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Persistent Storage Using GCE Persistent Disk

Overview

OKD supports GCE Persistent Disk volumes (gcePD). You can provision your OKD cluster with persistent storage using GCE. Some familiarity with Kubernetes and GCE is assumed.

Before creating persistent volumes using GCE, OKD must first be properly configured for GCE Persistent Disk.

The Kubernetes persistent volume framework allows administrators to provision a cluster with persistent storage and gives users a way to request those resources without having any knowledge of the underlying infrastructure. GCE Persistent Disk volumes can be provisioned dynamically. Persistent volumes are not bound to a single project or namespace; they can be shared across the OKD cluster. Persistent volume claims, however, are specific to a project or namespace and can be requested by users.

High-availability of storage in the infrastructure is left to the underlying storage provider.

Provisioning

Storage must exist in the underlying infrastructure before it can be mounted as a volume in OKD. After ensuring OKD is configured for GCE PersistentDisk, all that is required for OKD and GCE is an GCE Persistent Disk volume ID and the PersistentVolume API.

Creating the Persistent Volume

You must define your persistent volume in an object definition before creating it in OKD:

Example 1. Persistent Volume Object Definition Using GCE
apiVersion: "v1"
kind: "PersistentVolume"
metadata:
  name: "pv0001" (1)
spec:
  capacity:
    storage: "5Gi" (2)
  accessModes:
    - "ReadWriteOnce"
  gcePersistentDisk: (3)
    fsType: "ext4" (4)
    pdName: "pd-disk-1" (5)
1 The name of the volume. This will be how it is identified via persistent volume claims or from pods.
2 The amount of storage allocated to this volume.
3 This defines the volume type being used, in this case the gcePersistentDisk plug-in.
4 File system type to mount.
5 This is the GCE Persistent Disk volume that will be used.

Changing the value of the fstype parameter after the volume has been formatted and provisioned can result in data loss and pod failure.

Save your definition to a file, for example gce-pv.yaml, and create the persistent volume:

# oc create -f gce-pv.yaml
persistentvolume "pv0001" created

Verify that the persistent volume was created:

# oc get pv
NAME      LABELS    CAPACITY   ACCESSMODES   STATUS      CLAIM     REASON    AGE
pv0001    <none>    5Gi        RWO           Available                       2s

Users can then request storage using persistent volume claims, which can now utilize your new persistent volume.

Persistent volume claims only exist in the user’s namespace and can only be referenced by a pod within that same namespace. Any attempt to access a persistent volume from a different namespace causes the pod to fail.

Volume Format

Before OKD mounts the volume and passes it to a container, it checks that it contains a file system as specified by the fsType parameter in the persistent volume definition. If the device is not formatted with the file system, all data from the device is erased and the device is automatically formatted with the given file system.

This allows using unformatted GCE volumes as persistent volumes, because OKD formats them before the first use.