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State Persistence (8%)

Practice questions based on these concepts

  • Understand PersistentVolumeClaims for Storage

kubernetes.io > Documentation > Tasks > Configure Pods and Containers > Configure a Pod to Use a Volume for Storage

kubernetes.io > Documentation > Tasks > Configure Pods and Containers > Configure a Pod to Use a PersistentVolume for Storage

Define volumes

Create busybox pod with two containers, each one will have the image busybox and will run the 'sleep 3600' command. Make both containers mount an emptyDir at '/etc/foo'. Connect to the second busybox, write the first column of '/etc/passwd' file to '/etc/foo/passwd'. Connect to the first busybox and write '/etc/foo/passwd' file to standard output. Delete pod.

show

This question is probably a better fit for the 'Multi-container-pods' section but I'm keeping it here as it will help you get acquainted with state

Easiest way to do this is to create a template pod with:

kubectl run busybox --image=busybox --restart=Never -o yaml --dry-run=client -- /bin/sh -c 'sleep 3600' > pod.yaml
vi pod.yaml

Copy paste the container definition and type the lines that have a comment in the end:

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: busybox
  name: busybox
spec:
  dnsPolicy: ClusterFirst
  restartPolicy: Never
  containers:
  - args:
    - /bin/sh
    - -c
    - sleep 3600
    image: busybox
    imagePullPolicy: IfNotPresent
    name: busybox
    resources: {}
    volumeMounts: #
    - name: myvolume #
      mountPath: /etc/foo #
  - args:
    - /bin/sh
    - -c
    - sleep 3600
    image: busybox
    name: busybox2 # don't forget to change the name during copy paste, must be different from the first container's name!
    volumeMounts: #
    - name: myvolume #
      mountPath: /etc/foo #
  volumes: #
  - name: myvolume #
    emptyDir: {} #

Connect to the second container:

kubectl exec -it busybox -c busybox2 -- /bin/sh
cat /etc/passwd | cut -f 1 -d ':' > /etc/foo/passwd 
cat /etc/foo/passwd # confirm that stuff has been written successfully
exit

Connect to the first container:

kubectl exec -it busybox -c busybox -- /bin/sh
mount | grep foo # confirm the mounting
cat /etc/foo/passwd
exit
kubectl delete po busybox

Create a PersistentVolume of 10Gi, called 'myvolume'. Make it have accessMode of 'ReadWriteOnce' and 'ReadWriteMany', storageClassName 'normal', mounted on hostPath '/etc/foo'. Save it on pv.yaml, add it to the cluster. Show the PersistentVolumes that exist on the cluster

show 

vi pv.yaml
kind: PersistentVolume
apiVersion: v1
metadata:
  name: myvolume
spec:
  storageClassName: normal
  capacity:
    storage: 10Gi
  accessModes:
    - ReadWriteOnce
    - ReadWriteMany
  hostPath:
    path: /etc/foo

Show the PersistentVolumes:

kubectl create -f pv.yaml
# will have status 'Available'
kubectl get pv

Create a PersistentVolumeClaim for this storage class, called mypvc, a request of 4Gi and an accessMode of ReadWriteOnce, with the storageClassName of normal, and save it on pvc.yaml. Create it on the cluster. Show the PersistentVolumeClaims of the cluster. Show the PersistentVolumes of the cluster

show 

vi pvc.yaml
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: mypvc
spec:
  storageClassName: normal
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 4Gi

Create it on the cluster:

kubectl create -f pvc.yaml

Show the PersistentVolumeClaims and PersistentVolumes:

kubectl get pvc # will show as 'Bound'
kubectl get pv # will show as 'Bound' as well

Create a busybox pod with command 'sleep 3600', save it on pod.yaml. Mount the PersistentVolumeClaim to '/etc/foo'. Connect to the 'busybox' pod, and copy the '/etc/passwd' file to '/etc/foo/passwd'

show

Create a skeleton pod:

kubectl run busybox --image=busybox --restart=Never -o yaml --dry-run=client -- /bin/sh -c 'sleep 3600' > pod.yaml
vi pod.yaml

Add the lines that finish with a comment:

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: busybox
  name: busybox
spec:
  containers:
  - args:
    - /bin/sh
    - -c
    - sleep 3600
    image: busybox
    imagePullPolicy: IfNotPresent
    name: busybox
    resources: {}
    volumeMounts: #
    - name: myvolume #
      mountPath: /etc/foo #
  dnsPolicy: ClusterFirst
  restartPolicy: Never
  volumes: #
  - name: myvolume #
    persistentVolumeClaim: #
      claimName: mypvc #
status: {}

Create the pod:

kubectl create -f pod.yaml

Connect to the pod and copy '/etc/passwd' to '/etc/foo/passwd':

kubectl exec busybox -it -- cp /etc/passwd /etc/foo/passwd

Create a second pod which is identical with the one you just created (you can easily do it by changing the 'name' property on pod.yaml). Connect to it and verify that '/etc/foo' contains the 'passwd' file. Delete pods to cleanup. Note: If you can't see the file from the second pod, can you figure out why? What would you do to fix that?

show

Create the second pod, called busybox2:

vim pod.yaml
# change 'metadata.name: busybox' to 'metadata.name: busybox2'
kubectl create -f pod.yaml
kubectl exec busybox2 -- ls /etc/foo # will show 'passwd'
# cleanup
kubectl delete po busybox busybox2

If the file doesn't show on the second pod but it shows on the first, it has most likely been scheduled on a different node.

# check which nodes the pods are on
kubectl get po busybox -o wide
kubectl get po busybox2 -o wide

If they are on different nodes, you won't see the file, because we used the hostPath volume type. If you need to access the same files in a multi-node cluster, you need a volume type that is independent of a specific node. There are lots of different types per cloud provider (see here), a general solution could be to use NFS.

Create a busybox pod with 'sleep 3600' as arguments. Copy '/etc/passwd' from the pod to your local folder

show 

kubectl run busybox --image=busybox --restart=Never -- sleep 3600
kubectl cp busybox:etc/passwd ./passwd # kubectl cp command
# previous command might report an error, feel free to ignore it since copy command works
cat passwd

Additonal Practice Questions

List Persistent Volumes in the cluster 

kubectl get pv

Create a hostPath PersistentVolume named task-pv-volume with storage 10Gi, access modes ReadWriteOnce, storageClassName manual, and volume at /mnt/data and verify 

// task-pv-volume.yaml

apiVersion: v1
kind: PersistentVolume
metadata:
  name: task-pv-volume
  labels:
    type: local
spec:
  storageClassName: manual
  capacity:
    storage: 10Gi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: "/mnt/data"
    
kubectl create -f task-pv-volume.yaml

kubectl get pv

Create a PersistentVolumeClaim of at least 3Gi storage and access mode ReadWriteOnce and verify status is Bound 

// task-pv-claim.yaml

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: task-pv-claim
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 3Gi
      
kubectl create -f task-pv-claim.yaml

kubectl get pvc

Delete persistent volume and PersistentVolumeClaim we just created 

kubectl delete pvc task-pv-claim
kubectl delete pv task-pv-volume

Create a Pod with an image Redis and configure a volume that lasts for the lifetime of the Pod 

// emptyDir is the volume that lasts for the life of the pod

apiVersion: v1
kind: Pod
metadata:
  name: redis
spec:
  containers:
  - name: redis
    image: redis
    volumeMounts:
    - name: redis-storage
      mountPath: /data/redis
  volumes:
  - name: redis-storage
    emptyDir: {}
    
kubectl create -f redis-storage.yaml

Exec into the above pod and create a file named file.txt with the text ‘This is called the file’ in the path /data/redis and open another tab and exec again with the same pod and verifies file exist in the same path. 

// first terminal
kubectl exec -it redis-storage /bin/sh
cd /data/redis
echo 'This is called the file' > file.txt

//open another tab
kubectl exec -it redis-storage /bin/sh
cat /data/redis/file.txt

Delete the above pod and create again from the same yaml file and verifies there is no file.txt in the path /data/redis. 

kubectl delete pod redis

kubectl create -f redis-storage.yaml
kubectl exec -it redis-storage /bin/sh
cat /data/redis/file.txt // file doesn't exist

Create PersistentVolume named task-pv-volume with storage 10Gi, access modes ReadWriteOnce, storageClassName manual, and volume at /mnt/data and Create a PersistentVolumeClaim of at least 3Gi storage and access mode ReadWriteOnce and verify status is Bound 

kubectl create -f task-pv-volume.yaml
kubectl create -f task-pv-claim.yaml

kubectl get pv
kubectl get pvc

Create an nginx pod with containerPort 80 and with a PersistentVolumeClaim task-pv-claim and has a mouth path "/usr/share/nginx/html" 

// task-pv-pod.yaml

apiVersion: v1
kind: Pod
metadata:
  name: task-pv-pod
spec:
  volumes:
    - name: task-pv-storage
      persistentVolumeClaim:
        claimName: task-pv-claim
  containers:
    - name: task-pv-container
      image: nginx
      ports:
        - containerPort: 80
          name: "http-server"
      volumeMounts:
        - mountPath: "/usr/share/nginx/html"
          name: task-pv-storage
          
kubectl create -f task-pv-pod.yaml

Defining and Mounting a PersistentVolume

  1. Create a Persistent Volume named pv, access mode ReadWriteMany, storage class name shared, 512MB of storage capacity and the host path /data/config.
  2. Create a Persistent Volume Claim named pvc that requests the Persistent Volume in step 1. The claim should request 256MB. Ensure that the Persistent Volume Claim is properly bound after its creation.
  3. Mount the Persistent Volume Claim from a new Pod named app with the path /var/app/config. The Pod uses the image nginx.
  4. Check the events of the Pod after starting it to ensure that the Persistent Volume was mounted properly.
Show Solution

Create a YAML file for the Persistent Volume and create it with the command kubectl create command.

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv
spec:
  capacity:
    storage: 512m
  accessModes:
    - ReadWriteMany
  storageClassName: shared
  hostPath:
    path: /data/config

You will see that the Persistent Volume has been created but and is available to be claimed.

$ kubectl get pv
NAME   CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM   STORAGECLASS   REASON   AGE
pv     512m       RWX            Retain           Available           shared                  4s

Create a YAML file for the Persistent Volume Claim and create it with the command kubectl create command.

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: pvc
spec:
  accessModes:
    - ReadWriteMany
  resources:
    requests:
      storage: 256m
  storageClassName: shared

You will see that the Persisten Volume Claim has been created and has been bound to the Persisten Volume.

$ kubectl get pvc
NAME   STATUS   VOLUME   CAPACITY   ACCESS MODES   STORAGECLASS   AGE
pvc    Bound    pv       512m       RWX            shared         2s

$ kubectl get pv
NAME   CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM         STORAGECLASS   REASON   AGE
pv     512m       RWX            Retain           Bound    default/pvc   shared                  1m

Create a YAML file for the Pod and create it with the command kubectl create command.

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: app
  name: app
spec:
  containers:
  - image: nginx
    name: app
    volumeMounts:
      - mountPath: "/var/app/config"
        name: configpvc
    resources: {}
  volumes:
    - name: configpvc
      persistentVolumeClaim:
        claimName: pvc
  dnsPolicy: ClusterFirst
  restartPolicy: Never
status: {}

You can check the events of a Pod with the kubectl describe command. You should see an entry that indicates the successful mount.

$ kubectl describe pod app
...
Events:
  Type    Reason                 Age   From                         Message
  ----    ------                 ----  ----                         -------
  Normal  Scheduled              16s   default-scheduler            Successfully assigned app to docker-for-desktop
  Normal  SuccessfulMountVolume  16s   kubelet, docker-for-desktop  MountVolume.SetUp succeeded for volume "pv"
  Normal  SuccessfulMountVolume  16s   kubelet, docker-for-desktop  MountVolume.SetUp succeeded for volume "default-token-fsmmp"
  Normal  Pulling                15s   kubelet, docker-for-desktop  pulling image "nginx"
  Normal  Pulled                 14s   kubelet, docker-for-desktop  Successfully pulled image "nginx"
  Normal  Created                14s   kubelet, docker-for-desktop  Created container
  Normal  Started                13s   kubelet, docker-for-desktop  Started container