Persistent volume providers vary in capabilities. A volume uses access modes to specify the modes that it supports. For example, NFS can support multiple read/write clients, but a specific NFS PV might be exported on the server as read-only. OpenShift defines the following access modes, as summarized in the following table:

Developers must select a volume type that supports the required access level by the application. The following table shows some example supported access modes:

Kubernetes supports two volume modes for persistent volumes: Filesystem and Block. If the volume mode is not defined for a volume, then Kubernetes assigns the default volume mode, Filesystem, to the volume.

OpenShift Container Platform can provision raw block volumes. These volumes do not have a file system, and can provide performance benefits for applications that either write to the disk directly or that implement their own storage service. Raw block volumes are provisioned by specifying volumeMode: Block in the PV and PVC specification.

The following table provides examples of storage options with block volume support:

Use a PersistentVolume manifest file to manually create a persistent volume. The following example creates a persistent volume from a fiber channel storage device that uses block mode.

apiVersion: v1
kind: PersistentVolume 
metadata:
  name: block-pv 
spec:
  capacity:
    storage: 10Gi 
  accessModes:
    - ReadWriteOnce 
  volumeMode: Block 
  persistentVolumeReclaimPolicy: Retain 
  fc: 
    targetWWNs: ["50060e801049cfd1"]
    lun: 0
    readOnly: false

If the previous manifest is in a file named my-fc-volume.yaml, then the following command can create the PV resource on RHOCP:

[user@host]$ oc create -f my-fc-volume.yaml

To create a persistent volume from the web console, click the Storage → PersistentVolumes menu.

A PVC belongs to a specific project. To create a PVC, you must specify the access mode and size, among other options. A PVC cannot be shared between projects. Developers use a PVC to access a persistent volume (PV). Persistent volumes are not exclusive to projects, and are accessible across the entire OpenShift cluster. When a PV binds to a PVC, the PV cannot be bound to another PVC.

To add a volume to an application deployment, create a PersistentVolumeClaim resource, and add it to the application as a volume. Create the PVC by using either a Kubernetes manifest or the oc set volumes command. In addition to either creating a PVC or using an existing PVC, the oc set volumes command can modify a deployment to mount the PVC as a volume within the pod.

To add a volume to an application deployment, use the oc set volumes command:

[user@host ~]$ oc set volumes deployment/example-application \ 
--add \ 
--name example-pv-storage \ 
--type persistentVolumeClaim \ 
--claim-mode rwo \ 
--claim-size 15Gi \ 
--mount-path /var/lib/example-app \ 
--claim-name example-pv-claim 

The command creates a PVC resource and adds it to the application as a volume within the pod.

The command updates the deployment for the application with volumeMounts and volumes specifications.

apiVersion: apps/v1
kind: Deployment
metadata:
  ...output omitted...
  namespace: storage-volumes 
  ...output omitted...
spec:
  ...output omitted...
  template:
  ...output omitted...
    spec:
  ...output omitted...
        volumeMounts:
        - mountPath: /var/lib/example-app 
          name: example-pv-storage 
  ...output omitted...
      volumes:
      - name: example-pv-storage 
        persistentVolumeClaim:
          claimName: example-pv-claim 
  ...output omitted...

The following example specifies a PVC by using a YAML manifest to create a PersistentVolumeClaim API object:

---
apiVersion: v1
kind: PersistentVolumeClaim 
metadata:
  name: example-pv-claim 
  labels:
    app: example-application
spec:
  accessModes:
    - ReadWriteOnce 
  resources:
    requests:
      storage: 15Gi 

Use the oc create command to create the PVC from the manifest file.

[user@host ~]$ oc create -f pvc_file_name.yaml

Use oc get pvc to view the available PVCs in the current namespace.

[user@host ~]$ oc get pvc
NAME         STATUS   VOLUME          CAPACITY   ACCESS MODES   STORAGECLASS   ...
db-pod-pvc   Bound    pvc-13...ca45   1Gi        RWO            nfs-storage    ...

To create a persistent volume claim from the web console, click the Storage → PersistentVolumesClaims menu.

Click Create PersistentVolumeClaim and complete the form by adding the name, the storage class, the size, the access mode, and the volume mode.

PVs are defined by a PersistentVolume API object, which is from existing storage in the cluster. The cluster administrator must statically provision some storage types. Alternatively, the Kubernetes persistent volume framework can use a StorageClass object to dynamically provision PVs.

When you create a PVC, you specify a storage amount, the required access mode, and a storage class to describe and classify the storage. The control loop in the RHOCP control node watches for new PVCs, and binds the new PVC to an appropriate PV. If an appropriate PV does not exist, then a provisioner for the storage class creates one.

Claims remain unbound indefinitely if a matching volume does not exist or if a volume cannot be created with any available provisioner that services a storage class. Claims are bound when matching volumes become available. For example, a cluster with many manually provisioned 50 Gi volumes would not match a PVC that requests 100 Gi. The PVC can be bound when a 100 Gi PV is added to the cluster.

Use oc get storageclass to view the storage classes that the cluster provides.

[user@host ~]$ oc get storageclass
NAME                    PROVISIONER                                  ...
nfs-storage (default)   k8s-sigs.io/nfs-subdir-external-provisioner  ...
lvms-vg1             topolvm.io                           ...

In the example, the nfs-storage storage class is marked as the default storage class. When a default storage class is configured, the PVC must explicitly name any other storage class to use, or can set the storageClassName annotation to "", to be bound to a PV without a storage class.

The following oc set volume command example uses the claim-class option to specify a dynamically provisioned PV.

[user@host ~]$ oc set volumes deployment/example-application \
--add --name example-pv-storage --type pvc --claim-class nfs-storage \
--claim-mode rwo --claim-size 15Gi --mount-path /var/lib/example-app \
--claim-name example-pv-claim

When you create a PVC, you request a specific amount of storage, access mode, and storage class. Kubernetes binds the PVC to an appropriate PV. If an appropriate PV does not exist, then a provisioner for the storage class creates one.

Figure 5.17: PV lifecycle

PVs follow a lifecycle based on their relationship to the PVC.

To delete a volume, use the oc delete command to delete the PVC. The storage class reclaims the volume after removing the PVC.

[user@host ~]$ oc delete pvc/example-pvc-storage