Volumes

Essentialy - a directory that will be accessible by POD, preserving state independently of POD lifecycle.
Example (simplest) usage of volume

apiVersion: v1
kind: Pod
metadata:
  labels:
    app: pod
  name: example
spec:
  containers:
    - image: httpd
      name: example
      volumeMounts:
      - mountPath: /var/tmp
        name: vartmp
  volumes:
  - name: vartmp
    hostPath:
      path: /mnt/testing 

There are numerous volume types that can be used, hostPath is actually the most test-purpose only.
It is possible to mount ConfigMap as a volume.

Typically you have PersistentVolume (PV) resource (or have some Cloud provider prepare that for you) and you just mount it to the PODs. The persistentVolumeClaim (PVC) is used to represent the request for the storage (PV).

There are two main types of Persistent Volumes provisioning: 1. Static
It is the administrator that prepares volumes. Use selector in PVC, without providing storageClassName to use static provisioning

1. Dynamic
   Based on StorageClass the PersistentVolume will be automatically created for given PVC. Use storageClassName.

Example POD with PersistentVolume Statically provisioned (StorageClass is not used at all)

kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: local-storage
provisioner: kubernetes.io/no-provisioner
volumeBindingMode: WaitForFirstConsumer
---
apiVersion: v1
kind: PersistentVolume
metadata:
  name: example-pv
  labels:
    name: example-pv
spec:
  capacity:
    storage: 1Mi  
  volumeMode: Filesystem
  accessModes:
  - ReadWriteOnce
  persistentVolumeReclaimPolicy: Delete
  ##storageClassName: local-storage
  #hostPath:
  #  path: /mnt/testing
  local:
    path: /mnt/testing
  nodeAffinity:
    required:
      nodeSelectorTerms:
      - matchExpressions:
        - key: kubernetes.io/hostname
          operator: In
          values:
          - k8s1
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: example-pvc
spec:
  ##storageClassName: local-storage
  selector:
    matchLabels:
        name: example-pv
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 1Mi
--- 
apiVersion: v1
kind: Pod
metadata:
  labels:
    app: pod
  name: example
spec:
  containers:
    - image: httpd
      name: example
      volumeMounts:
      - mountPath: /var/tmp
        name: vartmp
  volumes:
  - name: vartmp
    persistentVolumeClaim:
      claimName: example-pvc

The persistentVolumeReclaimPolicy tells what happens to PersistentVolume after released from claim. Defaults:

Retain for statically provisioned PersistentVolume

Delete for dynamically provisioned PersistentVolume

The PersistentVolume must support different (than default) policy setting.