Storage: PV, PVC & StatefulSet

Storage: PV, PVC & StatefulSet

Step 1 of 11

replicas3

A database's replicas are not interchangeable: each needs its own name and its own data.

The Idea

A PersistentVolume is storage whose life does not depend on any Pod. A Pod asks for storage with a PersistentVolumeClaim — a size and an access mode — and a StorageClass can provision a matching volume on demand. A StatefulSet gives each replica a stable name (db-0, db-1) and its own claim from a volumeClaimTemplate.

Real-World Example

A three-replica database StatefulSet creates db-0, db-1 and db-2 in order, each bound to its own volume. When db-1's node fails, the replacement comes back as db-1 and mounts the same claim.

The Tradeoff

ReadWriteOnce means a single node mounts the volume read-write, so replicas do not share a disk — each gets its own. Scaling down keeps the claims by default, so cleanup is yours; and with reclaimPolicy: Delete, the default for dynamically provisioned volumes, deleting a claim deletes the data.

Hands-On

# illustrative — one claim per replica; serviceName is a headless Service
apiVersion: apps/v1
kind: StatefulSet
metadata: { name: db }
spec:
  serviceName: db
  replicas: 3
  selector: { matchLabels: { app: db } }
  template:
    metadata: { labels: { app: db } }
    spec:
      containers:
        - name: db
          image: registry.example.com/db:1.0
          volumeMounts: [{ name: data, mountPath: /data }]
  volumeClaimTemplates:
    - metadata: { name: data }
      spec: { accessModes: [ReadWriteOnce], resources: { requests: { storage: 10Gi } } }

Your turn

Put the steps in the right order.

1. 1A StorageClass provisions a volume and binds it to the claim↑↓
2. 2db-1's node fails; the new db-1 mounts data-db-1 again↑↓
3. 3The StatefulSet creates db-0 and its claim data-db-0↑↓
4. 4db-0 is Running and Ready, so db-1 is created next↑↓

Check order

Mini quiz

1 / 3

A PersistentVolumeClaim is:

• AA disk attached to one node
• BA request for storage: a size and an access mode
• CA StorageClass parameter
• DA snapshot of a volume

Sources

• Persistent Volumes — Kubernetes Documentation
• StatefulSets — Kubernetes Documentation
• Storage Classes — Kubernetes Documentation