Kubernetes Volumes (emptyDir,hostPath,Persistence Volume)

When a Container crashes, kubelet will restart it, but the files will be lost - the Container starts with a clean state. Second, when running Containers together in a Pod it is often necessary to share files between those Containers. The Kubernetes Volume abstraction solves both of these problems.

emptyDir

An emptyDir volume is first created when a Pod is assigned to a Node, and exists as long as that Pod is running on that node. As the name says, it is initially empty. Containers in the Pod can all read and write the same files in the emptyDir volume, though that volume can be mounted at the same or different paths in each Container. When a Pod is removed from a node for any reason, the data in the emptyDir is deleted forever

Lab

# nginx-emptydir.yaml

apiVersion: v1

kind: Pod

metadata:

  name: nginx-emptydir

spec:

  containers:

  - name: nginx-container

    image: nginx

    volumeMounts:

    - name: test-vol

      mountPath: /test-mnt

  volumes:

  - name: test-vol

    emptyDir: {}

2. Create & Display Pod with emptyDir volume

kubectl create -f nginx-emptydir.yaml

kubectl get po -o wide

kubectl exec nginx-emptydir df /test-mnt

kubectl describe pod nginx-emptydir

*******************************************************************

3. Cleanup

kubectl delete po nginx-emptydir

hostPath

A hostPath volume mounts a file or directory from the host node's filesystem into your Pod. This is not something that most Pods will need, but it offers a powerful escape hatch for some applications.

LAB

# 1. HostPath YAML file

apiVersion: v1

kind: Pod

metadata:

  name: nginx-hostpath

spec:

  containers:

    - name: nginx-container

      image: nginx

      volumeMounts:

      - mountPath: /test-mnt

        name: test-vol

  volumes:

  - name: test-vol

    hostPath:

      path: /test-vol

*******************************************************************

# 2. Create and Display HostPath

kubectl create -f nginx-hostpath.yaml

kubectl get po

kubectl exec nginx-hostpath df /test-mnt

*******************************************************************

3. Test: Creating "test" file underlying host dir & accessing from from pod

From HOST:

~~~~~~~~~~

cd /test-vol

echo "From Host" > from-host.txt

cat from-host.txt

From POD:

~~~~~~~~

kubectl exec nginx-hostpath cat /test-mnt/from-host.txt

*******************************************************************

4. Test: Creating "test" file inside the POD & accessing from underlying host dir

From POD:

~~~~~~~~~

kubectl exec nginx-hostpath -it -- /bin/sh

cd /test-mnt

echo "From Pod" > from-pod.txt

cat from-pod.txt

From Host:

~~~~~~~~~~

cd /test-vol

ls

cat from-pod.txt

*******************************************************************

5. Clean up

kubectl delete po nginx-hostpath

kubectl get po

ls /test-vol

PersistentVolume (PV)

A PersistentVolume (PV) is a piece of storage in the cluster that has been provisioned by an administrator or dynamically provisioned using Storage Classes. It is a resource in the cluster just like a node is a cluster resource. PVs are volume plugins like Volumes, but have a lifecycle independent of any individual Pod that uses the PV. This API object captures the details of the implementation of the storage, be that NFS, iSCSI, or a cloud-provider-specific storage system.

PersistentVolumeClaim (PVC) 

A PersistentVolumeClaim (PVC) is a request for storage by a user. It is similar to a Pod. Pods consume node resources and PVCs consume PV resources. Pods can request specific levels of resources (CPU and Memory). Claims can request specific size and access modes (e.g., they can be mounted ReadWriteOnce, ReadOnlyMany or ReadWriteMany

#pv.yaml

apiVersion: v1

kind: PersistentVolume

metadata:

  name: pv-hostpath

  labels:

    type: local

spec:

  storageClassName: manual

  capacity:

    storage: 1Gi

  accessModes:

    - ReadWriteOnce

  hostPath:

    path: "/kube"

#create pv with below command
kubectl create -f pv.yaml

#pvc.yaml

apiVersion: v1

kind: PersistentVolumeClaim

metadata:

  name: pvc-hostpath

spec:

  storageClassName: manual

  accessModes:

    - ReadWriteOnce

  resources:

    requests:

      storage: 100Mi

#Create pvc

kubectl create -f pvc.yaml

#pod.yaml

apiVersion: v1

kind: Pod

metadata:

  name: busybox

spec:

  volumes:

  - name: host-volume

    persistentVolumeClaim:

      claimName: pvc-hostpath

  containers:

  - image: busybox

    name: busybox

    command: ["/bin/sh"]

    args: ["-c", "sleep 600"]

    volumeMounts:

    - name: host-volume

      mountPath: /mydata

kubectl create -f pod.yaml

kubectl exec -it busybox  touch /mydata/test.txt

Example

Persistent volume with Delete Reclaim Policy Delete. The PV should be deleted when PVC is deleted.

pv.yaml

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv-hostpath
  labels:
    type: local
spec:
  storageClassName: manual
  capacity:
    storage: 1Gi
  persistentVolumeReclaimPolicy: Delete
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: "/tmp/kube"

pvc.yaml

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-hostpath
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 100Mi

pod.yaml

apiVersion: v1
kind: Pod
metadata:
  name: busybox
spec:
  volumes:
  - name: host-volume
    persistentVolumeClaim:
      claimName: pvc-hostpath
  containers:
  - image: busybox
    name: busybox
    command: ["/bin/sh"]
    args: ["-c", "sleep 600"]
    volumeMounts:
    - name: host-volume
      mountPath: /mydata