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Using Persistent Volumes

Overview

A PersistentVolume object is a storage resource in an OpenShift cluster. Storage is provisioned by an administrator by creating PersistentVolume objects from sources such as GCE Persistent Disks, AWS Elastic Block Stores (EBS), and NFS mounts.

Persistent volume plug-ins other than the supported NFS plug-in, such as AWS Elastic Block Stores (EBS), GCE Persistent Disks, GlusterFS, iSCSI, and RADOS (Ceph), are currently in Technology Preview. The Administrator Guide provides instructions on provisioning an OpenShift cluster with persistent storage using NFS.

Storage can be made available to you by laying claims to the resource. You can make a request for storage resources using a PersistentVolumeClaim object; the claim is paired with a volume that generally matches your request.

Requesting Storage

You can request storage by creating PersistentVolumeClaim objects in your projects:

Example 1. Persistent Volume Claim Object Definition
{
    "apiVersion": "v1",
    "kind": "PersistentVolumeClaim",
    "metadata": {
        "name": "claim1"
    },
    "spec": {
        "accessModes": [ "ReadWriteOnce" ],
        "resources": {
            "requests": {
                "storage": "5Gi"
            },
            "volumeName": "pv0001"
        }
    }
}

Volume and Claim Binding

A PersistentVolume is a specific resource. A PersistentVolumeClaim is a request for a resource with specific attributes, such as storage size. In between the two is a process that matches a claim to an available volume and binds them together. This allows the claim to be used as a volume in a pod. OpenShift finds the volume backing the claim and mounts it into the pod.

You can tell whether a claim or volume is bound by querying using the CLI:

$ oc get pvc
NAME        LABELS    STATUS    VOLUME
claim1      map[]     Bound     pv0001

$ oc get pv
NAME                LABELS              CAPACITY            ACCESSMODES         STATUS    CLAIM
pv0001              map[]               5368709120          RWO                 Bound     yournamespace / claim1

Claims as Volumes in Pods

A PersistentVolumeClaim is used by a pod as a volume. OpenShift finds the claim with the given name in the same namespace as the pod, then uses the claim to find the corresponding volume to mount.

Example 2. Pod Definition with a Claim
{
    "apiVersion": "v1",
    "kind": "Pod",
    "metadata": {
        "name": "mypod",
        "labels": {
            "name": "frontendhttp"
        }
    },
    "spec": {
        "containers": [{
            "name": "myfrontend",
            "image": "nginx",
            "ports": [{
                "containerPort": 80,
                "name": "http-server"
            }],
            "volumeMounts": [{
                "mountPath": "/var/www/html",
                "name": "pvol"
            }]
        }],
        "volumes": [{
            "name": "pvol",
            "persistentVolumeClaim": {
                "claimName": "claim1"
            }
        }]
    }
}

See Kubernetes Persistent Volumes for more information.