StorageClasses

Dynamic Provisioning on Demand

Before StorageClasses, admins pre-created a finite pool of PVs and users waited for a fit. A class inverts that: the claim names a class, its provisioner creates the volume on demand, and binding happens immediately.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata: { name: fast-ssd }
provisioner: pd.csi.storage.gke.io
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer
allowVolumeExpansion: true
parameters: { type: pd-ssd }

reclaimPolicy becomes the default for claims in the class; parameters are provisioner-specific and documented per CSI driver. Which classes exist depends on your cluster's drivers — copy the class only after confirming the provisioner is installed.

Binding Modes and Expansion

volumeBindingMode decides when provisioning runs:

• Immediate — provision at claim creation. Fine for zone-agnostic storage; wrong for zonal disks, because the pod may schedule in another zone.
• WaitForFirstConsumer — delay provisioning until a pod uses the claim, so topology constraints pick the correct zone. Recommended for zonal CSI disks.

Expansion flows through the chain when allowVolumeExpansion: true is set on the class (and the driver supports it):

kubectl edit pvc data    # raise spec.resources.requests.storage to 20Gi
kubectl get pvc data     # watch condition Resized=True

Shrinking is never permitted. Filesystem resize happens online for supported filesystems; some drivers still require a pod restart.

Pitfalls and Interview Angles

Pitfall 1 — Pending claims with no provisioner. Either the class does not exist, no default class carries the is-default-class: "true" annotation, or the CSI driver is not installed. kubectl describe pvc events name the failing class.

Pitfall 2 — zone mismatches with Immediate binding. A zonal disk created in zone A cannot attach to a node in zone B; pods stall in ContainerCreating with attach errors. Switch the class to WaitForFirstConsumer.

Interview-ready: contrast static PV pools with dynamic classes, explain why topology-aware binding exists, and describe expansion end to end — class flag, PVC size edit, Resized condition, and the explicit no-shrink rule.