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Chapter 6.2 - PersistentVolumes and PersistentVolumeClaims

Learning Objectives​

By the end of this chapter, you will be able to:

  • Understand PersistentVolumes (PV) and PersistentVolumeClaims (PVC)
  • Create and use PVs and PVCs
  • Understand access modes (RWO, ROX, RWX)
  • Manage the storage lifecycle
  • Understand reclaim policies
  • Choose the right type of storage based on the use case

Introduction​

Ephemeral volumes (emptyDir, tmpfs) are deleted when the Pod is terminated. For important data that needs to persist (databases, user files, etc.), Kubernetes uses PersistentVolumes and PersistentVolumeClaims.

Key Concepts​

PersistentVolume (PV)​

A PersistentVolume is a storage resource in the cluster that has been provisioned by an administrator. It is a cluster-level resource, independent of Pods.

PersistentVolumeClaim (PVC)​

A PersistentVolumeClaim is a storage request by a user. The PVC binds a PV to a Pod.

Creating a PersistentVolume​

Example: PV with hostPath (Local Development)​

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv-volume
  labels:
    type: local
spec:
  capacity:
    storage: 10Gi
  accessModes:
    - ReadWriteOnce
  persistentVolumeReclaimPolicy: Retain
  storageClassName: manual
  hostPath:
    path: /data/pv-volume

Note: hostPath is for local development only. In production, use cloud solutions (EBS, Azure Disk, GCE Persistent Disk) or NFS.

Example: PV with NFS​

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv-nfs
spec:
  capacity:
    storage: 50Gi
  accessModes:
    - ReadWriteMany
  persistentVolumeReclaimPolicy: Retain
  storageClassName: nfs
  nfs:
    server: nfs-server.example.com
    path: /exports/data

Creating a PersistentVolumeClaim​

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pv-claim
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 5Gi
  storageClassName: manual

Binding process:

Access Modes​

Access modes determine how the volume can be mounted:

ReadWriteOnce (RWO)​

  • Only one node can mount the volume for read-write
  • Multiple Pods on the same node can share the volume
  • Use case: Databases, stateful applications

ReadOnlyMany (ROX)​

  • Multiple nodes can mount the volume as read-only
  • Use case: Read-only configuration files, shared data

ReadWriteMany (RWX)​

  • Multiple nodes can mount the volume for read-write
  • Use case: Shared file systems (NFS, GlusterFS)

Usage in a Pod​

apiVersion: v1
kind: Pod
metadata:
  name: pv-pod
spec:
  containers:
  - name: app
    image: nginx:1.20
    volumeMounts:
    - name: data
      mountPath: /data
  volumes:
  - name: data
    persistentVolumeClaim:
      claimName: pv-claim

Full flow:

Reclaim Policies​

The reclaim policy determines what happens to the PV when the PVC is deleted.

Retain​

The PV is retained even after the PVC is deleted. Data remains intact.

Use case:

  • Important data to be saved
  • Need for manual recovery
  • Compliance and audit

Delete​

The PV is automatically deleted with the PVC. Data is lost.

Use case:

  • Temporary data
  • Development environments
  • Dynamic provisioning

Recycle (Deprecated)​

This policy is deprecated. Use Retain or Delete instead.

Complete Example: Database​

Step 1: Create the PersistentVolume​

apiVersion: v1
kind: PersistentVolume
metadata:
  name: postgres-pv
  labels:
    app: postgres
spec:
  capacity:
    storage: 20Gi
  accessModes:
    - ReadWriteOnce
  persistentVolumeReclaimPolicy: Retain
  storageClassName: fast-ssd
  hostPath:
    path: /data/postgres

Step 2: Create the PersistentVolumeClaim​

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: postgres-pvc
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 20Gi
  storageClassName: fast-ssd

Step 3: Use in a Deployment​

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:14
        env:
        - name: PGDATA
          value: /var/lib/postgresql/data/pgdata
        volumeMounts:
        - name: postgres-storage
          mountPath: /var/lib/postgresql/data
      volumes:
      - name: postgres-storage
        persistentVolumeClaim:
          claimName: postgres-pvc

Useful Commands​

View PVs and PVCs​

# List PVs
kubectl get pv

# Details of a PV
kubectl describe pv pv-volume

# List PVCs
kubectl get pvc

# Details of a PVC
kubectl describe pvc pv-claim

Check Binding​

# View binding status
kubectl get pv
kubectl get pvc

# View details
kubectl get pv pv-volume -o yaml
kubectl get pvc pv-claim -o yaml

Delete a PVC​

# Delete the PVC
kubectl delete pvc pv-claim

# Check PV status after deletion
kubectl get pv pv-volume

Storage Types​

Local Storage (hostPath)​

Advantages:

  • Simple for development
  • No external dependency

Disadvantages:

  • Not portable
  • Risk of data loss
  • Does not work with multiple nodes

Usage: Local development only

NFS (Network File System)​

Advantages:

  • Supports ReadWriteMany
  • Shareable between nodes
  • Easy to set up

Disadvantages:

  • Limited performance
  • Single point of failure
  • Requires an NFS server

Usage: Shared files, collaborative applications

Cloud Storage (EBS, Azure Disk, GCE PD)​

Advantages:

  • High performance
  • Cloud-native integration
  • Dynamic provisioning
  • Automatic snapshots

Disadvantages:

  • Cost based on usage
  • Dependency on cloud provider

Usage: Production, critical applications

Best Practices​

1. Use StorageClasses​

Prefer dynamic provisioning with StorageClasses rather than manual PV creation.

2. Choose the Right Reclaim Policy​

  • Retain: For important data
  • Delete: For temporary data

3. Appropriate Size​

Do not over-allocate. Start small and grow as needed (with allowVolumeExpansion: true).

4. Backups​

Implement regular backups for important data.

5. Monitoring​

Monitor storage usage to avoid exhaustion.

Summary​

In this chapter, you learned:

PersistentVolume (PV): Cluster-level storage resource
PersistentVolumeClaim (PVC): User's storage request
Access Modes: RWO (one node), ROX (multiple nodes read-only), RWX (multiple nodes read-write)
Reclaim Policy: Retain (keep) or Delete (remove)
Storage types: hostPath (dev), NFS (shared), Cloud (production)
Usage: Via persistentVolumeClaim in Pods
Best practices: StorageClasses, appropriate size, backups

Next Steps​

Chapter 6.3: StorageClasses and Dynamic Provisioning
Lab 6.2: PersistentVolumes and PVCs

Chapter created on: December 2024