What are Kubernetes Persistent Volumes?

Kubernetes Persistent Volumes (PVs) are a way to provide durable storage to your applications running in a Kubernetes cluster. They are a Kubernetes resource, just like Pods, Services, and Deployments, that represent a piece of storage in your cluster. PVs are independent of the lifecycle of a Pod, which means that even if a Pod is deleted, the data stored in the Persistent Volume will still be available.

Understanding Persistent Volumes

In a Kubernetes cluster, Pods are ephemeral and can be created, scaled, and deleted as needed. However, this means that any data stored within a Pod's containers is also ephemeral and will be lost when the Pod is deleted. Persistent Volumes provide a way to decouple storage from the lifecycle of a Pod, allowing your applications to persist data even when the underlying infrastructure changes.

Persistent Volumes can be provisioned in various ways, including:

Static Provisioning: The cluster administrator creates a Persistent Volume with a specific amount of storage, a reclaim policy, and other parameters. Pods can then request to use this Persistent Volume.
Dynamic Provisioning: Kubernetes can automatically create a Persistent Volume when a Pod requests storage, using a StorageClass to define the parameters of the storage to be provisioned.

Persistent Volumes can use a variety of storage backends, such as:

Local storage on a node
Network-attached storage (NAS) like NFS or iSCSI
Cloud storage providers like AWS EBS, Google Persistent Disks, or Azure Disks

graph TD A[Kubernetes Cluster] --> B[Persistent Volume] B --> C[Storage Backend] C --> D[Local Storage] C --> E[Network Storage] C --> F[Cloud Storage]

Persistent Volume Claims

Persistent Volume Claims (PVCs) are the way that Pods request storage from the Kubernetes cluster. A PVC specifies the amount of storage needed, the access mode (e.g., read-write, read-only), and other parameters. Kubernetes will then find a suitable Persistent Volume to bind to the PVC, either by using an existing one or dynamically provisioning a new one.

graph TD A[Pod] --> B[Persistent Volume Claim] B --> C[Persistent Volume] C --> D[Storage Backend]

Reclaim Policies

Persistent Volumes have a reclaim policy that determines what happens to the storage when the Persistent Volume Claim that was using it is deleted. The available reclaim policies are:

Retain: The underlying storage asset is kept, and you are responsible for manually cleaning it up.
Delete: The underlying storage asset is deleted when the PVC is deleted.
Recycle: The underlying storage is returned to the pool of available storage.

The reclaim policy is an important consideration when choosing a storage backend, as it can impact the long-term management of your cluster's storage resources.

Conclusion

Kubernetes Persistent Volumes provide a way to decouple storage from the lifecycle of your applications, allowing you to persist data even as Pods are created, scaled, and deleted. By understanding how Persistent Volumes and Persistent Volume Claims work, you can ensure that your applications have the durable storage they need to function reliably in a Kubernetes cluster.