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Storage

Container -> Persistent Volume Claim -> Persistent Volume -> External Storage

Regular volume can be set up relatively easily within a pod/container specification.

volumes: In the Pod spec, these specify the storage volumes available to the Pod. They specify the volumes available to the Pod. They specify the volumes type and other data that determines where and how the data is actually stored.

volumeMounts: In the Pod spec, these specify the location of the volumes within the container.

Volume Types

  • hostPath: Stores data in a specific directory on the k8s node.
  • emptyDir: Stores data in a directory created by the k8s node. When the pod is deleted, the directory is deleted.
  • gcePersistentDisk: Stores data on a GCE persistent disk.
apiVersion: v1
kind: Pod
metadata:
  labels:
    run: volume-pod
  name: volume-pod
  namespace: dev
spec:
  containers:
  - name: busybox
    image: busybox
    command: ['sh', '-c', 'echo Success > /output/success.txt']
    volumeMounts:
    - name: my-volume
      mountPath: /output
  restartPolicy: Never
  volumes:
    - name: my-volume
      hostPath:
        path: /var/data
status: {}

Then run get pod with wide to see the status of the pod.

kubectl get pod volume-pod -n dev -o wide

Then ssh to worker node and cat the output file.

emptyDir Example

apiVersion: v1
kind: Pod
metadata:
  labels:
    run: shared-volume-pod
  name: shared-volume-pod
  namespace: dev
spec:
  containers:
  - name: busybox1
    image: busybox
    command: ['sh', '-c', 'while true; do echo Success! > /output/output.txt; sleep 5; done']
    volumeMounts:
    - name: my-volume
      mountPath: /output
  - name: busybox2
    image: busybox
    command: ['sh', '-c', 'while true; do cat /input/output.txt; sleep 5; done']
    volumeMounts:
    - name: my-volume
      mountPath: /input
  volumes:
    - name: my-volume
      emptyDir: {}
  restartPolicy: Never

To view the second container logs

kubectl logs shared-volume-pod -c busybox2 -n dev

Persistent Volumes

Storage Classes

Storage classes allow k8s admins to specify the types of storage services they offer on their platform.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: localdisk
provisioner: kubernetes.io/no-provisioner

we can create StorageClass called slow to describe low performance storage but inexpensive storage. but another called fast for high performance storage but more constly resources.

Allow Volume Expansion

The allowVolumeExpansion field specifies whether the storage class allows volume expand. If set to false, the storage class will not allow volume expand. If set to true, the storage class will allow volume expand. If not specified, the default value is false.

ReClaim Policy

The reclaimPolicy field specifies the reclaim policy of the storage class. The default value is Delete.

  • Retain: The storage is not reclaimed by the system. Admins has to manually delete the storage.
  • Delete: The storage is reclaimed by the system.
  • Recycle: The storage is reclaimed and recycled.

Example

First create storage class

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: localdisk
provisioner: kubernetes.io/no-provisioner
allowVolumeExpansion: true

Then create a peristent volume

kind: PersistentVolume
apiVersion: v1
metadata:
  name: my-pv
spec:
  storageClassName: localdisk
  persistentVolumeClaim: Recycle
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: /var/output