Day 6: Storage and Data Persistence

What You'll Learn Today

• How Kubernetes storage differs from Docker volumes
• PersistentVolume (PV) and PersistentVolumeClaim (PVC)
• Dynamic provisioning with StorageClass
• Practical database data persistence

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From Docker Volumes to Kubernetes Storage

In Docker Day 6, you learned about Volumes, Bind Mounts, and tmpfs. Kubernetes also needs data persistence, but a multi-node cluster requires additional mechanisms.

Docker	Kubernetes	Description
Volume	PersistentVolume	Storage resource
docker volume create	PV / StorageClass	Storage provisioning
-v myvolume:/data	PersistentVolumeClaim	Storage request
tmpfs	emptyDir (medium: Memory)	Temporary in-memory storage

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Volume Types

emptyDir

A temporary volume that exists as long as the Pod. Used for sharing data between containers in a Pod.

apiVersion: v1
kind: Pod
metadata:
  name: shared-volume-pod
spec:
  containers:
    - name: writer
      image: busybox:1.37
      command: ["sh", "-c", "echo 'Hello' > /data/message.txt && sleep 3600"]
      volumeMounts:
        - name: shared-data
          mountPath: /data
    - name: reader
      image: busybox:1.37
      command: ["sh", "-c", "cat /data/message.txt && sleep 3600"]
      volumeMounts:
        - name: shared-data
          mountPath: /data
  volumes:
    - name: shared-data
      emptyDir: {}

Note: emptyDir data is lost when the Pod is deleted.

hostPath

Mounts the node's filesystem into the Pod. Equivalent to Docker's Bind Mount.

volumes:
  - name: host-data
    hostPath:
      path: /var/data
      type: DirectoryOrCreate

Note: hostPath is node-specific. If a Pod moves to another node, the data is inaccessible. Avoid in production.

---

PersistentVolume and PersistentVolumeClaim

Kubernetes manages storage through two resources: PersistentVolume (PV) and PersistentVolumeClaim (PVC).

flowchart LR
    ADMIN["Administrator"] -->|"creates"| PV["PersistentVolume\n(storage resource)"]
    DEV["Developer"] -->|"creates"| PVC["PersistentVolumeClaim\n(storage request)"]
    PVC -->|"binds"| PV
    POD["Pod"] -->|"mounts"| PVC
    style PV fill:#8b5cf6,color:#fff
    style PVC fill:#3b82f6,color:#fff
    style POD fill:#22c55e,color:#fff

PersistentVolume (PV)

A cluster storage resource, pre-provisioned by administrators or dynamically created via StorageClass.

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 10Gi
  accessModes:
    - ReadWriteOnce
  persistentVolumeReclaimPolicy: Retain
  hostPath:
    path: /mnt/data

PersistentVolumeClaim (PVC)

A developer's request for storage. Automatically binds to a matching PV.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: my-pvc
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 5Gi

Using PVC in a Pod

apiVersion: v1
kind: Pod
metadata:
  name: app-with-storage
spec:
  containers:
    - name: app
      image: nginx:1.27
      volumeMounts:
        - name: data
          mountPath: /usr/share/nginx/html
  volumes:
    - name: data
      persistentVolumeClaim:
        claimName: my-pvc

Access Modes

Mode	Abbreviation	Description
ReadWriteOnce	RWO	Read-write by a single node
ReadOnlyMany	ROX	Read-only by multiple nodes
ReadWriteMany	RWX	Read-write by multiple nodes
ReadWriteOncePod	RWOP	Read-write by a single Pod

Reclaim Policies

Policy	Description
Retain	Keep PV and data. Manual reuse required
Delete	Automatically delete PV and storage
Recycle	Clear data and reuse PV (deprecated)

---

StorageClass (Dynamic Provisioning)

Instead of pre-creating PVs, StorageClass automatically provisions PVs when PVCs are created.

flowchart LR
    PVC["PVC\n(5Gi request)"] -->|"references"| SC["StorageClass\n(standard)"]
    SC -->|"auto-creates"| PV["PersistentVolume\n(5Gi)"]
    PVC -->|"binds"| PV
    style SC fill:#f59e0b,color:#fff
    style PV fill:#8b5cf6,color:#fff
    style PVC fill:#3b82f6,color:#fff

# Check available StorageClasses
kubectl get storageclasses

PVC with StorageClass

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: dynamic-pvc
spec:
  accessModes:
    - ReadWriteOnce
  storageClassName: standard
  resources:
    requests:
      storage: 5Gi

Omitting storageClassName uses the default StorageClass.

---

Practical Example: PostgreSQL Data Persistence

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: postgres-pvc
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: postgres
spec:
  replicas: 1
  selector:
    matchLabels:
      app: postgres
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
        - name: postgres
          image: postgres:17
          ports:
            - containerPort: 5432
          env:
            - name: POSTGRES_DB
              value: myapp
            - name: POSTGRES_USER
              value: admin
            - name: POSTGRES_PASSWORD
              value: secret123
            - name: PGDATA
              value: /var/lib/postgresql/data/pgdata
          volumeMounts:
            - name: postgres-data
              mountPath: /var/lib/postgresql/data
          resources:
            requests:
              cpu: "250m"
              memory: "256Mi"
            limits:
              cpu: "500m"
              memory: "512Mi"
      volumes:
        - name: postgres-data
          persistentVolumeClaim:
            claimName: postgres-pvc
---
apiVersion: v1
kind: Service
metadata:
  name: postgres-service
spec:
  type: ClusterIP
  selector:
    app: postgres
  ports:
    - port: 5432
      targetPort: 5432

# Deploy
kubectl apply -f postgres-storage.yaml

# Verify PVC
kubectl get pvc

# Delete Pod — Deployment recreates it, data persists
kubectl delete pod -l app=postgres

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PV Lifecycle

flowchart LR
    A["Available"] --> B["Bound"]
    B --> C["Released"]
    C -->|"Retain"| D["Manual\nReuse"]
    C -->|"Delete"| E["Auto\nDeleted"]
    style A fill:#22c55e,color:#fff
    style B fill:#3b82f6,color:#fff
    style C fill:#f59e0b,color:#fff
    style E fill:#ef4444,color:#fff

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Summary

Concept	Description
emptyDir	Temporary shared volume within a Pod
hostPath	Mounts node filesystem (development only)
PersistentVolume (PV)	Cluster storage resource
PersistentVolumeClaim (PVC)	Storage request
StorageClass	Defines dynamic PV provisioning
Access Modes	RWO, ROX, RWX, RWOP

Key Takeaways

1. PV + PVC is the Kubernetes equivalent of Docker Volumes. PV is storage, PVC is the request
2. StorageClass enables dynamic PV creation — no manual PV provisioning needed
3. Use PVCs for database persistence so data survives Pod recreation

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Practice Exercises

Exercise 1: Basics

Create a 5Gi PVC, mount it to an Nginx Pod, and persist HTML files.

Exercise 2: Database

Create a MySQL Deployment with PVC. Insert data, delete the Pod, and verify data persists after recreation.

Challenge

Compare emptyDir and PVC. Write data to both, delete the Pod, and observe which data survives.

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References

• Persistent Volumes - Kubernetes Docs
• Storage Classes
• Volumes

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Next up: In Day 7, you'll learn about "ConfigMaps and Secrets" — how Kubernetes manages configuration that you previously handled with environment variables and .env files in Docker.