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Create a Burstable Managed CSI Premium Storage Class
This sample shows how to create a custom storage class that uses Azure Premium LRS to create a burstable managed disk.
burstable-managed-csi-premium

Create a Burstable Managed CSI Premium Storage Class

This sample shows how to create a custom storage class that uses Azure Premium LRS to dynamically create an Azure burstable managed disk as a persistent volume using a persistent volume claim in Azure Kubernetes Service.

Prerequisites

  • For online deployments, you need an Azure account. If you don't have one, create a free Azure account before you begin.
  • Install Visual Studio Code to edit YAML manifests and bash script on your local machine.
  • Install Azure CLI and kubectl to run commands against your Azure subscription and AKS cluster.
  • Run the bash script from a Linux/WSL/macOS terminal.

Azure Managed Disk Bursting

Currently, as documented in Managed disk bursting, there are two managed disk types that can burst: Premium SSDs and Standard SSDs. Other disk types cannot currently burst. There are two models of bursting for disks:

  • An on-demand bursting model, where the disk bursts whenever its needs exceed its current capacity. This model incurs additional charges anytime the disk bursts. On-demand bursting is only available for premium SSDs larger than 512 GiB. . On-demand bursting needs to be explicitly enabled. For more information, see Enable on-demand disk bursting.
  • A credit-based model, where the disk will burst only if it has burst credits accumulated in its credit bucket. This model does not incur additional charges when the disk bursts. Credit-based bursting is only available for premium SSDs 512 GiB and smaller, and standard SSDs 1024 GiB and smaller.

Azure Premium SSDs can use either bursting model, but Standard SSDs currently only offer credit-based bursting. Consider increasing the performance tier of your managed disks if your workload would otherwise be always running in burst mode.

Credit-based bursting On-demand bursting Changing performance tier
Scenarios Ideal for short-term scaling (30 minutes or less). Ideal for short-term scaling(Not time restricted). Ideal if your workload would otherwise continually be running in burst.
Cost Free Cost is variable, see the Billing section for details. The cost of each performance tier is fixed, see Managed Disks pricing for details.
Availability Only available for premium SSDs 512 GiB and smaller, and standard SSDs 1024 GiB and smaller. Only available for premium SSDs larger than 512 GiB. Available to all premium SSD sizes.
Enablement Enabled by default on eligible disks. Must be enabled by user. User must manually change their tier.

On-demand Bursting

Premium SSDs using the on-demand bursting model of disk bursting can burst beyond original provisioned targets, as often as needed by their workload, up to the max burst target. For example, on a 1 TiB P30 disk, the provisioned IOPS is 5000 IOPS. When disk bursting is enabled on this disk, your workloads can issue IOs to this disk up to the max burst performance of 30,000 IOPS and 1,000 MBps. For the max burst targets on each supported disk, see Scalability and performance targets for VM disks.

If you expect your workloads to frequently run beyond the provisioned perf target, disk bursting won't be cost-effective. In this case, we recommend that you change your disk's performance tier to a higher tier instead, for better baseline performance. Review your billing details and assess that against the traffic pattern of your workloads.

Before you enable on-demand bursting, understand the following:

  • On-demand bursting cannot be enabled on a premium SSD that has less than or equal to 512 GiB. Premium SSDs less than or equal to 512 GiB will always use credit-based bursting.
  • On-demand bursting is only supported on premium SSDs. If a premium SSD with on-demand bursting enabled is switched to another disk type, then disk bursting is disabled.
  • On-demand bursting doesn't automatically disable itself when the performance tier is changed. If you want to change your performance tier but do not want to keep disk bursting, you must disable it.
  • On-demand bursting can only be enabled when the disk is detached from a VM or when the VM is stopped. On-demand bursting can be disabled 12 hours after it has been enabled.

AKS Storage Options

Applications running in Azure Kubernetes Service (AKS) may need to store and retrieve data from persistent volumes attached to the worker nodes. While some application workloads can use local, fast storage on unneeded, emptied nodes, others require storage that persists on more regular data volumes within the Azure platform. Kubernetes typically treats individual pods as ephemeral, disposable resources. Applications have different approaches available to them for using and persisting data. A volume represents a way to store, retrieve, and persist data across pods and through the application lifecycle.

AKS Storage Options

Traditional volumes are created as Kubernetes resources backed by Azure Storage. You can manually create data volumes to be assigned to pods directly, or have Kubernetes automatically create them by declaring persistent volume claim. Data volumes can use the following storage types:

For more information, see Storage options for applications in Azure Kubernetes Service (AKS).

Persistent Volumes

Volumes defined and created as part of the pod lifecycle only exist until you delete the pod. Pods often expect their storage to remain if a pod is rescheduled on a different host during a maintenance event, especially in StatefulSets. A persistent volume (PV) is a storage resource created and managed by the Kubernetes API that can exist beyond the lifetime of an individual pod. You can use Azure Disks or Files to provide the PersistentVolume. The choice of Disks or Files is often determined by the need for concurrent access to the data or the performance tier.

Persistent volumes in an Azure Kubernetes Services (AKS) cluster

A PersistentVolume can be statically created by a cluster administrator, or dynamically created by the Kubernetes API server. If a pod is scheduled and requests currently unavailable storage, Kubernetes can create the underlying Azure Disk or Files storage and attach it to the pod. Dynamic provisioning uses a StorageClass to identify what type of Azure storage needs to be created.

Storage Classes

To define different tiers of storage, such as Premium and Standard, you can use an existing Storage Class or create your one. The StorageClass also defines the reclaimPolicy. When you delete the pod and the persistent volume is no longer required, the reclaimPolicy controls the behavior of the underlying Azure storage resource. The underlying storage resource can either be deleted or kept for use with a future pod. For clusters using the Container Storage Interface (CSI) drivers the following extra StorageClasses are created:

Permission Reason
managed-csi Uses Azure StandardSSD locally redundant storage (LRS) to create a Managed Disk. The reclaim policy ensures that the underlying Azure Disk is deleted when the persistent volume that used it is deleted. The storage class also configures the persistent volumes to be expandable, you just need to edit the persistent volume claim with the new size.
managed-csi-premium Uses Azure Premium locally redundant storage (LRS) to create a Managed Disk. The reclaim policy again ensures that the underlying Azure Disk is deleted when the persistent volume that used it is deleted. Similarly, this storage class allows for persistent volumes to be expanded.
azurefile-csi Uses Azure Standard storage to create an Azure File Share. The reclaim policy ensures that the underlying Azure File Share is deleted when the persistent volume that used it is deleted.
azurefile-csi-premium Uses Azure Premium storage to create an Azure File Share. The reclaim policy ensures that the underlying Azure File Share is deleted when the persistent volume that used it is deleted.

Unless you specify a StorageClass for a persistent volume, the default StorageClass will be used. Ensure volumes use the appropriate storage you need when requesting persistent volumes.

Important: Starting in Kubernetes version 1.21, AKS uses CSI drivers only and by default. In this case, the default storage class is managed-csi.

Burstable Managed CSI Premium Storage Class

If you plan to use a P30 or higher managed disk and you want to enable on-demand bursting, you can't use the default managed-csi-premium storage class in your persistent volume claim as this storage class has on-demand bursting disabled by default. You need to build your own storage class, and explicitly enable the on-demand bursting using the corresponding enableBursting parameter. This parameter enables on-demand bursting beyond the provisioned performance target of the managed disk. As mentioned above, on-demand bursting only be applied to Premium disk, disk size > 512GB, Ultra & shared disk is not supported. On-demand bursting is disabled by default, hence the default value of the enableBursting parameter for the managed-csi-premium storage class is false. Hence, if you use this built-in storage class in you persistent volume claim, on-demand bursting on the created managed disk will be disabled.

You can create a custom storage class as follows using kubectl and make sure to enable on-demand bursting. The following example uses Premium Managed Disks and specifies that on-demand bursting is enabled by setting the enableBursting parameter to true.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: burstable-managed-csi-premium
provisioner: disk.csi.azure.com
parameters:
  skuname: Premium_LRS
  enableBursting: "true"
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer
allowVolumeExpansion: true

Important: AKS reconciles the default storage classes and will overwrite any changes you make to those storage classes.

Persistent Volume Claims

A PersistentVolumeClaim requests storage of a particular StorageClass, access mode, and size. The Kubernetes API server can dynamically provision the underlying Azure storage resource if no existing resource can fulfill the claim based on the defined StorageClass. The pod definition includes the volume mount once the volume has been connected to the pod.

Persistent volume claims in an Azure Kubernetes Services (AKS) cluster

Once an available storage resource has been assigned to the pod requesting storage, PersistentVolume is bound to a PersistentVolumeClaim. Persistent volumes are 1:1 mapped to claims.

The following example YAML manifest shows a persistent volume claim that uses the newly created burstable-managed-csi-premium storage class and requests a 1TiB burstable Premium SSD.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: burstable-managed-csi-premium
spec:
  accessModes:
  - ReadWriteOnce
  storageClassName: burstable-managed-csi-premium
  resources:
    requests:
      storage: 1Ti

When you create a pod definition, you can specify:

  • A persistent volume claim to request the desired storage.
  • A volumeMount for your containerized application to read and write data from the persistent volume.

The following example YAML manifest shows how the previous persistent volume claim can be used to mount a volume at /mnt/azure:

kind: Pod
apiVersion: v1
metadata:
  name: nginx
spec:
  containers:
    - name: frontend
      image: mcr.microsoft.com/oss/nginx/nginx:1.15.5-alpine
      resources:
        requests:
          memory: "64Mi"
          cpu: "100m"
        limits:
          memory: "128Mi"
          cpu: "200m"
      volumeMounts:
      - mountPath: "/mnt/azure"
        name: volume
  volumes:
    - name: volume
      persistentVolumeClaim:
        claimName: burstable-managed-csi-premium

For mounting a volume in a Windows container, specify the drive letter and path. For example:

...      
       volumeMounts:
        - mountPath: "d:"
          name: volume
        - mountPath: "c:\k"
          name: k-dir
...

Run the Demo

You can run the following deploy.sh bash script to create the custom storage class, the persistent volume, and the pod that uses the persistent volume claim to create and mount a P30 managed disk as a volume.

#!/bin/bash

# Variables
namespace="burstable-csi"
persistentVolumeClaimTemplate="pvc.yml"
persistentVolumeClaimName="burstable-managed-csi-premium"
storageClassTemplate="sc.yml"
storageClassName="burstable-managed-csi-premium"
podTemplate="pod.yml"
podName="nginx"

# Create the storage class if doesn't already exists in the cluster
result=$(kubectl get sc -o jsonpath="{.items[?(@.metadata.name=='$storageClassName')].metadata.name}")

if [[ -n $result ]]; then
    echo "[$storageClassName] storage class already exists in the cluster"
else
    echo "[$storageClassName] storage class does not exist in the cluster"
    echo "creating [$storageClassName] storage class in the cluster..."
    kubectl apply -f $storageClassTemplate
fi

# Create the namespace if it doesn't already exists in the cluster
result=$(kubectl get namespace -o jsonpath="{.items[?(@.metadata.name=='$namespace')].metadata.name}")

if [[ -n $result ]]; then
    echo "[$namespace] namespace already exists in the cluster"
else
    echo "[$namespace] namespace does not exist in the cluster"
    echo "creating [$namespace] namespace in the cluster..."
    kubectl create namespace $namespace
fi

# Check if the persistent volume claim already exists
result=$(kubectl get pvc -n $namespace -o json | jq -r '.items[].metadata.name | select(. == "'$persistentVolumeClaimName'")')

if [[ -n $result ]]; then
    echo "[$persistentVolumeClaimName] persistent volume claim already exists"
    exit
else
    # Create the persistent volume claim 
    echo "[$persistentVolumeClaimName] persistent volume claim does not exist"
    echo "Creating [$persistentVolumeClaimName] persistent volume claim..."
    cat $persistentVolumeClaimTemplate | 
    yq "(.metadata.name)|="\""$persistentVolumeClaimName"\" |
    yq "(.spec.storageClassName)|="\""$storageClassName"\" |
    kubectl apply -n $namespace -f -
fi

# Check if the pod already exists
result=$(kubectl get pod -n $namespace -o json | jq -r '.items[].metadata.name | select(. == "'$podName'")')

if [[ -n $result ]]; then
    echo "[$podName] pod already exists"
    exit
else
    # Create the pod 
    echo "[$podName] pod does not exist"
    echo "Creating [$podName] pod..."
    cat $podTemplate | 
    yq "(.metadata.name)|="\""$podName"\" |
    yq "(.spec.volumes[0].persistentVolumeClaim.claimName)|="\""$persistentVolumeClaimName"\" |
    kubectl apply -n $namespace -f -
fi

Azure Portal

Use the Azure portal to check the newly created pod:

Persistent Volume Claim

Click the Volumes tab and then the persistent volume claim link to see the persistent volume claim.

Persistent Volume Claim

Now open the managed disk resource under the node resource group of your AKS cluster:

Persistent Volume Claim

In the overview page, you can observe:

  1. Disk size: is 1024 GiB == 1 TiB
  2. Disk sku: Premium SSD LRS
  3. Performance Tier: P30 – 5000 IOPS, 200 MBps

As shown at Premium SSD managed disks: Per-disk limits, a P30 managed disk can burst up to 30,000 IOPS and 1,000 MB/sec. Now, if you click Configuration under Settings , you can see that on-demand bursting is enabled.

Persistent Volume Claim

Premium SSDs using the on-demand bursting model are charged an hourly burst enablement flat fee and transaction costs apply to any burst transactions beyond the provisioned target. For more information about the cost model and some examples, see billing.

Next Steps

You can create custom storage classes and enable additional features in your persistent volumes. For example, you can use the diskEncryptionSetID parameter to specify the resource id of the disk encryption set to use for enabling server-side encryption of Azure managed disks. For more information, see Bring your own keys (BYOK) with Azure disks in Azure Kubernetes Service (AKS).

Useful Links