CKAD Ep 5: Storage & Persistent Workloads

TL;DR (Quick Summary)

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• PersistentVolume (PV): Physical storage resource provisioned in the cluster (cluster-scoped).
• PersistentVolumeClaim (PVC): Request for storage by a user/developer (namespace-scoped).
• Access Modes:
  • ReadWriteOnce (RWO): Mountable as read-write by a single node.
  • ReadOnlyMany (ROX): Mountable as read-only by many nodes.
  • ReadWriteMany (RWX): Mountable as read-write by many nodes (NFS, CephFS).
• Reclaim Policies: Retain (manual cleanup), Delete (auto-destroys backing storage).

1. Storage Abstraction Layer

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graph TD
    SC["StorageClass: fast-ssd
(provisioner: kubernetes.io/no-provisioner)"] -->|Provisions| PV["PersistentVolume: pv-data
(5Gi, hostPath: /data)"]
    PVC["PersistentVolumeClaim: pvc-app
(requests: 2Gi, accessModes: [ReadWriteOnce])"] -->|Binds to| PV
    Pod["Application Pod"] -->|volumeMounts: /usr/share/nginx/html| PVC

2. CKAD Terminal Hands-on Scenarios

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Scenario A: Create PersistentVolume

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Create a PersistentVolume named pv-analytics with 5Gi capacity, ReadWriteOnce access mode, Retain reclaim policy, using hostPath directory /mnt/data.

pv-analytics.yaml:

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv-analytics
spec:
  capacity:
    storage: 5Gi
  accessModes:
    - ReadWriteOnce
  persistentVolumeReclaimPolicy: Retain
  hostPath:
    path: /mnt/data

Apply and verify PV creation:

kubectl apply -f pv-analytics.yaml
kubectl get pv pv-analytics

Scenario B: Create PersistentVolumeClaim & Bind to Pod

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Create a PVC named pvc-analytics requesting 2Gi storage with ReadWriteOnce access mode, then mount it into a Pod named data-writer at /var/data.

pvc-analytics.yaml:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-analytics
  namespace: default
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 2Gi

data-writer-pod.yaml:

apiVersion: v1
kind: Pod
metadata:
  name: data-writer
spec:
  volumes:
    - name: storage-vol
      persistentVolumeClaim:
        claimName: pvc-analytics
  containers:
    - name: writer
      image: busybox:1.36
      command: ["sh", "-c", "echo 'Hello persistent storage' > /var/data/out.txt && sleep 3600"]
      volumeMounts:
        - name: storage-vol
          mountPath: /var/data

Verify binding status:

kubectl apply -f pvc-analytics.yaml
kubectl apply -f data-writer-pod.yaml
kubectl get pvc pvc-analytics
# Status must show: Bound

3. Storage Troubleshooting Matrix for CKAD

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Summary & Next Steps

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In this episode, we covered:

• Creating PersistentVolumes and requesting storage via PVCs.
• Access modes (ReadWriteOnce, ReadWriteMany) and reclaim policies.
• Mounting PVCs into Pod containers.
• Debugging PVC Pending status issues.

In CKAD Episode 6: Health Probes & Pod Debugging, we will master Liveness/Readiness/Startup probes and troubleshooting crashing containers!