Claiming Persistent Volumes

Usage of Persistent Volumes#

Kubernetes persistent volumes are useless if no one uses them. They exist only as objects with relation to, in our case, specific EBS volumes. They are waiting for someone to claim them through the PersistentVolumeClaim resource.

Just like Pods which can request specific resources like memory and CPU, PersistentVolumeClaims can request particular sizes and access modes. Both are, in a way, consuming resources, even though of different types. Just as Pods should not specify on which node they should run, PersistentVolumeClaims cannot define which volume they should mount. Instead, Kubernetes scheduler will assign them a volume depending on the claimed resources.

How to Claim Persistent Volumes?#

We’ll use pv/pvc.yml to explore how we could claim a persistent volume.

The output is as follows.

The YAML file defines a PersistentVolumeClaim with the storage class name manual-ebs. That is the same class as the persistent volumes manual-ebs-* we created earlier. The access mode and the storage request are also matching what we defined for the persistent volume.

Please note that we are not specifying which volume we’d like to use. Instead, this claim specifies a set of attributes (storageClassName, accessModes, and storage). Any of the volumes in the system that match those specifications might be claimed by the PersistentVolumeClaim named jenkins.

Bear in mind that resources do not have to be the exact match. Any volume that has the same or bigger amount of storage is considered a match. A claim for 1Gi can be translated to at least 1Gi. In our case, a claim for 1Gi matches all three persistent volumes since they are set to 5Gi.

Creating the Claim#

Now that we explored the definition of the claim, we can proceed, and create it.

The output indicates that the persistentvolumeclaim "jenkins" was created.

Verification#

Let’s list the claims and see what we got.

The output is as follows.

We see from the output that the status of the claim is bound. That means that the claim found a matching persistent volume and bounded it. We can confirm that by listing the volumes.

The output is as follows.

We can see that one of the volumes (manual-ebs-02) changed the status from available to bound. That is the volume bound to the claim we created a moment ago. We can see that the claim comes from jenkins Namespace and jenkins PersistentVolumeClaim.

📝 Please note that if a PersistentVolumeClaim cannot find a matching volume, it will remain unbound indefinitely, unless we add a new PersistentVolume with the matching specifications.

We still haven’t accomplished our goal. The fact that we claimed a volume does not mean that anyone uses it. On the other hand, our Jenkins needs to persist its state.

In the next lesson, we’ll join our PersistentVolumeClaim with a Jenkins container.