Persistent Volumes and Storage

# CHAPTER 9

Persistent Volumes and Storage #

1. Introduction #

2. Learning Objectives #

• Understand why local Pod storage is insufficient for databases.

• Define a Persistent Volume (PV).

• Define a Persistent Volume Claim (PVC).

• Understand the relationship between the Cluster Admin and the Developer regarding storage.

• Mount a PVC into a Pod to guarantee data persistence.

3. Beginner-Friendly Explanation #

• Pod Storage (Ephemeral): You use the mini-fridge in your specific room. When you check out (The Pod dies), the cleaning staff throws away everything in the fridge.

• Persistent Volume (PV): The hotel basement has 10 locked storage lockers. These represent physical hard drives (AWS EBS volumes, NFS drives). They exist independently of any hotel room.

• Persistent Volume Claim (PVC): You (The Developer) walk to the front desk and say, "I need a storage locker that holds exactly 10 Gigabytes." (You make a Claim). The front desk finds a basement locker that matches your request, gives you the key, and you take the key up to your room.

If you switch rooms (The Pod restarts on a different node), you still have the key to your basement locker, and your data remains perfectly safe.

4. Persistent Volume (PV) #

5. Persistent Volume Claim (PVC) #

6. Anatomy of Storage YAMLs #

The PVC (What the Developer writes):

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: mysql-pvc
spec:
  accessModes:
    - ReadWriteOnce # Only one node can mount this drive at a time
  resources:
    requests:
      storage: 5Gi

The Pod (Mounting the Claim):

# Inside the Pod spec...
  volumes:
    - name: mysql-storage # Define a volume based on the PVC
      persistentVolumeClaim:
        claimName: mysql-pvc
  containers:
    - name: mysql
      image: mysql:8.0
      volumeMounts:
        - mountPath: /var/lib/mysql # Where the container expects the data
          name: mysql-storage

7. Mini Project: Persist Database Data #

Step-by-Step Tutorial: *(Note: Minikube utilizes a dynamic provisioner, meaning if you create a PVC, Minikube automatically creates the underlying PV for you!)*

1. 1. Save the PVC YAML from Section 6 into pvc.yaml and apply it:

kubectl apply -f pvc.yaml

1. 2. Verify the PVC is "Bound":

kubectl get pvc

1. 3. Create a simple Pod that writes a permanent file (writer-pod.yaml):

apiVersion: v1
kind: Pod
metadata:
  name: writer-pod
spec:
  volumes:
    - name: my-storage
      persistentVolumeClaim:
        claimName: mysql-pvc
  containers:
    - name: alpine
      image: alpine
      command: ["/bin/sh", "-c"]
      args: ["echo 'THIS DATA SURVIVES' > /data/message.txt; sleep 3600"]
      volumeMounts:
        - mountPath: /data
          name: my-storage

1. 4. Apply the Pod (kubectl apply -f writer-pod.yaml).

1. 5. The Destruction: Delete the Pod! kubectl delete pod writer-pod. The Pod is dead.

1. 6. The Recovery: Apply the *exact same* writer-pod.yaml again to create a new Pod.

1. 7. Exec into the new Pod:

kubectl exec -it writer-pod -- cat /data/message.txt

*(You will see "THIS DATA SURVIVES". The data successfully lived on the PVC while the Pod was dead!)*

8. Real-World Scenarios #

9. Best Practices #

• StatefulSets for Databases: In this chapter, we attached a PVC to a basic Pod. In a true production environment, you deploy clustered databases (like a 3-node MongoDB cluster) using a StatefulSet controller instead of a Deployment. A StatefulSet guarantees strict ordering and assigns a unique, dedicated PVC to every single replica in the database cluster.

10. Common Mistakes #

• ReadWriteMany vs ReadWriteOnce: If you create a PVC with ReadWriteOnce (RWO), and try to mount that exact same PVC into two different Pods running on two different Worker Nodes, the second Pod will fail to start. A standard cloud hard drive can only be physically attached to one Node at a time. If multiple Pods on different nodes need to read the same files, you must use a Network File System (NFS) supporting ReadWriteMany (RWX).

11. Exercises #

1. 1. Explain the decoupling benefit of having separate PV and PVC objects, rather than allowing Pods to define physical hard drives directly in their manifests.

1. 2. If a Pod is deleted, what happens to the Persistent Volume Claim (PVC) attached to it?

12. FAQs #

13. Interview Questions #

• Q: Describe the architectural relationship between a StorageClass, a PersistentVolume (PV), and a PersistentVolumeClaim (PVC) in a dynamic provisioning environment like AWS EKS.

• Q: A developer attempts to scale a Deployment attached to a standard AWS EBS volume (ReadWriteOnce) from 1 replica to 3 replicas. Explain the storage failure that will occur and how it prevents the new Pods from entering the Running state.

14. Summary #

15. Next Chapter Recommendation #