Sharing an NFS mount across two persistent volume claims

Sharing an NFS mount across two persistent volume claims

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Overview

The following use case describes how a cluster administrator wanting to leverage shared storage for use by two separate containers would configure the solution. This example highlights the use of NFS, but can easily be adapted to other shared storage types, such as GlusterFS. In addition, this example will show configuration of pod security as it relates to shared storage.

Persistent Storage Using NFS provides an explanation of persistent volumes (PVs), persistent volume claims (PVCs), and using NFS as persistent storage. This topic shows and end-to-end example of using an existing NFS cluster and OKD persistent store, and assumes an existing NFS server and exports exist in your OKD infrastructure.

All oc commands are executed on the OKD master host.

Creating the Persistent Volume

Before creating the PV object in OKD, the persistent volume (PV) file is defined:

Example 1. Persistent Volume Object Definition Using NFS

apiVersion: v1
kind: PersistentVolume
metadata:
  name: nfs-pv (1)
spec:
  capacity:
    storage: 1Gi (2)
  accessModes:
    - ReadWriteMany (3)
  persistentVolumeReclaimPolicy: Retain (4)
  nfs: (5)
    path: /opt/nfs (6)
    server: nfs.f22 (7)
    readOnly: false

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	The volume reclaim policy `Retain` indicates that the volume will be preserved after the pods accessing it terminates.
5	This defines the volume type being used, in this case the NFS plug-in.
6	This is the NFS mount path.
7	This is the NFS server. This can also be specified by IP address.

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

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

Verify that the persistent volume was created:

# oc get pv
NAME         LABELS    CAPACITY   ACCESSMODES   STATUS      CLAIM     REASON    AGE
nfs-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. This is the use case we are highlighting in this example.

Example 2. PVC Object Definition

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: nfs-pvc  (1)
spec:
  accessModes:
  - ReadWriteMany      (2)
  resources:
     requests:
       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 right, 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 nfs-pvc.yaml, and create the PVC:

# oc create -f nfs-pvc.yaml
persistentvolumeclaim "nfs-pvc" created

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

# oc get pvc
NAME            LABELS    STATUS    VOLUME       CAPACITY   ACCESSMODES   AGE
nfs-pvc         <none>    Bound     nfs-pv       1Gi        RWX           24s
                                    (1)

1	The claim, nfs-pvc, was bound to the nfs-pv PV.

Ensuring NFS Volume Access

Access is necessary to a node in the NFS server. On this node, examine the NFS export mount:

[root@nfs nfs]# ls -lZ /opt/nfs/
total 8
-rw-r--r--. 1 root 100003  system_u:object_r:usr_t:s0     10 Oct 12 23:27 test2b
              (1)
                     (2)

1	the owner has ID 0.
2	the group has ID 100003.

In order to access the NFS mount, the container must match the SELinux label, and either run with a UID of 0, or with 100003 in its supplemental groups range. Gain access to the volume by matching the NFS mount’s groups, which will be defined in the pod definition below.

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

# setsebool -P virt_use_nfs on

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 NFS volume for read-write access:

Example 3. Pod Object Definition

apiVersion: v1
kind: Pod
metadata:
  name: hello-openshift-nfs-pod (1)
  labels:
    name: hello-openshift-nfs-pod
spec:
  containers:
    - name: hello-openshift-nfs-pod
      image: openshift/hello-openshift (2)
      ports:
        - name: web
          containerPort: 80
      volumeMounts:
        - name: nfsvol (3)
          mountPath: /usr/share/nginx/html (4)
  securityContext:
      supplementalGroups: [100003] (5)
      privileged: false
  volumes:
    - name: nfsvol
      persistentVolumeClaim:
        claimName: nfs-pvc (6)

1	The name of this pod as displayed by `oc get pod`.
2	The image run by this pod.
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 was created in the previous step.

Save the pod definition to a file, for example nfs.yaml, and create the pod:

# oc create -f nfs.yaml
pod "hello-openshift-nfs-pod" created

Verify that the pod was created:

# oc get pods
NAME                          READY     STATUS    RESTARTS   AGE
hello-openshift-nfs-pod       1/1       Running   0          4s

More details are shown in the oc describe pod command:

[root@ose70 nfs]# oc describe pod hello-openshift-nfs-pod
Name:                hello-openshift-nfs-pod
Namespace:            default (1)
Image(s):            fedora/S3
Node:                ose70.rh7/192.168.234.148 (2)
Start Time:            Mon, 21 Mar 2016 09:59:47 -0400
Labels:                name=hello-openshift-nfs-pod
Status:                Running
Reason:
Message:
IP:                10.1.0.4
Replication Controllers:    <none>
Containers:
  hello-openshift-nfs-pod:
    Container ID:    docker://a3292104d6c28d9cf49f440b2967a0fc5583540fc3b062db598557b93893bc6f
    Image:        fedora/S3
    Image ID:        docker://403d268c640894cbd76d84a1de3995d2549a93af51c8e16e89842e4c3ed6a00a
    QoS Tier:
      cpu:        BestEffort
      memory:        BestEffort
    State:        Running
      Started:        Mon, 21 Mar 2016 09:59:49 -0400
    Ready:        True
    Restart Count:    0
    Environment Variables:
Conditions:
  Type        Status
  Ready     True
Volumes:
  nfsvol:
    Type:    PersistentVolumeClaim (a reference to a PersistentVolumeClaim in the same namespace)
    ClaimName:    nfs-pvc (3)
    ReadOnly:    false
  default-token-a06zb:
    Type:    Secret (a secret that should populate this volume)
    SecretName:    default-token-a06zb
Events: (4)
  FirstSeen    LastSeen    Count    From            SubobjectPath                                      Reason        Message
  ─────────    ────────    ─────    ────            ─────────────                                      ──────        ───────
  4m        4m        1    {scheduler }                                                                  Scheduled    Successfully assigned hello-openshift-nfs-pod to ose70.rh7
  4m        4m        1    {kubelet ose70.rh7}    implicitly required container POD              Pulled        Container image "openshift3/ose-pod:v3.1.0.4" already present on machine
  4m        4m        1    {kubelet ose70.rh7}    implicitly required container POD              Created        Created with docker id 866a37108041
  4m        4m        1    {kubelet ose70.rh7}    implicitly required container POD              Started        Started with docker id 866a37108041
  4m        4m        1    {kubelet ose70.rh7}    spec.containers{hello-openshift-nfs-pod}        Pulled        Container image "fedora/S3" already present on machine
  4m        4m        1    {kubelet ose70.rh7}    spec.containers{hello-openshift-nfs-pod}        Created        Created with docker id a3292104d6c2
  4m        4m        1    {kubelet ose70.rh7}    spec.containers{hello-openshift-nfs-pod}        Started        Started with docker id a3292104d6c2

1	The project (namespace) name.
2	The IP address of the OKD 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 NFS volume being mounted. The container will not start correctly if the volume cannot mount.

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:

[root@ose70 nfs]# oc get pod hello-openshift-nfs-pod -o yaml
apiVersion: v1
kind: Pod
metadata:
  annotations:
    openshift.io/scc: restricted (1)
  creationTimestamp: 2016-03-21T13:59:47Z
  labels:
    name: hello-openshift-nfs-pod
  name: hello-openshift-nfs-pod
  namespace: default (2)
  resourceVersion: "2814411"
  selflink: /api/v1/namespaces/default/pods/hello-openshift-nfs-pod
  uid: 2c22d2ea-ef6d-11e5-adc7-000c2900f1e3
spec:
  containers:
  - image: fedora/S3
    imagePullPolicy: IfNotPresent
    name: hello-openshift-nfs-pod
    ports:
    - containerPort: 80
      name: web
      protocol: TCP
    resources: {}
    securityContext:
      privileged: false
    terminationMessagePath: /dev/termination-log
    volumeMounts:
    - mountPath: /usr/share/S3/html
      name: nfsvol
    - mountPath: /var/run/secrets/kubernetes.io/serviceaccount
      name: default-token-a06zb
      readOnly: true
  dnsPolicy: ClusterFirst
  host: ose70.rh7
  imagePullSecrets:
  - name: default-dockercfg-xvdew
  nodeName: ose70.rh7
  restartPolicy: Always
  securityContext:
    supplementalGroups:
    - 100003 (3)
  serviceAccount: default
  serviceAccountName: default
  terminationGracePeriodSeconds: 30
  volumes:
  - name: nfsvol
    persistentVolumeClaim:
      claimName: nfs-pvc (4)
  - name: default-token-a06zb
    secret:
      secretName: default-token-a06zb
status:
  conditions:
  - lastProbeTime: null
    lastTransitionTime: 2016-03-21T13:59:49Z
    status: "True"
    type: Ready
  containerStatuses:
  - containerID: docker://a3292104d6c28d9cf49f440b2967a0fc5583540fc3b062db598557b93893bc6f
    image: fedora/S3
    imageID: docker://403d268c640894cbd76d84a1de3995d2549a93af51c8e16e89842e4c3ed6a00a
    lastState: {}
    name: hello-openshift-nfs-pod
    ready: true
    restartCount: 0
    state:
      running:
        startedAt: 2016-03-21T13:59:49Z
  hostIP: 192.168.234.148
  phase: Running
  podIP: 10.1.0.4
  startTime: 2016-03-21T13:59:47Z

1	The SCC used by the pod.
2	The project (namespace) name.
3	The supplemental group ID for the pod (all containers).
4	The PVC name used by the pod.

Creating an Additional Pod to Reference the Same PVC

This pod definition, created in the same namespace, uses a different container. However, we can use the same backing storage by specifying the claim name in the volumes section below:

Example 4. Pod Object Definition

apiVersion: v1
kind: Pod
metadata:
  name: busybox-nfs-pod (1)
  labels:
    name: busybox-nfs-pod
spec:
  containers:
  - name: busybox-nfs-pod
    image: busybox (2)
    command: ["sleep", "60000"]
    volumeMounts:
    - name: nfsvol-2 (3)
      mountPath: /usr/share/busybox  (4)
      readOnly: false
  securityContext:
    supplementalGroups: [100003] (5)
    privileged: false
  volumes:
  - name: nfsvol-2
    persistentVolumeClaim:
      claimName: nfs-pvc (6)

1	The name of this pod as displayed by `oc get pod`.
2	The image run by this pod.
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 was created earlier and is also being used by a different container.

Save the pod definition to a file, for example nfs-2.yaml, and create the pod:

# oc create -f nfs-2.yaml
pod "busybox-nfs-pod" created