Volume

• A volume is a persistent storage which could be created and mounted at a location inside the containers of a pod. This allows the pod to persist the storage at that location even if it is restarted.

• Volume could be:
• Local (on the same node as the pod) - This is not acceptable if the cluster has multiple worker nodes as each node will store different data in their volumes.
• Remote (outside the cluster) - This works with multiple worker nodes as the storage is being managed remotely. The remote storage provider must follow the Container Storage Interface (CSI) standards.

Creating a local volume on the node

apiVersion: v1
kind: Pod
metadata:
  labels:
    name: frontend
spec:
  containers:
	  - name: httpd
	    image: httpd:2.4-alpine
			volumeMounts:
				- name: data-volume
					mountPath: /opt

	volumes:
		- name: data-volume
			hostPath: 
				path: /data
				type: Directory

Creating a shared remote volume on EBS

apiVersion: v1
kind: Pod
metadata:
  labels:
    name: frontend
spec:
  containers:
	  - name: httpd
	    image: httpd:2.4-alpine
			volumeMounts:
				- name: data-volume
					mountPath: /opt

	volumes:
		- name: data-volume
			awsElasticBlockStore: 
				volumeId: <volume-id>
				fsType: ext4

Persistent Volumes

• Persistent Volumes (PVs) are cluster wide storage volumes configured by the admin. This allows the volumes to be centrally configured and managed by the admin. The developer creating application (pods) can claim these persistent volumes by creating Persistent Volume Claims (PVCs).

• Once the PVCs are created, K8s binds the PVCs with available PVs based on the requests in the PVCs and the properties set on the volumes. A PVC can bind with a single PV only (there is a 1:1 relationship between a PV and a PVC). If multiple PVs match a PVC, we can label a PV and select it using label selectors in PVC.

• A smaller PVC can bind to a larger PV if all the other criteria in the PVC match the PV’s properties and there is no better option.

• When a PV is created, it is in Available state until a PVC binds to it, after which it goes into Bound state. If the PVC is deleted while the reclaim policy was set to Retain, the PV goes into Released state.

• If no PV matches the given criteria for a PVC, the PVC remains in Pending state until a PV is created that matches its criteria. After this, the PVC will be bound to the newly created PV.

• The properties involved in binding between PV and PVC are: Capacity, Access Modes, Volume Modes, Storage Class and Selector.

• List persistent volumes - k get persistentvolume or k get pv

• List persistent volume claims - k get persistentvolumeclaim or k get pvc

PV definition file

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

PVC definition file

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

Using PVCs in Pods

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
    - name: frontend
      image: nginx
      volumeMounts:
	      - mountPath: "/var/www/html"
	        name: mypd
  volumes:
    - name: mypd
      persistentVolumeClaim:
        claimName: myclaim

Reclaim Policy

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