Storage

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This section is a refresher that provides an overview of the primary Kubernetes resources related to storage. At the end of this section, please complete the exercises to put these concepts into practice.

Volume is a property that can be defined in a Pod’s specification (at .spec.volumes) and mounted in a container’s filesystem (.spec.containers.volumeMounts). It allows decoupling a container from the storage solution and containers of the same pod to share data.

Among the available types of volumes:

  • emptyDir
  • configMap
  • Secret
  • hostPath
  • downwardAPI

The following specification defines a MongoDB Pod with an emptyDir volume mounted at /data/db in the container filesystem. It allows the data to persist outside the container’s filesystem but still on the host filesystem.

apiVersion: v1
kind: Pod
metadata:
  name: mongo
spec:
  containers:
  - name: mongo
    image: mongo:7.0
    volumeMounts:
    - name: data
      mountPath: /data/db
  volumes:
  - name: data
    emptyDir: {}

We should not use a volume for persistent storage; instead we should use PersistentVolume and PersistentVolumeClaim.

A PersistentVolume (PV) is a resource used to provide storage, either statically or dynamically. It decouples an application from the storage solution.

Various types of PersistentVolume are available, including:

  • cephfs
  • csi
  • fc
  • hostPath
  • iscsi
  • local
  • nfs
  • rbd

The main properties of a PV are the following:

  • volumeMode defines how the volume is presented to a Pod

    • Filesystem (default)
    • Block

  • accessModes specifies how we can mount the volume in the Pod

    • ReadWriteOnce (RWO): the volume can be mounted as read-write by a single Node
    • ReadOnlyMany (ROX): the volume can be mounted as read-only by multiple Nodes at the same time
    • ReadWriteMany (RWX): the volume can be mounted as read-write by multiple Nodes at the same time
    • ReadWriteOncePod (RWOP): the volume can be mounted as read-write by a single Pod only

  • persistentVolumeReclaimPolicy defines what happens to the PersistentVolume after the associated PersistentVolumeClaim is deleted

    • Retain (default if manually created): the PersistentVolume is not deleted, a manual data recovery is possible
    • Delete (default if dynamically created): the PersistentVolume and the underlying storage are deleted

The specification below defines a PersistentVolume that will be tied to a single Node and offer 1G of storage from this Node’s filesystem.

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv
spec:
  accessModes:
  - ReadWriteOnce
  capacity:
    storage: 1Gi
  hostPath:
    path: /tmp/data

A PersisentVolumeClaim (PVC) is a storage request. It specifies the storage requirements regqrding size and access mode, and is bound to a PersistentVolume that meets those requirements.

The following specification defines a PersistentVolumeClaim, which requests 500 MB of storage with a RWO mode.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 500Mi

A Pod can request storage by referencing the PVC in its specification as follows. When a PVC is bound to a PV, the PV is mounted into the container’s filesystem.

apiVersion: v1
kind: Pod
metadata:
  name: db
spec:
  containers:
  - image: mongo:5.0
    name: mongo
    volumeMounts:
    - name: data
      mountPath: /data/db
  volumes:
  - name: data
    persistentVolumeClaim:
      claimName: pvc

A StorageClass defines how to dynamically create PersistentVolumes without needing to pre-provision them.

More information in the official documentation at https://kubernetes.io/docs/concepts/storage/

— Practice — 🔗

You can now jump to the Exercises part to learn and practice the concepts above.

Exercises