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CSI inline ephemeral volumes

Container Storage Interface (CSI) inline ephemeral volumes allow you to define a Pod spec that creates inline ephemeral volumes when a pod is deployed and delete them when a pod is destroyed.

This feature is only available with supported Container Storage Interface (CSI) drivers:

  • Shared Resource CSI driver

  • Azure File CSI driver

  • Secrets Store CSI driver

Overview of CSI inline ephemeral volumes

Traditionally, volumes that are backed by Container Storage Interface (CSI) drivers can only be used with a PersistentVolume and PersistentVolumeClaim object combination.

This feature allows you to specify CSI volumes directly in the Pod specification, rather than in a PersistentVolume object. Inline volumes are ephemeral and do not persist across pod restarts.

Support limitations

The Shared Resource CSI Driver feature is now generally available in Builds for Red Hat OpenShift 1.1. This feature is now deprecated in OpenShift Container Platform. To use this feature, ensure you are using Builds for Red Hat OpenShift 1.1 or a more recent version.

By default, OpenShift Container Platform supports CSI inline ephemeral volumes with these limitations:

  • Support is only available for CSI drivers. In-tree and FlexVolumes are not supported.

  • The Shared Resource CSI Driver supports using inline ephemeral volumes only to access Secrets or ConfigMaps across multiple namespaces as a Technology Preview feature in OpenShift Container Platform.

  • Community or storage vendors provide other CSI drivers that support these volumes. Follow the installation instructions provided by the CSI driver provider.

CSI drivers might not have implemented the inline volume functionality, including Ephemeral capacity. For details, see the CSI driver documentation.

CSI Volume Admission plugin

The Container Storage Interface (CSI) Volume Admission plugin allows you to restrict the use of an individual CSI driver capable of provisioning CSI ephemeral volumes on pod admission. Administrators can add a csi-ephemeral-volume-profile label, and this label is then inspected by the Admission plugin and used in enforcement, warning, and audit decisions.

Overview

To use the CSI Volume Admission plugin, administrators add the security.openshift.io/csi-ephemeral-volume-profile label to a CSIDriver object, which declares the CSI driver’s effective pod security profile when it is used to provide CSI ephemeral volumes, as shown in the following example:

kind: CSIDriver
metadata:
  name: csi.mydriver.company.org
  labels:
    security.openshift.io/csi-ephemeral-volume-profile: restricted (1)
1 CSI driver object YAML file with the csi-ephemeral-volume-profile label set to "restricted"

This “effective profile” communicates that a pod can use the CSI driver to mount CSI ephemeral volumes when the pod’s namespace is governed by a pod security standard.

The CSI Volume Admission plugin inspects pod volumes when pods are created; existing pods that use CSI volumes are not affected. If a pod uses a container storage interface (CSI) volume, the plugin looks up the CSIDriver object and inspects the csi-ephemeral-volume-profile label, and then use the label’s value in its enforcement, warning, and audit decisions.

Pod security profile enforcement

When a CSI driver has the csi-ephemeral-volume-profile label, pods using the CSI driver to mount CSI ephemeral volumes must run in a namespace that enforces a pod security standard of equal or greater permission. If the namespace enforces a more restrictive standard, the CSI Volume Admission plugin denies admission. The following table describes the enforcement behavior for different pod security profiles for given label values.

Table 1. Pod security profile enforcement
Pod security profile Driver label: restricted Driver label: baseline Driver label: privileged

Restricted

Allowed

Denied

Denied

Baseline

Allowed

Allowed

Denied

Privileged

Allowed

Allowed

Allowed

Pod security profile warning

The CSI Volume Admission plugin can warn you if the CSI driver’s effective profile is more permissive than the pod security warning profile for the pod namespace. The following table shows when a warning occurs for different pod security profiles for given label values.

Table 2. Pod security profile warning
Pod security profile Driver label: restricted Driver label: baseline Driver label: privileged

Restricted

No warning

Warning

Warning

Baseline

No warning

No warning

Warning

Privileged

No warning

No warning

No warning

Pod security profile audit

The CSI Volume Admission plugin can apply audit annotations to the pod if the CSI driver’s effective profile is more permissive than the pod security audit profile for the pod namespace. The following table shows the audit annotation applied for different pod security profiles for given label values.

Table 3. Pod security profile audit
Pod security profile Driver label: restricted Driver label: baseline Driver label: privileged

Restricted

No audit

Audit

Audit

Baseline

No audit

No audit

Audit

Privileged

No audit

No audit

No audit

Default behavior for the CSI Volume Admission plugin

If the referenced CSI driver for a CSI ephemeral volume does not have the csi-ephemeral-volume-profile label, the CSI Volume Admission plugin considers the driver to have the privileged profile for enforcement, warning, and audit behaviors. Likewise, if the pod’s namespace does not have the pod security admission label set, the Admission plugin assumes the restricted profile is allowed for enforcement, warning, and audit decisions. Therefore, if no labels are set, CSI ephemeral volumes using that CSI driver are only usable in privileged namespaces by default.

The CSI drivers that ship with OpenShift Container Platform and support ephemeral volumes have a reasonable default set for the csi-ephemeral-volume-profile label:

  • Shared Resource CSI driver: restricted

  • Azure File CSI driver: privileged

An admin can change the default value of the label if desired.

Embedding a CSI inline ephemeral volume in the pod specification

You can embed a CSI inline ephemeral volume in the Pod specification in OpenShift Container Platform. At runtime, nested inline volumes follow the ephemeral lifecycle of their associated pods so that the CSI driver handles all phases of volume operations as pods are created and destroyed.

Procedure

  1. Create the Pod object definition and save it to a file.

  2. Embed the CSI inline ephemeral volume in the file.

    my-csi-app.yaml
    kind: Pod
apiVersion: v1
metadata:
  name: my-csi-app
spec:
  containers:
    - name: my-frontend
      image: busybox
      volumeMounts:
      - mountPath: "/data"
        name: my-csi-inline-vol
      command: [ "sleep", "1000000" ]
  volumes: (1)
    - name: my-csi-inline-vol
      csi:
        driver: inline.storage.kubernetes.io
        volumeAttributes:
          foo: bar
    1 The name of the volume that is used by pods.

  3. Create the object definition file that you saved in the previous step.

    $ oc create -f my-csi-app.yaml

Additional resources