What are persistent volumes in Kubernetes?

Persistent volumes are storage resources in Kubernetes that exist independently of pod lifecycles, enabling you to retain data even when containers restart or terminate. They solve a fundamental challenge: containers are ephemeral by nature and lose all data when they stop running. This guide answers the most common questions about implementing persistent storage in your Kubernetes clusters.

What are persistent volumes in Kubernetes and why do you need them?

Persistent volumes are storage resources in Kubernetes that remain available regardless of what happens to the pods using them. When a container restarts or terminates, any data stored in its local filesystem disappears. Persistent volumes prevent this data loss by providing storage that lives outside the container lifecycle.

Containers are designed to be temporary and replaceable. This works well for stateless applications like web servers that don't need to remember anything between restarts. However, many applications require data persistence. Databases need to retain records, file systems must preserve uploaded documents, and applications often require configuration data that survives restarts.

Without persistent volumes, you would lose all your database records every time a pod restarts. Customer uploads would vanish. Application state would reset constantly. Persistent volumes enable stateful applications to run reliably in Kubernetes by separating data storage from compute resources.

The difference between ephemeral container storage and persistent storage is straightforward. Ephemeral storage exists only while the container runs and disappears when the container stops. Persistent storage remains available across container restarts, pod deletions, and even node failures. Your application can reconnect to the same data after any disruption.

How do persistent volumes actually work in Kubernetes?

Persistent volumes operate through a workflow involving three main components that work together. The PersistentVolume (PV) represents actual storage capacity available in your cluster. The PersistentVolumeClaim (PVC) represents a request for storage by an application. The StorageClass defines different types of storage available and how to provision them automatically.

The workflow follows a clear pattern. Cluster administrators provision PersistentVolumes that represent physical storage resources. Developers create PersistentVolumeClaims specifying their storage requirements (size, access mode, performance characteristics). Kubernetes then binds appropriate PVs to PVCs, matching claims to available volumes that meet the requirements.

Persistent volumes move through four lifecycle stages:

• Provisioning: Creating the storage resource, either manually by administrators or automatically through StorageClasses
• Binding: Matching a PVC to an available PV that meets the claim requirements
• Using: Pods mount the volume and read or write data to it
• Reclaiming: Determining what happens to the storage after the PVC is deleted

Access modes determine how pods can use the storage. ReadWriteOnce (RWO) allows one pod to mount the volume for reading and writing, suitable for databases that need exclusive access. ReadOnlyMany (ROX) allows multiple pods to mount the volume in read-only mode, useful for shared configuration data. ReadWriteMany (RWX) allows multiple pods to mount the volume for reading and writing simultaneously, necessary for shared file systems.

Your choice of access mode affects application design significantly. If you need multiple pods to write to the same storage simultaneously, you must use RWX-capable storage. If your application requires exclusive write access, RWO provides better performance and simpler consistency guarantees.

What's the difference between persistent volumes and persistent volume claims?

PersistentVolumes (PVs) are actual storage resources, whilst PersistentVolumeClaims (PVCs) are requests for those resources. Think of PVs as available servers in your infrastructure and PVCs as resource requests from applications. The PV represents the physical storage capacity, whilst the PVC represents the application's need for storage.

This separation creates a useful abstraction. Cluster administrators manage the storage infrastructure by creating and configuring PersistentVolumes. They handle the technical details: which storage backend to use, how to connect to it, what performance characteristics it offers, and how to manage its lifecycle.

Developers simply create PersistentVolumeClaims describing what they need: storage size, access mode, and performance requirements. They don't need to know whether the storage comes from local disks, network-attached storage, or cloud provider services. The claim abstracts away these implementation details.

This separation of concerns benefits both teams. Operations teams control storage infrastructure, costs, and policies without changing application configurations. Development teams request storage resources without needing deep infrastructure knowledge. When you move applications between environments, you change the PV configuration but keep the same PVC specifications.

How do you choose the right storage type for your Kubernetes application?

Selecting appropriate storage depends on your application's specific requirements and usage patterns. Block storage provides high-performance volumes that attach to individual pods, suitable for databases and applications needing fast, exclusive access. Network file systems like NFS offer shared storage that multiple pods can access simultaneously, useful for shared file repositories and content management systems.

Common storage backends each serve different purposes. Block storage delivers excellent performance for single-pod access. Distributed storage systems provide high availability and scalability for demanding workloads. Cloud provider storage integrates directly with your infrastructure provider's storage services. Network file systems enable simple shared access across multiple pods.

Consider these factors when selecting storage:

• Performance needs: Databases require high IOPS and low latency, whilst file archives prioritize capacity over speed
• Access patterns: Single-pod applications use RWO storage, shared file systems need RWX capability
• Data durability: Production databases need replicated storage, development environments can use simpler options
• Cost considerations: High-performance storage costs more, so match storage tier to actual requirements
• Backup and recovery: Some storage types offer built-in snapshots, others require separate backup solutions

Match storage characteristics to your application type. Databases benefit from block storage with high IOPS and low latency. Applications sharing files between pods need network file systems with RWX access. Caching layers can use faster but less durable storage since cache data can be regenerated. Content delivery systems need high-throughput storage for serving files efficiently.

What happens to your data when a pod or persistent volume is deleted?

Deleting a pod does not delete the persistent volume or your data. The volume remains intact and can be reattached to new pods. This separation between compute and storage means you can restart, reschedule, or replace pods without losing data. Your application reconnects to the same storage with all data preserved.

When you delete a PersistentVolumeClaim, the reclaim policy determines what happens to the data. The Retain policy preserves the data and the PV after claim deletion, requiring manual cleanup. This protects against accidental data loss. The Delete policy automatically removes both the PV and the underlying storage, cleaning up resources but permanently deleting data. The Recycle policy (now deprecated) attempted to scrub data for reuse.

Understanding the distinction between PVC and PV deletion prevents data loss. Deleting a PVC triggers the reclaim policy but doesn't immediately delete data if the policy is Retain. Deleting a PV directly bypasses reclaim policies and removes the storage resource, though this requires removing the PVC reference first.

Protect your data in production environments with these practices:

• Use the Retain reclaim policy for production volumes containing important data
• Implement regular backup schedules using volume snapshots or backup tools
• Test your restore procedures before you need them in an emergency
• Document which volumes contain stateful data requiring careful handling
• Use role-based access control to prevent unauthorized volume deletions
• Tag volumes clearly to identify their purpose and importance

Configure your backup strategy based on data importance and change frequency. Databases need frequent backups with point-in-time recovery capability. Configuration data might only need daily backups. Development environments can use simpler backup approaches than production systems.

Persistent volumes provide the foundation for running stateful applications reliably in Kubernetes. They separate storage lifecycle from container lifecycle, enabling data to survive pod restarts and failures. Understanding how PVs, PVCs, and StorageClasses work together helps you design robust applications that preserve data whilst maintaining Kubernetes' flexibility. At Falconcloud, we provide block storage solutions that integrate with your Kubernetes infrastructure, offering expandable volumes from 10 GB to 1 TB with the performance and reliability your applications need.