Setting-up a Persistent Volume OVHcloud Managed Kubernetes

Last updated March 25^th, 2020.

In this tutorial we are going to guide you a simple example of setting-up a Persistent Volume (PV) on your OVHcloud Managed Kubernetes Service.

Before you begin

This tutorial presupposes that you already have a working OVHcloud Managed Kubernetes cluster, and some basic knowledge of how to operate it. If you want to know more on those topics, please look at the OVHcloud Managed Kubernetes Service Quickstart.

It also supposes you have read our Persistent Volumes on OVHcloud Managed Kubernetes guide.

Persistent Volumes (PV) and Persistent Volume Claims (PVC)

As the official documentation states:

A PersistentVolume (PV) is a piece of storage in the cluster that has been provisioned by an administrator or dynamically provisioned using Storage Classes. It is a resource in the cluster just like a node is a cluster resource.

A PersistentVolumeClaim (PVC) is a request for storage by a user. It is similar to a pod. Pods consume node resources and PVCs consume PV resources. Pods can request specific levels of resources (CPU and Memory). Claims can request specific size and access modes (e.g., can be mounted once read/write or many times read-only).

Or, if you prefer an analogy, PVC are to PV like pods are to nodes. PVC consume abstract storage resources (the PVs) as Pods consume node resources.

So you want some persistent storage on your cluster

Let's say you need some persistent storage on your cluster, some kind of network storage (for OVHcloud Managed Kubernetes that currently means a storage based on Cinder. In Kubernetes terms you will need two objects: a PersistentVolumeClaim and its associated PersistentVolume.

How do you get them? You simply need to create the PVC object in your cluster. Kubernetes will see your claim and, according to its available resources, allocate a PV corresponding to your claim.

Let's create a PVC

Copy the next YAML fragment into a test-pvc.yaml file:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: test-pvc
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 10Gi
  storageClassName: csi-cinder-high-speed

And apply it to your cluster:

kubectl apply -f test-pvc.yaml

Then you can see the PVC and the associated PV using kubectl:

kubectl get pvc
kubectl get pv

$ kubectl apply -f test-pvc.yaml
persistentvolumeclaim/test-pvc created

$ kubectl get pvc
NAME      STATUS  VOLUME                                     CAPACITY  ACCESS MODES  STORAGECLASS           AGE
test-pvc  Bound   ovh-managed-kubernetes-btw8lc-pvc-LONG-ID  10Gi      RWO           csi-cinder-high-speed  5s

$ kubectl get pv
NAME                                       CAPACITY  ACCESS MODES  RECLAIM POLICY  STATUS  CLAIM             STORAGECLASS           AGE
ovh-managed-kubernetes-btw8lc-pvc-LONG-ID  10Gi      RWO           Delete          Bound   default/test-pvc  csi-cinder-high-speed  15s

Using the PVC

Pods access storage by using the PVC as a volume. In the pod manifest you declare a volume and associate it to a PVC. The volume is then mounted to the host and into the pod.

For our example, let's create a test-pvc-pod.yaml file: that deploys a simple Nginx server using our test-pvc PVC as external volume:

apiVersion: v1
kind: Pod
metadata:
  name: test-pvc-pod
spec:
  containers:
    - name: myfrontend
      image: nginx
      volumeMounts:
      - mountPath: "/var/www/html"
        name: test-volume
  volumes:
    - name: test-volume
      persistentVolumeClaim:
        claimName: test-pvc

And apply it to the cluster:

kubectl apply -f test-pvc-pod.yaml

$ kubectl apply -f test-pvc-pod.yaml
pod/test-pvc-pod created
$ kubectl describe pod test-pvc-pod
Name:               test-pvc-pod
Namespace:          default
Priority:           0
PriorityClassName:  <none>
Node:               node-03/51.75.199.0
Start Time:         Mon, 06 Jan 2020 11:38:16 +0100
Labels:             <none>
[...]
Volumes:
  test-volume:
    Type:       PersistentVolumeClaim (a reference to a PersistentVolumeClaim in the same namespace)
    ClaimName:  test-pvc
    ReadOnly:   false
[...]
Events:
  Type    Reason                  Age   From                     Message
  ----    ------                  ----  ----                     -------
  Normal  Scheduled               70s   default-scheduler        Successfully assigned default/test-pvc-pod to node-03
  Normal  SuccessfulAttachVolume  68s   attachdetach-controller  AttachVolume.Attach succeeded for volume "ovh-managed-kubernetes-btw8lc-pvc-LONG-ID"
  Normal  Pulling                 60s   kubelet, node-03         Pulling image "nginx"
  Normal  Pulled                  59s   kubelet, node-03         Successfully pulled image "nginx"
  Normal  Created                 59s   kubelet, node-03         Created container myfrontend
  Normal  Started                 59s   kubelet, node-03         Started container myfrontend

Storage Classes

We currently support two Storage Classes on OVHcloud Managed Kubernetes: csi-cinder-high-speed and csi-cinder-classic, both based on Cinder, the OpenStack Block Storage service. The difference between them is the associated physical storage device. csi-cinder-high-speed uses SSD, while csi-cinder-classic uses traditional spinning disks. Both are distributed transparently, on three physical local replicas.

When you create a Persistent Volume Claim on your Kubernetes cluster, we provision the Cinder storage into your account. This storage is charged according to the OVH flexible cloud storage prices.

Since Kubernetes 1.11, support for expanding PersistentVolumeClaims (PVCs) is enabled by default, and it works on Cinder volumes. In order to learn how to resize them, please refer to the Resizing Persistent Volumes tutorial. Kubernetes PVCs resizing only allows to expand volumes, not to decrease them.

Access Modes

The way a PV can be mounted on a host depends on the capabilities of the resource provider. Each PV gets its own set of access modes describing that specific PV’s capabilities:

• ReadWriteOnce: the PV can be mounted as read-write by a single node
• ReadOnlyMany: the PV can be mounted read-only by many nodes
• ReadWriteMany: the PV can be mounted as read-write by many nodes

Our storage resource, Cinder, doesn't allow to mount a PV on several nodes at the same time, so you need to use the ReadWriteOnce access mode.

Reclaim policies

When you are done with a volume, you can delete the PVC, to liberate the resource. At that instant you have an unbounded PV object on your cluster, and its fate depends on the reclaim policy you have chosen in your PVC.

There are 2 possible reclaim policies:

• Retain: When the PVC is deleted, the PV still exists. The volume is considered released, but it is not yet available because the previous data remains on the volume. If you want to delete it, you must do manually.

• Delete: when the PVC is deleted, the PV and the associated storage in the external infrastructure (i.e. the Cinder storage in our case) are both deleted.

For every Storage Class there is a reclaim policy set by default, that can be changed for individual instances of PV. On our Cinder based storage classes, the reclaim policy by default is Delete, as you can verify:

kubectl get pv

$ kubectl get pv
NAME                                       CAPACITY  ACCESS MODES  RECLAIM POLICY  STATUS  CLAIM             STORAGECLASS           AGE
ovh-managed-kubernetes-btw8lc-pvc-LONG-ID  10Gi      RWO           Delete          Bound   default/test-pvc  csi-cinder-high-speed  15s

If you delete the PVC, the associated PV is also deleted:

kubectl delete pvc test-pvc
kubectl get pvc
kubectl get pv

$ kubectl delete pvc test-pvc
persistentvolumeclaim "test-pvc" deleted

$ kubectl get pvc
No resources found.

$ kubectl get pv
No resources found.

Changing the reclaim policy

To illustrate how to change the reclaim policy, let's begin by creating a new PVC using the test-pvc.yaml file:

kubectl apply -f test-pvc.yaml

List the PV and get its name:

kubectl get pv

And patch it to change its reclaim policy:

kubectl patch pv <your-pv-name> -p '{"spec":{"persistentVolumeReclaimPolicy":"Retain"}}'

where <your-pv-name> is the name of your chosen PersistentVolume.

Now you can verify that the PV has the right policy:

kubectl get pv

$ kubectl apply -f test-pvc.yaml
persistentvolumeclaim/test-pvc created

$ kubectl get pv
NAME                                      CAPACITY  ACCESS MODES  RECLAIM POLICY  STATUS  CLAIM             STORAGECLASS           AGE
ovh-managed-kubernetes-btw8lc-pvc-LONG-ID 10Gi      RWO           Delete          Bound   default/test-pvc  csi-cinder-high-speed  19s

$ kubectl patch pv ovh-managed-kubernetes-btw8lc-pvc-LONG-ID -p '{"spec":{"persistentVolumeReclaimPolicy":"Retain"}}'
persistentvolume/ovh-managed-kubernetes-btw8lc-pvc-LONG-ID patched

$ kubectl get pv
NAME                                       CAPACITY  ACCESS MODES  RECLAIM POLICY  STATUS  CLAIM             STORAGECLASS           AGE
ovh-managed-kubernetes-btw8lc-pvc-LONG-ID  10Gi      RWO           Retain          Bound   default/test-pvc  csi-cinder-high-speed  19s

In the preceding output, you can see that the volume bound to PVC default/test-pvc has reclaim policy Retain.
It will not be automatically deleted when a user deletes PVC default/test-pvc

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