PVC constraints — bounding storage requests

PVC constraints in LimitRange bound the storage requests per PVC. This lesson walks the PVC limits, the use case, the interaction with quotas, and the discipline.

The PVC limits

apiVersion: v1
kind: LimitRange
metadata:
  name: pvc-limits
  namespace: tenant-a-prod
spec:
  limits:
    - type: PersistentVolumeClaim
      max:
        storage: 100Gi
      min:
        storage: 1Gi

The PVC fields:

• max. Maximum storage request. A PVC requesting 200Gi would be rejected.
• min. Minimum storage request. A PVC requesting 100Mi would be rejected.

The use case

flowchart LR
    A["PVC request: 10Gi"] --> B{Within max?}
    B -->|Yes| C[Accepted]
    B -->|No| D["Rejected: above max"]
    E["PVC request: 200Gi"] --> B

The use case: a tenant requests a 10TiB PVC for a Postgres database. The quota allows 1TiB total. The PVC max is 100Gi. The request is rejected at PVC admission.

Without PVC max, the request would be rejected at quota admission (later in the process), but the error is less specific. PVC max gives a clearer error message.

Interaction with storage quotas

flowchart LR
    A[PVC creation request] --> B{Within PVC max?}
    B -->|No| C["Rejected: PVC max"]
    B -->|Yes| D{Within storage quota?}
    D -->|No| E["Rejected: storage quota"]
    D -->|Yes| F[Accepted]

The admission sequence:

1. The PVC request is checked against PVC max.
2. If within max, the storage quota is checked.
3. If within quota, the PVC is accepted.

PVC max is an early check; storage quota is a later check. Both are needed for full enforcement.

PVC max in production

flowchart LR
    A[Production PVC max] --> B["Database: 500Gi"]
    A --> C["Application: 100Gi"]
    A --> D["Cache: 50Gi"]
    B --> E["Tier 1: high"]
    C --> F["Tier 2: medium"]
    D --> G["Tier 3: low"]

In practice, production deployments have different PVC max for different namespaces:

• Database namespace. PVC max 500Gi (Postgres databases can be large).
• Application namespace. PVC max 100Gi (most apps don’t need more).
• Cache namespace. PVC max 50Gi (caches are bounded).

The max is per-namespace; each namespace has its own LimitRange.

Quiz

The operational discipline

PVC constraints in production rest on five non-negotiable elements:

• Set PVC max for every namespace. Prevent oversized PVCs.
• Set PVC min. Prevent zero-sized PVCs that may not bind.
• Tune per namespace. Database namespaces need higher max.
• Account for growth. PVCs can be expanded (StorageClass allows it); the max accommodates the expanded size.
• Test the constraints. Quarterly: create a PVC that exceeds max; verify rejection.

PVC constraints prevent oversized storage requests that can exhaust the backend. The discipline is to set the max based on workload characteristics and to monitor the storage quota.