Container Storage

Volumes

Volumes

The containers have a temporary file system, if you restart the container the file system will be deleted.

Ephemeral Volumes: exist for the lifespan of a pod. Useful for sharing data between multiple containers running in a pod
Persistent Volumes: preserve data beyond the lifespan of a pod.

Define a volume using spec.volumes[] and then reference it in a container with spec.containers[].volume.volumeMounts

Volume Types

There are a lot, some are only offered by certain cloud providers.

Some of the common ones:

emptyDir: Empty dir in a pod with read/write access. Ephemeral.
hostPath: Point to a file in the host.
configMap: Mount config data
nfs: provide a network file system. Persistent.

Ephemeral Volume

We need to define the volume first in spec.volumes and then reference it inside spec.containers.volumeMounts

Here is an example:

apiVersion: v1
kind: Pod
metadata: Pod
    name: my-container
spec:
    volumes:
    - name: logs-volume
      emptyDir: {}
    containers:
    - image: nginx
      name: my-container
      volumeMounts:
      - mounthPath: /var/logs
        name: logs-volume

Persistent Volume

We need to create two more objects to create this type of volume.

PersistentVolume (PV): piece of storage in the cluster provisioned by an admin
PersistentVolumeClaim (PVC): consumes PV, similar to how a pod consumes node resources (cpu, memory) here the pvc consumes storage from a pv

There are different ways on how we can provision this type of storage.

static provisioning: we create the PersistentVolume object by ourselves.
dynamic provisioning: automatically creates the PersistentVolume object
storage class: obj/s that already exist in the k8s cluster. (setup by admin)

Defining a PersistentVolume

We need to set capacity, access mode, and host path

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

Access Modes:

ReadWriteOnce: rw by single node
ReadOnlyMany: r by many nodes
ReadWriteMany: rw by many nodes
ReadWriteOncePod: rw mounted by a single pod

Reclaim Policy:

Retain: default. When PVC (persisted volume claim) is deleted the pv is released and can be reclaimed.
Delete: Deletion removes PV and associated storage.

How to use it.

So once we have our pv, we need to create a pvc to claim storage from that pv.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: db-pvc
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 8Gi
  storageClassName: "" # empty means we will use a statically-created pv

When we create it we need to check that the state is Bound.

In the pod, we create the volume as regular and assign it to the container.

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  volumes:
    - name: app-storage
      persistentVolumeClaim:
        claimName: db-pvc # reference the volume by pvc name
  containers:
    - name: myfrontend
      image: nginx
      volumeMounts:
      - mountPath: "/mnt/data"
        name: app-storage

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