Kubernetes 101 : Storage : Persistent volumes (PV) and Persistent Volume Claim (PVC)

When requesting kubernetes for storage, we would need to know the technology behind the storage ( NFS, iSCSI, SSD, ....) to be able to make the right choices regarding the storage that would fit our application needs.

Persistent Volume Clain ( PVC ) allows application running inside a pod to request storage ( Persistent volumes ) without worrying about the underlying technology.
The task of providing storage that fits the application will be in hands of the kubernetes administrator.

The Persistent volume ( PV ) could either be  provided automatically through storage classes or manually through a kubernetes cluster administrator.

Persistent volumes are not attached to the life-cycle of the pods that use them, they persist even if the pod exits or crashes.

Persistent volume claim and persistent volumes:

A PVC is a request for storage.

The PVC specifies the amount of storage a pod needs and also the access rights needed for the storage.

Kubernetes tries to find the most suitable PV that matches the characteristics of a PVC. 

The characteristics of the PV must match or be greater than what the PVC requires.

Remark:

The PVCs must exist in the same namespace as the pods attached to it.
Pods mount a PVCs in the same way they would mount a volume.

Deleting a PVC:

Once the PVC is deleted, the storage space associated with the PV is reclaimed by kubernetes.

The reclaim policies:

Below are the reclaim policies:

• Retain: the PV is not reclaimed by kubernetes after the PVC is deleted
• Delete: kubernetes deletes the PVC along with the storage attached to the PV

Persistent volume configuration file:

We create a storage "persistent volume" on the host to be mounted on the "hostPath" path and to be used later by pods, after requesting it through a PVC.

Access mode:

Below are the different access modes for the Persistent volumes:

• ReadWriteOnce (RWO) : the volume can be mounted in read-write mode by one nodes.
• ReadWriteOnce (RWX) : the volume can be mounted in read-write mode by many nodes.
• ReadOnlyMany (ROX) :  the volume can be mounted in read-only mode by many nodes.

We can create the persistent volume using the below command:

We can check our created persistent volume using the below command:

We notice that the status is still "available" because it is not bound to any PVC ( CLAIM ).

Remark:

After the persistent volume claim is created kubernetes binds the persistent volume claim to a "matching" persistent volume. 

The persistent volume claim:

Below is  a Yaml configuration file for a PVC:

We create the persistent volume claim from the Yaml file using the below command:

We can check our created persistent volume using the below command:

We notice that the status is "bound" and the claim is "default/pvc_1".

The "default" is the default namespace.

 A pod configuration file and its PVC:

Below is the pod configuration file with the persistent volume claim ( PVC ) to request storage ( PV ):

We can create our pod using the below command:

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IkbalC

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