Persistent Storage in Kubernetes

What You’ll Learn 📚

• Core concepts of persistent storage in Kubernetes
• Key terminology and definitions
• Step-by-step examples from simple to complex
• Common questions and detailed answers
• Troubleshooting tips for common issues

Introduction to Persistent Storage

In Kubernetes, applications are often deployed in containers. These containers are ephemeral, meaning they can be created and destroyed frequently. But what if your application needs to store data that persists beyond the life of a container? That’s where persistent storage comes in! 🗄️

Core Concepts

Let’s break down the core concepts:

• Persistent Volume (PV): A piece of storage in the cluster that has been provisioned by an administrator or dynamically provisioned using Storage Classes.
• Persistent Volume Claim (PVC): A request for storage by a user. It is similar to a pod. Pods consume node resources, and PVCs consume PV resources.
• Storage Class: A way to describe the “classes” of storage available. Different classes might map to quality-of-service levels, or to backup policies, or to arbitrary policies determined by the cluster administrators.

Key Terminology

• Ephemeral: Temporary and short-lived. In Kubernetes, containers are ephemeral.
• Provisioning: The process of setting up and configuring storage resources.

Simple Example: Creating a Persistent Volume

apiVersion: v1
kind: PersistentVolume
metadata:
  name: example-pv
spec:
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: "/mnt/data"

Progressively Complex Examples

Example 1: Creating a Persistent Volume Claim

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: example-pvc
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi

Example 2: Using a PVC in a Pod

apiVersion: v1
kind: Pod
metadata:
  name: example-pod
spec:
  containers:
  - name: example-container
    image: nginx
    volumeMounts:
    - mountPath: "/usr/share/nginx/html"
      name: example-volume
  volumes:
  - name: example-volume
    persistentVolumeClaim:
      claimName: example-pvc

Example 3: Dynamic Provisioning with Storage Classes

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast
provisioner: kubernetes.io/aws-ebs
parameters:
  type: gp2
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: example-pvc
spec:
  storageClassName: fast
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi

Common Questions and Answers

1. What is the difference between a PV and a PVC?

   A PV is a piece of storage in the cluster, while a PVC is a request for storage by a user. Think of a PV as a piece of land and a PVC as a lease agreement for that land.

2. Can a PVC be resized?

   Yes, but it depends on the storage provider and the Kubernetes version. Make sure to check the documentation for your specific setup.

3. What happens if a PVC requests more storage than available?

   The PVC will remain in a pending state until enough storage is available.

4. How do I delete a PV or PVC?

   Use the kubectl delete command. For example, kubectl delete pvc example-pvc.

Troubleshooting Common Issues

If your PVC is stuck in a pending state, check if there are available PVs that match the PVC’s requirements.

Always double-check your YAML files for syntax errors. A missing space or incorrect indentation can cause issues! 🧐

Practice Exercises

1. Create a PV and PVC, then use them in a Pod. Verify that the data persists even if the Pod is deleted and recreated.
2. Experiment with different access modes and storage classes to see how they affect your setup.

For more information, check out the Kubernetes documentation on persistent volumes.