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How to Set Up and Manage Persistent Volumes on OVHcloud Managed Kubernetes

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Michael M.
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Before you begin

This tutorial assumes 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 about these topics, please look at the OVHcloud Managed Kubernetes Service Quickstart.

When a Persistent Volume resource is created inside a Managed Kubernetes cluster, an associated Public Cloud Block Storage volume is automatically created with it, with a lifespan depending on the parent cluster’s lifespan. This volume is charged hourly and will appear in your Public Cloud project. For more information, please refer to the following documentation: Volume Block Storage price

Persistent Volumes

Before going further, let’s review how Kubernetes deals with data storage. There are currently two kinds of storage available with Kubernetes: Volumes and Persistent Volumes.

Kubernetes Volumes exist only while the Pod (containers) exists, and are deleted when it is deleted.
As a result, Kubernetes Volumes are only useful for storing temporary data.

Kubernetes Persistent Volumes allow us to work with non-volatile data in Kubernetes. Persistent Volumes are not tied to the pod lifecycle, or to a single Pod. Pods can claim Persistent Volumes, thus making the data available to them.

You must be wondering how Persistent Volumes are compatible with the rule that containers should be stateless – one of the most important principles of best practice for containers. It’s important to note that as the Kubernetes ecosystem has matured, and persistent storage solutions have emerged, this rule is no longer universally applicable.

What are the use cases for Persistent Volumes in Kubernetes?
Well, the most common application is databases. Database data, by definition, is meant to be persistent, and not linked to a specific pod, so Persistent Volumes are needed to deploy them in Kubernetes.

When deploying a database in Kubernetes, we follow these steps:

  • Create and configure a Pod for the database engine
  • Attach a Persistent Volume to the Pod using a Persistent Volume Claim
  • Mount the claimed volume in the Pod

In order to use a Persistent Volume (PV) on a Kubernetes cluster, you must create a Persistent Volume Claim (PVC). Persistent Volume Claims are requests to provision a Persistent Volume of a specific type and configuration. The different kinds of persistent storage are defined by cluster admins, using Storage Classes.

When you need a Persistent Volume, you create a Persistent Volume Claim, and choose a Storage Class from those made available by the cluster administrators. Depending on the Storage Class, an actual infrastructure volume storage device is provisioned on your account and a Persistent Volume is created on this physical device. The Persistent Volume is a sort of virtual storage instance over the infrastructure virtual storage.

pv1.png

Persistent Volumes on OVHcloud Managed Kubernetes

We currently support several Storage Classes on OVHcloud Managed Kubernetes:

  • csi-cinder-classic
  • csi-cinder-high-speed

You can display them with the kubectl get storageclass command:

$ kubectl get storageclass
NAME                              PROVISIONER                RECLAIMPOLICY   VOLUMEBINDINGMODE   ALLOWVOLUMEEXPANSION   AGE
csi-cinder-classic                cinder.csi.openstack.org   Delete          Immediate           true                   42d
csi-cinder-high-speed (default)   cinder.csi.openstack.org   Delete          Immediate           true                   42d

All of them are based on Cinder, the OpenStack Block Storage service.

The difference between them is the associated physical storage device. The csi-cinder-high-speed use SSD, while csi-cinder-classic use 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 OVHcloud Flexible Cloud Block Storage Policy.

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, nor to decrease them.

Setting up a Persistent Volume

In this guide, we are going to use a simple example: a small Nginx web server, running in a Pod, created by a Deployment, attached to a Persistent Volume.

pv2.png

Create a namespace:

kubectl create ns nginx-example

Define a Persistent Volume Claim (PVC) in a file named pvc.yaml with the following content:

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: nginx-logs
  namespace: nginx-example
  labels:
    app: nginx
spec:
  storageClassName: csi-cinder-high-speed
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi

Apply the YAML manifest:

kubectl apply -f pvc.yaml

This PVC will create a PV, with a size of 1GB, dynamically according to the storage class csi-cinder-high-speed.

Check the new PVC and PV have been correctly created:

kubectl get pvc -n nginx-example
kubectl get pv

You should see the following:

$ kubectl create ns nginx-example
namespace/nginx-example created

$ kubectl apply -f pvc.yaml
persistentvolumeclaim/nginx-logs created

$ kubectl get pvc -n nginx-example
NAME         STATUS   VOLUME                                                                   CAPACITY   ACCESS MODES   STORAGECLASS            AGE
nginx-logs   Bound    ovh-managed-kubernetes-d6r47l-pvc-a6025a24-c572-4c28-b5e7-c6f8311aa47f   1Gi        RWO            csi-cinder-high-speed   21s

$ kubectl get pv
NAME                                                                     CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                      STORAGECLASS            REASON   AGE
ovh-managed-kubernetes-d6r47l-pvc-a6025a24-c572-4c28-b5e7-c6f8311aa47f   1Gi        RWO            Delete           Bound    nginx-example/nginx-logs   csi-cinder-high-speed            19s

As you can see, the Persistent Volume is created and is bound to the Persistent Volume Claim you created.

Now create a file named deployment.yaml with the following content:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  namespace: nginx-example
spec:
  strategy:
    type: Recreate
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      volumes:
        - name: nginx-logs
          persistentVolumeClaim:
            claimName: nginx-logs
      containers:
        - image: nginx:1.7.9
          name: nginx
          ports:
            - containerPort: 80
          volumeMounts:
            - mountPath: "/var/log/nginx"
              name: nginx-logs
              readOnly: false

Apply it:

kubectl apply -f deployment.yaml

If you look at the deployment part of this manifest, you will see that we have defined a .spec.strategy.type. It specifies the strategy used to replace old Pods with new ones, and we have set it to Recreate, so all existing Pods are killed before new ones are created.

We do so as the Storage Class we are using, csi-cinder-high-speed, only supports a ReadWriteOnce, so we can only have one pod writing on the Persistent Volume at any given time.

Thanks to the Deployment, Kubernetes will create one Pod with one container of Nginx and mount a volume on it in the path /var/log/nginx. The Nginx container will have the permission to write in this folder.

Create a service for the Nginx container in a file named svc.yaml:

apiVersion: v1
kind: Service
metadata:
  labels:
    app: nginx
  name: nginx-service
  namespace: nginx-example
spec:
  ports:
    - port: 80
      targetPort: 80
  selector:
    app: nginx
  type: LoadBalancer

Apply it:

kubectl apply -f svc.yml

Wait until you get an external IP:

kubectl -n nginx-example get svc/nginx-service -w

And do some calls to the URL to generate some access logs:

export NGINX_URL=$(kubectl get svc nginx-service -n nginx-example -o jsonpath='{.status.loadBalancer.ingress[].ip}')
echo Nginx URL: http://$NGINX_URL/

curl -I http://$NGINX_URL/

You should see the following:

$ kubectl apply -f deployment.yaml

kubedeployment.apps/nginx-deployment created

$ kubectl get po -n nginx-example
NAME                                READY   STATUS    RESTARTS   AGE
nginx-deployment-766444c4d9-bqnz7   1/1     Running   0          41s

$ kubectl apply -f svc.yaml
service/nginx-service created

$ kubectl -n nginx-example get svc/nginx-service -w

NAME            TYPE           CLUSTER-IP     EXTERNAL-IP   PORT(S)        AGE
nginx-service   LoadBalancer   10.3.128.254        80:31622/TCP   30s
nginx-service   LoadBalancer   10.3.128.254        80:31622/TCP   51s
nginx-service   LoadBalancer   10.3.128.254   152.228.168.120   80:31622/TCP   51s

$ export NGINX_URL=$(kubectl get svc nginx-service -n nginx-example -o jsonpath='{.status.loadBalancer.ingress[].ip}')
echo Nginx URL: http://$NGINX_URL/
Nginx URL: http://152.228.168.120/

$ curl -I http://$NGINX_URL/
HTTP/1.1 200 OK
Server: nginx/1.7.9
Date: Thu, 24 Mar 2022 12:31:12 GMT
Content-Type: text/html
Content-Length: 612
Last-Modified: Tue, 23 Dec 2014 16:25:09 GMT
Connection: keep-alive
ETag: "54999765-264"
Accept-Ranges: bytes

$ curl -I http://$NGINX_URL/
HTTP/1.1 200 OK
Server: nginx/1.7.9
Date: Thu, 24 Mar 2022 12:31:25 GMT
Content-Type: text/html
Content-Length: 612
Last-Modified: Tue, 23 Dec 2014 16:25:09 GMT
Connection: keep-alive
ETag: "54999765-264"
Accept-Ranges: bytes

Now we need to connect to the pod to read the log file and verify that our logs are written.

First, get the name of the Nginx running pod:

export POD_NAME=$(kubectl get po -n nginx-example -o name)

And then connect to it and see your access logs:

kubectl -n nginx-example exec $POD_NAME -c nginx -- cat /var/log/nginx/access.log

You should see the following:

$ export POD_NAME=$(kubectl get po -n nginx-example -o name)

$ kubectl -n nginx-example exec $POD_NAME -c nginx -- cat /var/log/nginx/access.log
10.2.1.0 - - [24/Mar/2022:12:31:03 +0000] "GET / HTTP/1.1" 200 612 "-" "curl/7.64.1" "-"
10.2.2.0 - - [24/Mar/2022:12:31:12 +0000] "HEAD / HTTP/1.1" 200 0 "-" "curl/7.64.1" "-"

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