Kubernetes Ep 8: Persistent Volumes, PVCs & StorageClasses

TL;DR (Quick Summary)

#

• PersistentVolume (PV): A cluster-wide storage resource provisioned by an administrator or dynamically created by a CSI plugin (AWS EBS, GCP PD, NFS).
• PersistentVolumeClaim (PVC): A request for storage by a user/Pod (specifying size, access modes, and StorageClass).
• StorageClass: Enables dynamic provisioning of PVs on-demand when a PVC is submitted.
• Access Modes:
  • ReadWriteOnce (RWO): Volume can be mounted read-write by a single Node.
  • ReadOnlyMany (ROX): Volume can be mounted read-only by many Nodes.
  • ReadWriteMany (RWX): Volume can be mounted read-write by many Nodes (e.g., NFS, Ceph, EFS).

1. Storage Abstraction Architecture

#

graph TD
    Developer["Developer / Pod Manifest"] -->|Requests 10Gi RWO| PVC["PersistentVolumeClaim (PVC)
(Claim: app-storage-pvc)"]
    PVC -->|Binds to| PV["PersistentVolume (PV)
(10Gi RWO, Bound)"]
    SC["StorageClass (gp3 / standard)
(Provisioner: ebs.csi.aws.com)"] -.->|Dynamically Creates| PV
    PV -->|Mounts underlying storage| Disk["Cloud Disk / Storage LUN
(AWS EBS Volume vol-0a1b2c)"]

2. Dynamic Storage Provisioning with StorageClass

#

In modern Kubernetes clusters, administrators configure a default StorageClass so users don’t have to manually pre-provision static PVs.

Inspect cluster StorageClasses:

kubectl get storageclass

Expected Terminal Output:

NAME                 PROVISIONER                  RECLAIMPOLICY   VOLUMEBINDINGMODE   ALLOWVOLUMEEXPANSION
standard (default)   k8s.io/minikube-hostpath     Delete          Immediate           true
gp3-csi              ebs.csi.aws.com             Delete          WaitForFirstConsumer true

3. Step-by-Step Stateful Storage Workflow

#

Step 1: Submit a PersistentVolumeClaim (PVC)

#

Create pvc.yaml:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: postgres-data-pvc
  namespace: default
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 5Gi
  storageClassName: standard # Uses cluster default if omitted

Apply PVC:

kubectl apply -f pvc.yaml
kubectl get pvc postgres-data-pvc

Expected Terminal Output:

NAME                STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGE
postgres-data-pvc   Bound    pvc-8d7e9f1a-2b3c-4d5e-6f7a-8b9c0d1e2f3a   5Gi        RWO            standard       12s

Notice how the STATUS instantly changed to Bound because the StorageClass dynamically provisioned a matching PV!

Step 2: Attach PVC to a Pod

#

Create pod-storage.yaml:

apiVersion: v1
kind: Pod
metadata:
  name: postgres-pod
spec:
  containers:
  - name: postgres
    image: postgres:15-alpine
    env:
    - name: POSTGRES_PASSWORD
      value: "SecretDBPass123"
    ports:
    - containerPort: 5432
    volumeMounts:
    - name: postgres-persistent-storage
      mountPath: /var/lib/postgresql/data
  volumes:
  - name: postgres-persistent-storage
    persistentVolumeClaim:
      claimName: postgres-data-pvc

Apply manifest:

kubectl apply -f pod-storage.yaml

4. Reclaim Policies (Retain vs Delete)

#

What happens to the underlying PV and cloud storage disk when a user deletes a PVC?

• Delete (Default for cloud StorageClasses): Deleting the PVC automatically destroys the bound PV and erases the underlying cloud volume (EBS/GCP PD).
• Retain: Deleting the PVC leaves the PV intact (Released status). Data on disk is preserved so an administrator can manually recover or re-bind it.

To change reclaim policy on an existing PV:

kubectl patch pv <pv-name> -p '{"spec":{"persistentVolumeReclaimPolicy":"Retain"}}'

5. Summary & Next Steps

#

Persistent Volumes ensure your data survives Pod deletions and node crashes. However, deploying databases using a standard Deployment resource can lead to split-brain scenarios or volume attachment errors during node failures.

In Episode 09: StatefulSets & Stateful Database Deployments, we will master StatefulSets to manage stateful workloads requiring unique network identities, ordered deployments, and persistent storage per replica!