Storage in Kubernetes

The following items are related to data storage in Kubernetes:

• Volume : shares data between containers in a Pod
• PersistentVolume (PV) : storage resource
• PersistentVolumeClaim (PVC) : request for storage
• StorageClass : dynamique storage provisioning
• StatefulSet : used to manage stateful applications

PersistentVolume / PersistentVolumeClaim

A PersistentVolume provides the storage, it can be provisioned either statically (by an administrator) or dynamically (by a StorageClass).
A PersistentVolumeClaim is a request for storage, it decouples an application from the storage solution and allows storage to be consumed by a Pod

StorageClass

A StorageClass is a resource which allows PV to be dynamically created. It is referenced within the specification of a PVC.

Provisioning with a StorageClass

The local cluster, based on k3s, used in this section has a default storage class based on the local-path provisioner. It allows to create storage on the VM filesystem. The specification of this StorageClass is the following one:

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: local-path
provisioner: rancher.io/local-path
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer
...

In order to use this StorageClass, we define a PVC (PersistentVolumeClaim) referencing the StorageClass:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: mysql-pv-claim
spec:
  accessModes:
    - ReadWriteOnce
  storageClassName: local-path
  resources:
    requests:
      storage: 1Gi

Next we reference this PVC in our application:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: mysql
spec:
  selector:
    matchLabels:
      app: mysql
  template:
    metadata:
      labels:
        app: mysql
    spec:
      containers:
      - image: mysql:5.6
        name: mysql
        env:
        - name: MYSQL_ROOT_PASSWORD
          valueFrom:
            secretKeyRef:
              name: mysql-pass
              key: password
        volumeMounts:
        - name: mysql-storage
          mountPath: /var/lib/mysql
      volumes:
      - name: mysql-storage
        persistentVolumeClaim:
          claimName: mysql-pv-claim

Creating the Deployment and the PVC triggers the creation of a PV of 1G in size.

Using Storage in the VotingApp

The previous exercise left the app with the following components:

We will now use the existing StorageClass and define PVCs to request storage for both databases used in the VotingApp: