Kubernetes Volumes

  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

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