Understanding Kubernetes Storage: From Persistent Volumes to Storage Classes

In Kubernetes, managing storage for applications running in pods is achieved through Persistent Volumes (PV), Persistent Volume Claims (PVC), and Storage Classes (SC). Here's a quick breakdown:

1. Persistent Volume (PV):

A Persistent Volume is a piece of storage in the cluster provisioned by an administrator or dynamically using Storage Classes. It defines where the storage is located.

Example PV YAML:

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  persistentVolumeReclaimPolicy: Retain
  storageClassName: manual
  hostPath:
    path: "/mnt/data"

This example creates a PV of 1Gi, with ReadWriteOnce access, meaning it can be mounted as read-write by a single node. It uses the host's local storage at /mnt/data.

2. Persistent Volume Claim (PVC):

A PVC is how a pod requests storage. The claim specifies the size and access mode for the required storage.

Example PVC YAML:

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

This PVC requests 1Gi of storage and matches any PV with the manual storage class.

3. Storage Class (SC) for Dynamic Provisioning:

A Storage Class automates PV provisioning. When a PVC references a Storage Class, Kubernetes creates the PV dynamically.

Example SC YAML:

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast-storage
provisioner: kubernetes.io/aws-ebs  # Replace with your cloud provider or storage type
parameters:
  type: gp2  # AWS EBS volume type, modify for your storage type
  fsType: ext4
reclaimPolicy: Retain

This example Storage Class is for AWS EBS (gp2 volume type) with ext4 as the file system. You can modify this for other storage backends (e.g., GCE, Azure).

4. Pod with Persistent Volume (Bound by PVC):

A pod accesses a PV through the PVC. Here's an example pod definition:

Example Pod YAML:

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

This pod runs an nginx container, and the /usr/share/nginx/html directory inside the container is mounted from the storage specified by the PVC my-pvc.

Example Flow:

1. Without Storage Class:

   • Manually create a PV.
   • Create a PVC to claim that PV.
   • Assign the PVC to a pod, which uses the storage from the PV.

2. With Storage Class:

   • Create a PVC referencing a Storage Class.
   • Kubernetes automatically provisions a PV based on the Storage Class.
   • The PVC is bound to the dynamically created PV, and the pod uses this storage.

Using Storage Classes simplifies the process in dynamic environments where storage needs change frequently.