Persistent Storage Using OpenStack Cinder

Overview

You can provision your OKD cluster with persistent storage using OpenStack Cinder. Some familiarity with Kubernetes and OpenStack is assumed.

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. You can provision OpenStack Cinder volumes 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.

Provisioning Cinder PVs

Storage must exist in the underlying infrastructure before it can be mounted as a volume in OKD. After ensuring that OKD is configured for OpenStack, all that is required for Cinder is a Cinder volume ID and the **PersistentVolume** API.

Creating the Persistent Volume

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

1. Save your object definition to a file, for example cinder-pv.yaml:

   apiVersion: "v1"
   kind: "PersistentVolume"
   metadata:
     name: "pv0001" (1)
   spec:
     capacity:
       storage: "5Gi" (2)
     accessModes:
       - "ReadWriteOnce"
     cinder: (3)
       fsType: "ext3" (4)
       volumeID: "f37a03aa-6212-4c62-a805-9ce139fab180" (5)

   1	The name of the volume that is used by persistent volume claims or pods.
   2	The amount of storage allocated to this volume.
   3	The volume type, in this case cinder.
   4	File system type to mount.
   5	The Cinder volume to use.

   Do not change the fstype parameter value after the volume is formatted and provisioned. Changing this value can result in data loss and pod failure.

2. Create the persistent volume:

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

3. Verify that the persistent volume exists:

   # 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.

Cinder PV 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 Cinder volumes as persistent volumes, because OKD formats them before the first use.

Cinder volume security

If you use Cinder PVs in your application, configure security for their deployment configurations.

1. Create an SCC that uses the appropriate **fsGroup** strategy.

2. Create a service account and add it to the SCC:

   [source,bash]
   $ oc create serviceaccount <service_account>
   $ oc adm policy add-scc-to-user <new_scc> -z <service_account> -n <project>

3. In your application’s deployment configuration, provide the service account name and securityContext:

   apiVersion: v1
   kind: ReplicationController
   metadata:
     name: frontend-1
   spec:
     replicas: 1  (1)
     selector:    (2)
       name: frontend
     template:    (3)
       metadata:
         labels:  (4)
           name: frontend (5)
       spec:
         containers:
         - image: openshift/hello-openshift
           name: helloworld
           ports:
           - containerPort: 8080
             protocol: TCP
         restartPolicy: Always
         serviceAccountName: <service_account> (6)
         securityContext:
           fsGroup: 7777 (7)

   1	The number of copies of the pod to run.
   2	The label selector of the pod to run.
   3	A template for the pod the controller creates.
   4	The labels on the pod must include labels from the label selector.
   5	The maximum name length after expanding any parameters is 63 characters.
   6	Specify the service account you created.
   7	Specify an fsGroup for the pods.