Last modified November 18, 2020

Using Persistent Volumes on Azure

If your cluster is running in the cloud on Azure, it comes with four dynamic storage provisioners using different flavours of Azure Disks and Azure File shares. This enables you to store data beyond the lifetime of a Pod.

Storage Classes

The default Storage Classes are:

managed-premium (the default one): Provisions a Premium LRS Managed disk and thus can be used only on kubernetes nodes that run on supported VM types (those with an s in their name).
managed-standard: Provisions a Standard LRS Managed disk and can be used on any VM instance type.
af-premium: Provisions a volume backed by the Azure File Share service within a Premium LRS storage account.
af-standard: Provisions a volume backed by the Azure File Share service within a Standard LRS storage account.

Creating Persistent Volumes

The most straight forward way to create a Persistent Volume is to create a Persistent Volume Claim object. Please ensure to specify the right Storage Class to have the desired type of volume automatically provisioned for you.

Alternatively, to be able to set more specific parameters on your PV you can first create a Persistent Volume object and then claim that PV using a Persistent Volume Claim (PVC).

Under the hood, the Dynamic Storage Provisioner will take care that a corresponding Azure File Share or Managed Disk gets created.

The Volume and its data will persist as long as the corresponding PV resource exists. Deleting the resource will also delete the corresponding volume, which means that all stored data will be lost at that point.

Using Persistent Volumes in a Pod

Once you have a Persistent Volume Claim you can claim it as a Volume in your Pods.

Note that an Azure Disk volumes can only be used by a single Pod at a time. Thus, the access mode of your PVC can only be ReadWriteOnce.

This limitation doesn’t apply to Azure File Share volumes, that can be attached with the ReadWriteMany policy.

Example

First we create a PVC:

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: myclaim
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 6Gi
  storageClassName: managed-standard

Now we can create a Pod that uses our PVC:

kind: Pod
apiVersion: v1
metadata:
  name: mypod
spec:
  containers:
    - name: myfrontend
      image: nginx
      volumeMounts:
      - mountPath: "/var/www/html"
        name: mypd
  volumes:
    - name: mypd
      persistentVolumeClaim:
        claimName: myclaim

Now we have an NGINX Pod which serves the contents of our Azure Managed Disk Volume.

Expanding Persistent Volume Claims

Starting with volumes created on clusters using tenant cluster release v13.0.0 for Azure, Persistent Volume Claims can be expanded by simply editing the claim and requesting a larger size. It will trigger an update in the underlying Persistent Volume and Azure Volume (Kubernetes always uses the existing one).

Please note that for Managed Disk-based volumes, it is mandatory to release the PVC (i.e. stop the Pod that is mounting it) before the resize operation begins. This is a limitation of Azure Cloud that does not apply to Azure File Share-based volumes.

Deleting Persistent Volumes

By default the Reclaim Policy of Persistent Volumes in your cluster is set to Delete. Thus, deleting the PersistentVolume resource will also delete the respective Azure Disk or Azure File Share. Similarly, by default if you delete a PersistenVolumeClaim resource the respective Persistent Volume and Azure resource will get deleted.

Note that deleting an application that is using Persistent Volumes might not automatically also delete its storage, so in most cases you will have to manually delete the PersistenVolumeClaim resources to clean up.