A Persistent Volume (PV) separates storage from Pods, and its lifecycle is independent of any Pod that uses it. The administrator can create it or dynamically provision it using Storage Classes.
When you create a PersistentVolumeClaim, Kubernetes looks for a PV with the same StorageClass and AccessMode to bind it to. It will then mount this PV as a volume in your deployment workloads.
Persistent Volumes (PVs)
A Kubernetes persistent volume claim represents a physical storage resource in a Kubernetes cluster. It is decoupled from the lifecycle of Pods, allowing them to specify their storage requirements as a Persistent Volume Claim (PVC). This abstraction layer lets developers request and use storage without knowing the details of the underlying hardware.
PVCs can be provisioned statically or dynamically. An administrator creates static PVs and then exists in the cluster, waiting to be bound to a PVC. When a PVC that specifies a static PV is created, the storage resources in the PV are reserved and become available to the Pods set by the claim. This binding happens regardless of the size and access modes of the backing volume, but the control plane still checks that the storage class, requested storage size, and access mode of the PVC are valid.
To provide a more resilient and flexible experience, administrators can enable Persistent Volumes to be automatically backed by multiple storage providers. This provides a failover capability to restore storage during a primary provider failure. When this feature is enabled, a reclaim policy must be set to determine what happens to the volume after a claim is deleted. Options include retaining the volume, deleting it, or recycling the volume.
Persistent Volume Claims (PVCs)
Application developers or administrators create Persistent Volume Claims to request application storage. They abstract the underlying storage details from the application and allow applications to be relocated between nodes or even across clusters without losing data. PVCs have fields such as name, capacity, access modes, and an optional storage class.
When a PVC is bound to a pod, it becomes a reference for the Pod to use the requested storage. PVCs can be used statically or dynamically. The administrator may statically provision PVs for cluster users, or they can use the storage class feature to dynamically provision PVs using a physical storage system.
The capacity of a persistent volume can be resized when a PVC is edited to set a larger size. However, the Kubernetes control plane must match the desired size of the PVC and the backing volume to perform the resize. Otherwise, no resizing is performed, and the underlying persistent volume remains at its original size.
The PVC can also be reclaimed, and its physical storage can be released by setting a policy on the persistent volume claim. Policies can include “Retain” (keep the data even after the PV is unbound), “Recycle” (overwrite the data and release the PV), or “Delete” (delete the data and detach the PV from the cluster). Retain and recycle are the default policies, but you can specify other options in the persistent-volume-claim-config YAML file.
Storage Class
The Kubernetes system supports several ways to provision persistent storage. The first is statically provisioning, which uses PVs that represent real storage. These PVs exist in the Kubernetes API and are manually deployed in advance by a cluster administrator. When a user’s PersistentVolumeClaim matches one of these static PVs, the cluster binds that container to the PV. The second method involves dynamically provisioning a PV based on a pre-configured StorageClass. The user’s PVC must request a StorageClass for this to occur.
When a PVC is bound to a volume, the Pod’sPod’s container can use that volume as its storage. The Pod can use any access mode supported by the PV, which may be read-only, read-write, or ReadWriteOnce. If a Pod has a PersistentVolumeClaim that requests a ReadWriteOnce access mode, the underlying file system will expand when the Pod needs additional space.
Users should monitor persistent storage to ensure it is utilized correctly and efficiently. Open-source tools can help track storage and provide visibility into persistent volumes and PersistentVolumeClaims. In addition, users should back up persistent storage to ensure they don’t lose any data. Velero is an example of a storage solution that enables users to automatically back up and restore their persistent storage, including PersistentVolumeClaims and volumes. Implementing a quality of service (QoS) policy for persistent storage is also a good idea, allowing administrators to limit resource usage, such as memory, processing, and storage.
Access Modes
Kubernetes has a variety of access modes that storage, application, and platform administrators can use. Pods only need to specify the required PVC, and the cluster will automatically find the physical volume to satisfy the request.
PVCs describe the storage capacity and characteristics a Pod requires and are used to provision storage on the cluster using Storage Classes. When a PVC is created, the cluster searches for an existing PersistentVolume (PV) with the specified requirements, such as size and availability. Once the control plane finds a PV with the required specifications, it binds the claim to the PV and sets its status to Bound.
If a container needs more storage space, the administrator can create another PVC for the same PV by specifying a new size. The cluster will then attempt to expand the underlying storage system to the new size until it succeeds or fails. If expansion fails, the PVC will be continuously retried until the user or the cluster administrator takes action to recover it.
The cluster will mark the PV with a Retain reclaim policy if the underlying storage fails to expand. Then, when the PVC is deleted, Kubernetes will connect the volume to a temporary pod and issue a rm -rf command to scrub the data before making it available to be claimed by another Pod. If the reclaim policy is not set to Retain, the volume will be deleted from the Kubernetes storage object and removed from the external infrastructure when released.