Simplifying Persistent Storage in Kubernetes: A Deep Dive into PVs, PVCs, and SCs
Tags: kubernetes, storage, persistent, volume
Published: 2024-06-22 13:19:39

In the world of Kubernetes, managing persistent storage efficiently stands as a cornerstone for deploying resilient and scalable applications. Kubernetes not only orchestrates containers but also offers robust solutions for handling persistent data across these containers. 

This blog dives into the critical components of Kubernetes storage management: Persistent Volumes (PV), Persistent Volume Claims (PVC), Storage Classes (SC), and Volume Claim Templates. These elements are pivotal in making Kubernetes a powerhouse for maintaining stateful applications amidst the dynamic nature of containerized environments.

## Persistent Volumes (PV)

Persistent Volumes are one of the building blocks of storage in Kubernetes. A PV is a networked storage unit in the cluster that has been provisioned by an administrator or automatically provisioned via Storage Classes. It represents a piece of storage that is physically backed by some underlying mass storage system, like NFS, iSCSI, or a cloud provider-specific storage system.

### Characteristics of PVs:

- Lifecycle Independence: PVs exist independently of pods' lifecycles. This means that the storage persists even after the pods that use them are deleted.
- Storage Abstraction: PVs abstract the details of how storage is provided from how it is consumed, allowing for a separation of concerns between administrators and users.
- Multiple Access Modes: PVs support different access modes like ReadWriteOnce, ReadOnlyMany, and ReadWriteMany, which dictate how the volume can be mounted on a node.

## Persistent Volume Claims (PVC)

Persistent Volume Claims are essentially requests for storage by a pod. PVCs consume PV resources by specifying size and access modes, like a kind of "storage lease" that a user requests to store their data.

### How PVCs Work:

- Binding: When a PVC is created, Kubernetes looks for a PV that matches the PVC’s requirements and binds them together. If no suitable PV exists, the PVC will remain unbound until a suitable one becomes available or is dynamically provisioned.
- Dynamic Provisioning: If a PVC specifies a Storage Class, and no PV matches its requirements, a new PV is dynamically created according to the specifics of the Storage Class.

## Storage Classes (SC)

Storage Classes define and classify the types of storage available within a Kubernetes cluster. They enable dynamic volume provisioning by describing the "classes" of storage (different levels of performance, backups, and policies).

### Features of SCs:

- Provisioning: Admins can define as many Storage Classes as needed, each specifying a different quality of service or backup policy.
- Automation: Based on the Storage Class specified in a PVC, Kubernetes automates the volume provisioning, without manual PV creation by the administrator.

## Example:
Consider a scenario where a Kubernetes cluster needs to dynamically provide storage for a database application:

```
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: db-storage
spec:
  accessModes:
    - ReadWriteOnce
  storageClassName: fast-disk
  resources:
    requests:
      storage: 100Gi
```
This PVC requests a 100 GiB disk with read-write access on a single node. The fast-disk Storage Class is designed to provision high-performance SSD-based storage, tailored for database applications.

## How It Works:

- PVC Creation: The above PVC is created, requesting specific storage characteristics.
- Dynamic Provisioning: If no existing PV matches the PVC, the Storage Class fast-disk triggers the dynamic creation of a new PV that fits the criteria.
- Binding: The newly created PV is automatically bound to the PVC, ensuring the database application has the necessary storage.

## Conclusion
Understanding PVs, PVCs, and SCs is crucial for effectively managing storage in Kubernetes. These components offer a flexible, powerful way to handle persistent data, ensuring applications can be highly available and resilient. As Kubernetes continues to evolve, the capabilities and complexity of managing storage will likely increase, offering even more robust solutions for cloud-native environments.

## In a nutshell
- PVs act as a bridge between the physical storage and the pods, offering a lifecycle independent of the pods.
- PVCs allow pods to request specific sizes and access modes from the available PVs.
- SCs automate the provisioning of storage based on the desired characteristics, facilitating dynamic storage allocation without manual intervention.