Dynamically create and use a persistent volume with Azure Disks in Azure Kubernetes Service (AKS)

A persistent volume represents a piece of storage that has been provisioned for use with Kubernetes pods. A persistent volume can be used by one or many pods, and can be dynamically or statically provisioned. This article shows you how to dynamically create persistent volumes with Azure Disks for use by a single pod in an Azure Kubernetes Service (AKS) cluster.

For more information on Kubernetes volumes, see Storage options for applications in AKS.

This article assumes that you have an existing AKS cluster with 1.21 or later version. If you need an AKS cluster, see the AKS quickstart using the Azure CLI, using Azure PowerShell, or using the Azure portal.

You also need the Azure CLI version 2.0.59 or later installed and configured. Run az --version to find the version. If you need to install or upgrade, see Install Azure CLI.

The Azure Disks CSI driver has a limit of 32 volumes per node. The volume count will change based on the size of the node/node pool. Run the following command to determine the number of volumes that can be allocated per node:

kubectl get CSINode <nodename> -o yaml

A storage class is used to define how a unit of storage is dynamically created with a persistent volume. For more information on Kubernetes storage classes, see Kubernetes Storage Classes.

Each AKS cluster includes four pre-created storage classes, two of them configured to work with Azure Disks:

• The default storage class provisions a standard SSD Azure Disk.
  • Standard storage is backed by Standard SSDs and delivers cost-effective storage while still delivering reliable performance.
• The managed-csi-premium storage class provisions a premium Azure Disk.
  • Premium disks are backed by SSD-based high-performance, low-latency disk. Perfect for VMs running production workload. If the AKS nodes in your cluster use premium storage, select the managed-premium class.

If you use one of the default storage classes, you can't update the volume size after the storage class is created. To be able to update the volume size after a storage class is created, add the line allowVolumeExpansion: true to one of the default storage classes, or you can create your own custom storage class. It's not supported to reduce the size of a PVC (to prevent data loss). You can edit an existing storage class by using the kubectl edit sc command.

For example, if you want to use a disk of size 4 TiB, you must create a storage class that defines cachingmode: None because disk caching isn't supported for disks 4 TiB and larger.

For more information about storage classes and creating your own storage class, see Storage options for applications in AKS.

Use the kubectl get sc command to see the pre-created storage classes. The following example shows the pre-create storage classes available within an AKS cluster:

kubectl get sc

The output of the command resembles the following example:

NAME                PROVISIONER                AGE
default (default)   disk.csi.azure.com         1h
managed-csi         disk.csi.azure.com         1h

A persistent volume claim (PVC) is used to automatically provision storage based on a storage class. In this case, a PVC can use one of the pre-created storage classes to create a standard or premium Azure managed disk.

Create a file named azure-pvc.yaml, and copy in the following manifest. The claim requests a disk named azure-managed-disk that is 5 GB in size with ReadWriteOnce access. The managed-csi storage class is specified as the storage class.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: azure-managed-disk
spec:
  accessModes:
  - ReadWriteOnce
  storageClassName: managed-csi
  resources:
    requests:
      storage: 5Gi

Create the persistent volume claim with the kubectl apply command and specify your azure-pvc.yaml file:

kubectl apply -f azure-pvc.yaml

The output of the command resembles the following example:

persistentvolumeclaim/azure-managed-disk created

Once the persistent volume claim has been created and the disk successfully provisioned, a pod can be created with access to the disk. The following manifest creates a basic NGINX pod that uses the persistent volume claim named azure-managed-disk to mount the Azure Disk at the path /mnt/azure. For Windows Server containers, specify a mountPath using the Windows path convention, such as 'D:'.

Create a file named azure-pvc-disk.yaml, and copy in the following manifest.

kind: Pod
apiVersion: v1
metadata:
  name: mypod
spec:
  containers:
  - name: mypod
    image: mcr.azk8s.cn/oss/nginx/nginx:1.15.5-alpine
    resources:
      requests:
        cpu: 100m
        memory: 128Mi
      limits:
        cpu: 250m
        memory: 256Mi
    volumeMounts:
    - mountPath: "/mnt/azure"
      name: volume
  volumes:
    - name: volume
      persistentVolumeClaim:
        claimName: azure-managed-disk

Create the pod with the kubectl apply command, as shown in the following example:

kubectl apply -f azure-pvc-disk.yaml

pod/mypod created

You now have a running pod with your Azure Disk mounted in the /mnt/azure directory. This configuration can be seen when inspecting your pod via kubectl describe pod mypod, as shown in the following condensed example:

kubectl describe pod mypod

The output of the command resembles the following example:

[...]
Volumes:
  volume:
    Type:       PersistentVolumeClaim (a reference to a PersistentVolumeClaim in the same namespace)
    ClaimName:  azure-managed-disk
    ReadOnly:   false
  default-token-smm2n:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-smm2n
    Optional:    false
[...]
Events:
  Type    Reason                 Age   From                               Message
  ----    ------                 ----  ----                               -------
  Normal  Scheduled              2m    default-scheduler                  Successfully assigned mypod to aks-nodepool1-79590246-0
  Normal  SuccessfulMountVolume  2m    kubelet, aks-nodepool1-79590246-0  MountVolume.SetUp succeeded for volume "default-token-smm2n"
  Normal  SuccessfulMountVolume  1m    kubelet, aks-nodepool1-79590246-0  MountVolume.SetUp succeeded for volume "pvc-faf0f176-8b8d-11e8-923b-deb28c58d242"
[...]

To back up the data in your persistent volume, take a snapshot of the managed disk for the volume. You can then use this snapshot to create a restored disk and attach to pods as a means of restoring the data.

First, get the volume name with the kubectl get pvc command, such as for the PVC named azure-managed-disk:

$ kubectl get pvc azure-managed-disk

NAME                 STATUS    VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS      AGE
azure-managed-disk   Bound     pvc-faf0f176-8b8d-11e8-923b-deb28c58d242   5Gi        RWO            managed-premium   3m

This volume name forms the underlying Azure Disk name. Query for the disk ID with az disk list and provide your PVC volume name, as shown in the following example:

az disk list --query '[].id | [?contains(@,`pvc-faf0f176-8b8d-11e8-923b-deb28c58d242`)]' -o tsv

/subscriptions/<guid>/resourceGroups/MC_MYRESOURCEGROUP_MYAKSCLUSTER_chinaeast2/providers/MicrosoftCompute/disks/kubernetes-dynamic-pvc-faf0f176-8b8d-11e8-923b-deb28c58d242

Use the disk ID to create a snapshot disk with az snapshot create. The following example creates a snapshot named pvcSnapshot in the same resource group as the AKS cluster (MC_myResourceGroup_myAKSCluster_chinaeast2). You may encounter permission issues if you create snapshots and restore disks in resource groups that the AKS cluster doesn't have access to.

az snapshot create \
    --resource-group MC_myResourceGroup_myAKSCluster_chinaeast2 \
    --name pvcSnapshot \
    --source /subscriptions/<guid>/resourceGroups/MC_myResourceGroup_myAKSCluster_chinaeast2/providers/MicrosoftCompute/disks/kubernetes-dynamic-pvc-faf0f176-8b8d-11e8-923b-deb28c58d242

Depending on the amount of data on your disk, it may take a few minutes to create the snapshot.

To restore the disk and use it with a Kubernetes pod, use the snapshot as a source when you create a disk with az disk create. This operation preserves the original resource if you then need to access the original data snapshot. The following example creates a disk named pvcRestored from the snapshot named pvcSnapshot:

az disk create --resource-group MC_myResourceGroup_myAKSCluster_chinaeast2 --name pvcRestored --source pvcSnapshot

To use the restored disk with a pod, specify the ID of the disk in the manifest. Get the disk ID with the az disk show command. The following example gets the disk ID for pvcRestored created in the previous step:

az disk show --resource-group MC_myResourceGroup_myAKSCluster_chinaeast2 --name pvcRestored --query id -o tsv

Create a pod manifest named azure-restored.yaml and specify the disk URI obtained in the previous step. The following example creates a basic NGINX web server, with the restored disk mounted as a volume at /mnt/azure:

kind: Pod
apiVersion: v1
metadata:
  name: mypodrestored
spec:
  containers:
  - name: mypodrestored
    image: mcr.azk8s.cn/oss/nginx/nginx:1.15.5-alpine
    resources:
      requests:
        cpu: 100m
        memory: 128Mi
      limits:
        cpu: 250m
        memory: 256Mi
    volumeMounts:
    - mountPath: "/mnt/azure"
      name: volume
  volumes:
    - name: volume
      azureDisk:
        kind: Managed
        diskName: pvcRestored
        diskURI: /subscriptions/<guid>/resourceGroups/MC_myResourceGroupAKS_myAKSCluster_chinaeast2/providers/Microsoft.Compute/disks/pvcRestored

Create the pod with the kubectl apply command, as shown in the following example:

$ kubectl apply -f azure-restored.yaml

The output of the command resembles the following example:

pod/mypodrestored created

You can use kubectl describe pod mypodrestored to view details of the pod, such as the following condensed example that shows the volume information:

kubectl describe pod mypodrestored

The output of the command resembles the following example:

[...]
Volumes:
  volume:
    Type:         AzureDisk (an Azure Data Disk mount on the host and bind mount to the pod)
    DiskName:     pvcRestored
    DiskURI:      /subscriptions/19da35d3-9a1a-4f3b-9b9c-3c56ef409565/resourceGroups/MC_myResourceGroupAKS_myAKSCluster_chinaeast2/providers/Microsoft.Compute/disks/pvcRestored
    Kind:         Managed
    FSType:       ext4
    CachingMode:  ReadWrite
    ReadOnly:     false
[...]

For more information on using Azure tags, see Use Azure tags in Azure Kubernetes Service (AKS).

For associated best practices, see Best practices for storage and backups in AKS.

Learn more about Kubernetes persistent volumes using Azure Disks.