Karan Sharma

Ramblings on tech, cycling, and finance

14 Jan 20

Resize PVC in Kubernetes

Well, the title is self explanatory so let's begin!

First off, we need to ensure that the StorageClass which was used to provision the PVC has the correct configuration. From the official docs:

You can only expand a PVC if its storage class’s allowVolumeExpansion field is set to true.

So, let's inspect our storage class:

$ kubectl get sc # sc is short for storageclass
NAME            PROVISIONER             AGE
gp2 (default)   kubernetes.io/aws-ebs   8d

$ kubectl describe sc/gp2
# output redacted to focus only on the field we're concerned with
Name:            gp2
AllowVolumeExpansion:  True

If AllowVolumeExpansion is set to True you can skip the below step. If it's not true, you need edit the field allowVolumeExpansion as true:

apiVersion: storage.k8s.io/v1
kind: StorageClass
    storageclass.kubernetes.io/is-default-class: "true"
  name: gp2
  fsType: ext4
  type: gp2
provisioner: kubernetes.io/aws-ebs
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer
allowVolumeExpansion: true

Once the StorageClass config is correct, all we need to do is update with the new size.

So, for example if size of the PVC was 15GB orginally:

      storage: 15Gi

To update it to 30GB, you simply need to edit spec.resources.requests field:

      storage: 30Gi

We now need to "apply" the updated PVC manifest.

$ kubectl apply -f pvc.yml

Let's take a look at the PVC:

$ kubectl describe pvc/myclaim
# output redacted for brevity
  Type                      Status  LastProbeTime                     LastTransitionTime                Reason  Message
  ----                      ------  -----------------                 ------------------                ------  -------
  FileSystemResizePending   True    Mon, 01 Jan 0001 00:00:00 +0000   Tue, 14 Jan 2020 20:52:21 +0530           Waiting for user to (re-)start a pod to finish file system resize of volume on node.

So, basically what FileSystemResizePending means is that while PVC is in use, we have to either restart or delete the underlying Pod using the PVC. At the time of writing this, ExpandInUsePersistentVolumes is still in beta and has to be enabled as a feature gate. Sadly, EKS is still on 1.14 (while the world has moved to 1.17, such sloooow release cycles!), so I couldn't enable this in my case.

Once the pod is restarted, the expanded disk is automagically available! Let's verify this:

kc get pvc/myclaim -o=jsonpath="{.status.capacity.storage}"

Now, compare this with the standard way of resizing an EBS volume in EC2 instance. You need to first modify the volume size using AWS EBS API and then in the EC2 instance, use a combination of growpart and resize2fs to extend the resized volume. This sounds much more cumbersome than simply updating the storage field in PVC manifest!