PersistentVolume

Persistent-Volume documentation:

• Online

A PersistentVolume (PV) is a place where you can store stuff. It can be mounted into a pod, so your application use it as storage. It doesn’t disappear when the pod is deleted. In other words, it’s persistent.

PersistentVolumes are created by PersistentVolumeClaims (PVC).

A PersistentVolumeClaim is a claim for a PersistentVolume from a storage provider.

With a PersistentVolumeClaim your app can request, for instance, 10GB of space. And it will automagically provision a volume that can be used without any concern of what the underlying technology is.

Caveat:

Default Persistent Volume on GCP does not allow multiple pods to mount a PVC at a time. Access mode must be ReadWriteOnce which means that you will have to scale your deployment down to 1 for as long as you are doing this task. If you use more than one replica, only one will be able to start successfully.

So, in your deployment.yaml (either one, for example for the workshop-api / backend), edit replicas to be 1:

spec:
  replicas: 1

Task: Create a PersistentVolumeClaim

Let’s make a PersistentVolumeClaim, requesting 10Mb for our app. Create a file, pvc.yaml with the contents:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: my-claim
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 10Mi

In your deployment.yaml (same deployment you edited above), add a volume from the claim:

...
    spec:
      ...
      volumes:
      - name: my-persistent-volume
        persistentVolumeClaim:
          claimName: my-claim
...

and then mount the volume at a path:

...
    containers:
      ...
      volumeMounts:
        - name: my-persistent-volume
          mountPath: /mydata
...

Let’s apply the claim:

kubectl apply -f pvc.yaml

Then verify that it has been “bound”:

kubectl get pvc

NAME       STATUS    VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
my-claim   Bound     pvc-10a0ecc2-4962-11e8-978c-e4b318337de0   10Mi       RWO            hostpath       7m

Now we can apply our updated deployment.yaml:

kubectl apply -f deployment.yaml

Since the deployment has been updated since last time we applied it, Kubernetes should restart the pod in the deployment.

Let’s verify that a PersistentVolume has been created as the claim requested:

kubectl get pvc
NAME                                       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS    CLAIM                               STORAGECLASS   REASON    AGE
pvc-10a0ecc2-4962-11e8-978c-e4b318337de0   10Mi       RWO            Delete           Bound     guybrush/my-claim                   hostpath                 15m
...

Task: Verify that the PersistentVolumeClaim works

You can do this by

• writing to a file in the now mounted volume in the pod
• delete the pod
• verify that the file still exists when a new pod is started

Tip: Use kubectl exec -it my-pod-name-8d77fb47-p529c sh to open a shell to the pod’s container.

kubectl get po
kubectl exec -it sample-app-deployment-7756ccb788-vflmz sh
/ # cd mydata
/mydata # ls
/mydata # echo hello there > hello
/mydata # exit

kubectl delete pod sample-app-deployment-7756ccb788-vflmz

kubectl exec -it sample-app-deployment-a9w84f98n-fsdfm sh
/ # cd mydata
/mydata # ls
hello
/mydata # cat hello
hello there
/mydata # exit