Certified Kubernetes Administrator (CKA) : Part-2: Storage

3 min readJun 5, 2022

Background

This blog is a part of a mini-series for preparing for the CKA exam. This blog would focus on the concepts related to storage.

How to read through the various blogs in this mini-series

Please feel free to go through all blogs (or) a particular blog, for a concept that You want to have a quick review. However, I would recommend going through the exam tips, as it may help You better organize Your time.

Part-1 : Tips
Part-2: Storage — You are reading this
Part-3 to Part-8: would update the links here as I publish them

Environment

Examples are executed using Docker Desktop on Mac, with Kubernetes.

% kubectl get nodes
NAME             STATUS   ROLES           AGE     VERSION
docker-desktop   Ready    control-plane   4h33m   v1.24.0

Storage Usecases

Although Kubernetes supports many types of volumes, I would discuss on three particular use cases that could probably come up in the exam. Since exam does not assume familiarity with a particular storage technology, focussing on these limited storage options may be fine.

emtpydir volume mounted inside a pod
hostpath volume mounted inside a pod
Persistent Volume Claim used to provide storage to a pod

emptydir volume mounted inside a pod

An emptydir volume lasts for the life of the Pod, even if the Container terminates and restarts.
volumes are defined in the pod spec using volumes and containers.volumeMounts fields.
The following yaml is available at the following link.

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: {}

Create Pod using above yaml
Now, describe the pod and check that volume is mounted:

% kubectl describe pod redis
...
    Mounts:
      /data/redis from redis-storage (rw)
...

hostPath volume mounted inside a pod

storage is provided from the host node’s filesystem
update the volumes.hostPath.path to your local filesystem

apiVersion: v1
kind: Pod
metadata:
  name: task-pv-hostpath
spec:
  volumes:
    - name: task-pv-hostpath
      hostPath:
        path: /Users/sriramcvn/k8s_hostpath
        type: Directory
  containers:
    - name: task-pv-container
      image: nginx
      ports:
        - containerPort: 80
          name: "http-server"
      volumeMounts:
        - mountPath: "/usr/share/nginx/html"
          name: task-pv-hostpath

Create Pod using above yaml
Now, describe the pod and check that volume is mounted:

% kubectl describe pod task-pv-hostpath
...
    Mounts:
      /usr/share/nginx/html from task-pv-hostpath (rw)
...

PersistentVolume used to provide storage for a Pod

PersistentVolume related api objects abstracts how storage is provided from how it is consumed.
we would first create a local storage class (notice the VolumeBindingMode setting — storage is not bound until pod is created)

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: local-storage
provisioner: kubernetes.io/no-provisioner
volumeBindingMode: WaitForFirstConsumer

then, we would create persistent volume
update local.path to your local filesystem

apiVersion: v1
kind: PersistentVolume
metadata:
  name: local-pv
spec:
  capacity:
    storage: 2Gi
  volumeMode: Filesystem
  accessModes:
  - ReadWriteOnce
  persistentVolumeReclaimPolicy: Delete
  storageClassName: local-storage
  local:
    path: /Users/sriramcvn/k8s_local
  nodeAffinity:
    required:
      nodeSelectorTerms:
      - matchExpressions:
        - key: kubernetes.io/hostname
          operator: In
          values:
          - docker-desktop

then, we would create persistent volume claim
storageClassName is used to map to the available persistent volumes

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: task-pv-claim
spec:
  storageClassName: local-storage
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi

Check that storage is not yet bound as pod is not created

% kubectl get sc
NAME                 PROVISIONER                    RECLAIMPOLICY   VOLUMEBINDINGMODE      ALLOWVOLUMEEXPANSION   AGE
local-storage        kubernetes.io/no-provisioner   Delete          WaitForFirstConsumer   false                  4s% kubectl get pv
NAME       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM   STORAGECLASS    REASON   AGE
local-pv   2Gi        RWO            Delete           Available           local-storage            5s% kubectl get pvc
NAME            STATUS    VOLUME   CAPACITY   ACCESS MODES   STORAGECLASS    AGE
task-pv-claim   Pending                                      local-storage   5s

Now create the pod using following 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

check that volume is bound

% kubectl apply -f pv-pod.yml 
pod/task-pv-pod created
% kubectl get pv
NAME       CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                   STORAGECLASS    REASON   AGE
local-pv   2Gi        RWO            Delete           Bound    default/task-pv-claim   local-storage            63s
% kubectl get pvc
NAME            STATUS   VOLUME     CAPACITY   ACCESS MODES   STORAGECLASS    AGE
task-pv-claim   Bound    local-pv   2Gi        RWO            local-storage   41s

References

Originally published at https://sriram.uscconsulting.net.