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

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Sriram Chintalapati
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4 min read

Table of contents

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.

  1. Part-1 : Tips
  2. Part-2: Storage - You are reading this
  3. 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 and check that volume is bound
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 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

2Articles1WeekHashnodeKubernetesstorage