Storage: PVs and PVCs

Key Takeaways for AI & Readers

• Persistence Strategy: PVs and PVCs separate the physical storage (PV) from the user's request for storage (PVC), ensuring data survives container restarts.
• The Binding Handshake: A PVC "binds" to a PV only if the PV meets the requested capacity and access mode requirements.
• Dynamic Provisioning: StorageClasses automate PV creation, eliminating the need for administrators to manually pre-provision physical disks.
• Access Modes: Understanding ReadWriteOnce (single node) vs. ReadWriteMany (shared across nodes) is critical for architectural planning.

Containers are ephemeral. When they die, their data dies with them. To save data permanently (like a database), Kubernetes uses PersistentVolumes (PV) and PersistentVolumeClaims (PVC).

The Handshake Visualizer

• PV: A piece of storage in the cluster (Administrator created).
• PVC: A request for storage (User created).

Try requesting storage below. Notice that a Claim will only Bind to a Volume if the Volume is big enough and available.

Create PersistentVolumeClaim (PVC)

Active Claims (Click to Delete)

No active PVCs

> Cluster initialized with 2 PersistentVolumes.

💾 pv-fast-1 (10Gi) [Available]

💾 pv-slow-1 (50Gi) [Available]

React Flow

Press enter or space to select a node. You can then use the arrow keys to move the node around. Press delete to remove it and escape to cancel.

Press enter or space to select an edge. You can then press delete to remove it or escape to cancel.

Key Concepts

1. PersistentVolume (PV): A resource in the cluster (like a node) that is a volume plugin like EBS, NFS, or local disk.
2. PersistentVolumeClaim (PVC): A request for storage by a user. It is similar to a Pod. Pods consume node resources and PVCs consume PV resources.
3. StorageClass: A "template" that allows PVs to be dynamically provisioned when a PVC is created (so you don't have to manually create PVs).

3. Storage Types: Block vs. File

When choosing a StorageClass, you must understand the underlying technology:

Block Storage (ReadWriteOnce)

• Examples: AWS EBS, Azure Disk, GCE Persistent Disk.
• Performance: High performance, low latency.
• Limitation: Usually can only be attached to one Node at a time. If your pod moves to a different node, the disk must be detached and re-attached (takes ~30s).

File Storage (ReadWriteMany)

• Examples: NFS, AWS EFS, Azure Files.
• Performance: Slightly higher latency.
• Benefit: Can be shared by many Pods on many Nodes simultaneously. Perfect for shared assets or log storage.

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4. Binding Rules

• Capacity: The PV must have at least as much storage as the PVC requested.
• Access Modes: Must match (e.g., ReadWriteOnce).
• Status: The PV must be Available.