose a suitable node for the pod.

In terms of resource ownership, a pod that has generic ephemeral storage is the owner of the PVCs that provide that ephemeral storage. When the pod is deleted, the Kubernetes garbage collector deletes the PVC, which then usually triggers deletion of the volume because the default reclaim policy of storage classes is to delete volumes. You can create quasi-ephemeral local storage by using a storage class with a reclaim policy of retain. The storage outlives the pod, and in this case, you must ensure that volume clean-up happens separately. While these PVCs exist, they can be used like any other PVC. In particular, they can be referenced as data sources in volume cloning or snapshotting. The PVC object also holds the current status of the volume.

Additional resources

Creating generic ephemeral volumes

Security

You can enable the generic ephemeral volume feature so that if a user can create pods, they can also create persistent volume claims (PVCs) indirectly.

The generic ephemeral volume feature works even if these users do not have permission to create PVCs directly. Cluster administrators must be aware of this. If this does not fit your security model, use an admission webhook that rejects objects such as pods that have a generic ephemeral volume.

The normal namespace quota for PVCs still applies, so even if users are allowed to use this new mechanism, they cannot use it to circumvent other policies.

Persistent volume claim naming

To avoid resource conflicts, review the naming convention for automatically created persistent volume claims (PVCs). Because the system generates names by combining the pod name and volume name with a hyphen, you must ensure manually created resources do not inadvertently match this pattern.

For example, pod-a with volume scratch and pod with volume a-scratch both end up with the same PVC name, pod-a-scratch.

Such conflicts are detected, and a PVC is only used for an ephemeral volume if it was created for the pod. This check is based on the ownership relationship. An existing PVC is not overwritten or modified, but this does not resolve the conflict. Without the right PVC, a pod cannot start.

Be careful when naming pods and volumes inside the same namespace so that naming conflicts do not occur.

Creating generic ephemeral volumes

Configure temporary storage by creating generic ephemeral volumes that use drivers that support dynamic provisioning.

Procedure

Create the pod object definition and save it to a file.

Include the generic ephemeral volume information in the file.

my-example-pod-with-generic-vols.yaml

kind: Pod
apiVersion: v1
metadata:
  name: my-app
spec:
  containers:
    - name: my-frontend
      image: busybox:1.28
      volumeMounts:
      - mountPath: "/mnt/storage"
        name: data
      command: [ "sleep", "1000000" ]
  volumes:
    - name: data
      ephemeral:
        volumeClaimTemplate:
          metadata:
            labels:
              type: my-app-ephvol
          spec:
            accessModes: [ "ReadWriteOnce" ]
            storageClassName: "gp2-csi"
            resources:
              requests:
                storage: 1Gi

volumes.name:: Specifies the name for the generic ephemeral volume.