This topic provides an end-to-end example of how to use an existing Gluster cluster as an OpenShift Enterprise persistent store. It is assumed that a working Gluster cluster is already set up. If not, consult the Red Hat Gluster Storage Administration Guide.

Persistent Storage Using GlusterFS provides an explanation of persistent volumes (PVs), persistent volume claims (PVCs), and using GlusterFS as persistent storage.

All oc …​ commands are executed on the OpenShift Enterprise master host.

Installing the glusterfs-fuse Package

The glusterfs-fuse library must be installed on all schedulable OpenShift Enterprise nodes:

# yum install -y glusterfs-fuse

The OpenShift Enterprise all-in-one host is often not used to run pod workloads and, thus, is not included as a schedulable node.

Creating the Gluster Endpoints

The named endpoints define each node in the Gluster-trusted storage pool:

Example 1. GlusterFS Endpoint Definition
apiVersion: v1
kind: Endpoints
  name: gluster-endpoints (1)
- addresses:              (2)
  - ip:
  ports:                  (3)
  - port: 1
    protocol: TCP
- addresses:
  - ip:
  - port: 1
    protocol: TCP
1 The name of the endpoints is used in the PV definition below.
2 An array of IP addresses for each node in the Gluster pool. Currently, host names are not supported.
3 The port numbers are ignored, but must be legal port numbers. The value 1 is commonly used.

Save the endpoints definition to a file, for example gluster-endpoints.yaml, then create the endpoints object:

# oc create -f gluster-endpoints.yaml
endpoints "gluster-endpoints" created

Verify that the endpoints were created:

# oc get endpoints gluster-endpoints
NAME                ENDPOINTS                           AGE
gluster-endpoints,   1m

Creating the Persistent Volume

Next, before creating the PV object, define the persistent volume in OpenShift Enterprise:

Example 2. Persistent Volume Object Definition Using GlusterFS
apiVersion: v1
kind: PersistentVolume
  name: gluster-pv   (1)
    storage: 1Gi     (2)
  - ReadWriteMany    (3)
  glusterfs:         (4)
    endpoints: gluster-endpoints        (5)
    path: /HadoopVol (6)
    readOnly: false
  persistentVolumeReclaimPolicy: Retain (7)
1 The name of the PV, which is referenced in pod definitions or displayed in various oc volume commands.
2 The amount of storage allocated to this volume.
3 accessModes are used as labels to match a PV and a PVC. They currently do not define any form of access control.
4 This defines the volume type being used. In this case, the glusterfs plug-in is defined.
5 This references the endpoints named above.
6 This is the Gluster volume name, preceded by /.
7 A volume reclaim policy of retain indicates that the volume will be preserved after the pods accessing it terminate.

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

# oc create -f gluster-pv.yaml
persistentvolume "gluster-pv" created

Verify that the persistent volume was created:

# oc get pv
gluster-pv   <none>    1Gi        RWX           Available                       37s

Creating the Persistent Volume Claim

A persistent volume claim (PVC) specifies the desired access mode and storage capacity. Currently, based on only these two attributes, a PVC is bound to a single PV. Once a PV is bound to a PVC, that PV is essentially tied to the PVC’s project and cannot be bound to by another PVC. There is a one-to-one mapping of PVs and PVCs. However, multiple pods in the same project can use the same PVC.

Example 3. PVC Object Definition
apiVersion: v1
kind: PersistentVolumeClaim
  name: gluster-claim  (1)
  - ReadWriteMany      (2)
       storage: 1Gi    (3)
1 The claim name is referenced by the pod under its volumes section.
2 As mentioned above for PVs, the accessModes do not enforce access rights, but rather act as labels to match a PV to a PVC.
3 This claim will look for PVs offering 1Gi or greater capacity.

Save the PVC definition to a file, for example gluster-claim.yaml, and create the PVC:

# oc create -f gluster-claim.yaml
persistentvolumeclaim "gluster-claim" created

Verify the PVC was created and bound to the expected PV:

# oc get pvc
gluster-claim   <none>    Bound     gluster-pv   1Gi        RWX           24s
1 The claim was bound to the gluster-pv PV.

Defining GlusterFS Volume Access

Access is necessary to a node in the Gluster-trusted storage pool. On this node, examine the glusterfs-fuse mount:

# ls -lZ /mnt/glusterfs/
drwxrwx---. yarn hadoop system_u:object_r:fusefs_t:s0    HadoopVol

# id yarn
uid=592(yarn) gid=590(hadoop) groups=590(hadoop)
1 The owner has ID 592.
2 The group has ID 590.

In order to access the HadoopVol volume, the container must match the SELinux label, and either run with a UID of 592, or with 590 in its supplemental groups. It is recommended to gain access to the volume by matching the Gluster mount’s groups, which is defined in the pod definition below.

By default, SELinux does not allow writing from a pod to a remote Gluster server. To enable writing to GlusterFS volumes with SELinux enforcing on each node, run:

# setsebool -P virt_sandbox_use_fusefs on

The virt_sandbox_use_fusefs boolean is defined by the docker-selinux package. If you get an error saying it is not defined, ensure that this package is installed.

Creating the Pod

A pod definition file or a template file can be used to define a pod. Below is a pod specification that creates a single container and mounts the Gluster volume for read-write access:

Example 4. Pod Object Definition
apiVersion: v1
kind: Pod
  name: gluster-pod1
    name: gluster-pod1   (1)
  - name: gluster-pod1
    image: busybox       (2)
    command: ["sleep", "60000"]
    - name: gluster-vol1 (3)
      mountPath: /usr/share/busybox (4)
      readOnly: false
    supplementalGroups: [590]       (5)
    privileged: false
  - name: gluster-vol1   (3)
      claimName: gluster-claim      (6)
1 The name of this pod as displayed by oc get pod.
2 The image run by this pod. In this case, we are telling busybox to sleep.
3 The name of the volume. This name must be the same in both the containers and volumes sections.
4 The mount path as seen in the container.
5 The group ID to be assigned to the container.
6 The PVC that is bound to the Gluster cluster.

Save the pod definition to a file, for example gluster-pod1.yaml, and create the pod:

# oc create -f gluster-pod1.yaml
pod "gluster-pod1" created

Verify the pod was created:

# oc get pod
gluster-pod1   1/1       Running   0          31s

1 After a minute or so, the pod will be in the Running state.

More details are shown in the oc describe pod command:

# oc describe pod gluster-pod1
Name:				gluster-pod1
Namespace:			default   (1)
Image(s):			busybox
Node:				rhel7.2-dev/
Start Time:			Tue, 22 Mar 2016 10:55:57 -0700
Labels:				name=gluster-pod1
Status:				Running
IP:  (2)
Replication Controllers:	<none>
    Container ID:	docker://acc0c80c28a5cd64b6e3f2848052ef30a21ee850d27ef5fe959d11da4e5a3f4f
    Image:		busybox
    Image ID:		docker://964092b7f3e54185d3f425880be0b022bfc9a706701390e0ceab527c84dea3e3
    QoS Tier:
      cpu:		BestEffort
      memory:		BestEffort
    State:		Running
      Started:		Tue, 22 Mar 2016 10:56:00 -0700
    Ready:		True
    Restart Count:	0
    Environment Variables:
  Type		Status
  Ready 	True
    Type:	PersistentVolumeClaim (a reference to a PersistentVolumeClaim in the same namespace)
    ClaimName:	gluster-claim  (3)
    ReadOnly:	false
    Type:	Secret (a secret that should populate this volume)
    SecretName:	default-token-rbi9o

Events:                        (4)
  FirstSeen	LastSeen	Count	From			SubobjectPath	Reason		Message
  ─────────	────────	─────	────			─────────────	──────		───────
  2m		2m		1	{scheduler }				Scheduled	Successfully assigned gluster-pod1 to rhel7.2-dev
  2m		2m		1	{kubelet rhel7.2-dev}	implicitly required container POD	Pulled		Container image "openshift3/ose-pod:v3.1.1.6" already present on machine
  2m		2m		1	{kubelet rhel7.2-dev}	implicitly required container POD	Created		Created with docker id d5c66b4f3aaa
  2m		2m		1	{kubelet rhel7.2-dev}	implicitly required container POD	Started		Started with docker id d5c66b4f3aaa
  2m		2m		1	{kubelet rhel7.2-dev}	spec.containers{gluster-pod1}		Pulled		Container image "busybox" already present on machine
  2m		2m		1	{kubelet rhel7.2-dev}	spec.containers{gluster-pod1}		Created		Created with docker id acc0c80c28a5
  2m		2m		1	{kubelet rhel7.2-dev}	spec.containers{gluster-pod1}		Started		Started with docker id acc0c80c28a5
1 The project (namespace) name.
2 The IP address of the OpenShift Enterprise node running the pod.
3 The PVC name used by the pod.
4 The list of events resulting in the pod being launched and the Gluster volume being mounted.

There is more internal information, including the SCC used to authorize the pod, the pod’s user and group IDs, the ⁠SELinux label, and more shown in the oc get pod <name> -o yaml command:

# oc get pod gluster-pod1 -o yaml
apiVersion: v1
kind: Pod
    openshift.io/scc: restricted  (1)
  creationTimestamp: 2016-03-22T17:55:57Z
    name: gluster-pod1
  name: gluster-pod1
  namespace: default              (2)
  resourceVersion: "511908"
  selflink: /api/v1/namespaces/default/pods/gluster-pod1
  uid: 545068a3-f057-11e5-a8e5-5254008f071b
  - command:
    - sleep
    - "60000"
    image: busybox
    imagePullPolicy: IfNotPresent
    name: gluster-pod1
    resources: {}
      privileged: false
      runAsUser: 1000000000      (3)
        level: s0:c1,c0          (4)
    terminationMessagePath: /dev/termination-log
    - mountPath: /usr/share/busybox
      name: gluster-vol1
    - mountPath: /var/run/secrets/kubernetes.io/serviceaccount
      name: default-token-rbi9o
      readOnly: true
  dnsPolicy: ClusterFirst
  host: rhel7.2-dev
  - name: default-dockercfg-2g6go
  nodeName: rhel7.2-dev
  restartPolicy: Always
      level: s0:c1,c0            (4)
    - 590                        (5)
  serviceAccount: default
  serviceAccountName: default
  terminationGracePeriodSeconds: 30
  - name: gluster-vol1
      claimName: gluster-claim   (6)
  - name: default-token-rbi9o
      secretName: default-token-rbi9o
  - lastProbeTime: null
    lastTransitionTime: 2016-03-22T17:56:00Z
    status: "True"
    type: Ready
  - containerID: docker://acc0c80c28a5cd64b6e3f2848052ef30a21ee850d27ef5fe959d11da4e5a3f4f
    image: busybox
    imageID: docker://964092b7f3e54185d3f425880be0b022bfc9a706701390e0ceab527c84dea3e3
    lastState: {}
    name: gluster-pod1
    ready: true
    restartCount: 0
        startedAt: 2016-03-22T17:56:00Z
  phase: Running
  startTime: 2016-03-22T17:55:57Z
1 The SCC used by the pod.
2 The project (namespace) name.
3 The UID of the busybox container.
4 The ⁠SELinux label for the container, and the default ⁠SELinux label for the entire pod, which happen to be the same here.
5 The supplemental group ID for the pod (all containers).
6 The PVC name used by the pod.