Myth 1: Kubernetes Cluster Can Have Only One Default StorageClass

You're defining a new StorageClass and set:

metadata:
  annotations:
    storageclass.kubernetes.io/is-default-class: "true"

You assume Kubernetes will reject or warn if another default already exists. After all, "default" means only one, right?

So you apply multiple StorageClass resources with the same default annotation — and nothing happens. No error. No warning.

You run:

kubectl get sc -o custom-columns=NAME:.metadata.name,DEFAULT:.metadata.annotations.storageclass\.kubernetes\.io/is-default-class

And you see two or more default classes marked "true".

Why This Myth Exists?

• The term “default” creates the impression that only one can exist.
• Most tutorials and cluster setups only define one, reinforcing this assumption.
• The lack of enforcement is not widely documented.
• Engineers expect the system to prevent ambiguous behavior—but Kubernetes allows it.

The Reality

You can create multiple default StorageClasses in a Kubernetes cluster. The "storageclass.kubernetes.io/is-default-class": "true" annotation is not enforced by Kubernetes — it's simply a marker.

• When a PVC omits storageClassName, the system looks for a StorageClass with that annotation.
• If more than one exists, the behavior is undefined and provider-specific.
• Kubernetes does not raise any warning or error when multiple defaults exist.
• It's up to cluster admins to ensure only one default exists for consistent PVC provisioning.

Experiment & Validate

Step1: Create Two Default StorageClasses

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast-default
  annotations:
    storageclass.kubernetes.io/is-default-class: "true"
provisioner: kubernetes.io/no-provisioner
---
apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: slow-default
  annotations:
    storageclass.kubernetes.io/is-default-class: "true"
provisioner: kubernetes.io/no-provisioner

Step 2: Apply

kubectl apply -f dual-default-sc.yaml

Step 3: List All Defaults

kubectl get sc -o custom-columns=NAME:.metadata.name,DEFAULT:.metadata.annotations.storageclass\.kubernetes\.io/is-default-class

You’ll see both fast-default and slow-default marked as default. No error. No warning.

Key Takeaways

• Kubernetes allows multiple default StorageClasses without any warning.
• The “default” is an annotation, not a rule.
• If more than one default exists, PVCs without storageClassName may be handled unpredictably.
• Always ensure only one default StorageClass exists to avoid confusion.