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Persistent_Volumes_and_Claims

Persistent Volumes and Claims​

Persistent Volumes​

A persistent volume (pv) is a storage abstraction used to retain data longer than the Pod using it. Pods define a volume of type persistentVolumeClaim (pvc). The cluster then attaches the persistentVolume. PVs are not namespaced while PVCs are.

The following are the phases of Persistent Storage:

  1. Provision: Provisioning can be from the PVs created in advance by the cluster administrator or requested from a dynamic provider (Google/AWS, etc).

  2. Bind: Binding occurs when a control loop on the master notices the PVC and locates a matching PV.

  3. Use: When the bound volume is mounted for the Pod to use and is kept in this phase as long as the Pod requires.

  4. Release: Happens when the Pod is done with the volume and an API request to delete the PVC is sent.

  5. Reclaim: could be:

    • Retain: keeps data intact allowing an admin to handle storage and data.
    • Delete: tells volume plugin to delete the API object as well as storage.
    • Recycle: run rm -rf /mountpoint and then makes it available to a new claim.

We can see the PV and PVC:

kubectl get pv
kubectl get pvc

The following specification is a basic declaration of a PV using hostPath:

apiVersion: v1
kind: PersistentVolume
metadata:
  name: 10Gpv01
  labels:
    type: local
spec:
  capacity:
    storage: 10Gi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: "/somepath/data01"

Persistent Volume Claim​

With a created PV in the cluster, we can write a manifest for a claim and use that claim in the Pod specification using persistentVolumeClaim.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: myclaim
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 8Gi

In the Pod:

spec:
  containers:
...
  volumes:
    - name: test-volume
      persistentVolumeClaim:
        claimName: myclain