Directly specifying a persistent volume in a configuration file makes a tight couple with a particular infrastructure. For the preceding example (tomcat-log volume), pdName is gce-pd-1 and Volume type is gcePersistentDisk. From a container management point of view, the pod definition shouldn't be locked into the specific environment because the infrastructure could be different based on the environment. The ideal pod definition should be flexible or abstract the actual infrastructure and specify only volume name and mount point.

Consequently, Kubernetes provides an abstraction layer that associates the pod with the persistent volume, which is called the Persistent Volume Claim (PVC). This allows us to decouple from the infrastructure. The Kubernetes administrator just needs to pre-allocate the size of the persistent volume in advance. Then Kubernetes will bind the persistent volume and the PVC as follows:

The following example is a definition of a pod that uses a PVC; let's reuse the previous example (gce-pd-1) to register with Kubernetes first:

$ cat pv-gce-pd-1.yml 
apiVersion: "v1"
kind: "PersistentVolume"
metadata:
  name: pv-1
spec:
  storageClassName: "my-10g-pv-1"
  capacity:
    storage: "10Gi"
  accessModes:
    - "ReadWriteOnce"
  gcePersistentDisk:
    fsType: "ext4"
    pdName: "gce-pd-1"

$ kubectl create -f pv-gce-pd-1.yml 
persistentvolume/pv-1 created

$ kubectl get pv
NAME      CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM     STORAGECLASS   REASON    AGE
pv-1      10Gi       RWO            Retain           Available             my-10g-pv-1              11s 

//create PVC specify storageClassName as "my-10g-pv-1"
$ cat pvc-1.yml 
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
    name: pvc-1
spec:
  storageClassName: "my-10g-pv-1"
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 10Gi

$ kubectl create -f pvc-1.yml 
persistentvolumeclaim/pvc-1 created


//check PVC status is "Bound"
$ kubectl get pvc
NAME      STATUS    VOLUME    CAPACITY   ACCESS MODES   STORAGECLASS   AGE
pvc-1     Bound     pv-1      10Gi       RWO            my-10g-pv-1    7s


//check PV status is also "Bound"
$ kubectl get pv
NAME      CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS    CLAIM           STORAGECLASS   REASON    AGE
pv-1      10Gi       RWO            Retain           Bound     default/pvc-1   my-10g-pv-1              2m

Now the tomcat setting has been decoupled from the GCE persistent volume, and bound to the abstracted volume, pvc-1:

$ cat tomcat-pvc.yml 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: tomcat
spec:
  replicas: 1
  selector:
    matchLabels:
      run: tomcat
  template:
    metadata:
      labels:
        run: tomcat
    spec:
      containers:
        - image: tomcat
          name: tomcat
          ports:
          - containerPort: 8080
          volumeMounts:
          - mountPath: /usr/local/tomcat/logs
            name: tomcat-log
      volumes:
      - name: tomcat-log
        persistentVolumeClaim:
          claimName: "pvc-1"