Storage options for applications in Azure Kubernetes Service (AKS)

• 03/11/2021
• 7 minutes to read

In this article

Applications running in Azure Kubernetes Service (AKS) may need to store and retrieve data. 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.

Multiple pods may need to:

• Share the same data volumes.
• Reattach data volumes if the pod is rescheduled on a different node.

Finally, you may need to inject sensitive data or application configuration information into pods.

This article introduces the core concepts that provide storage to your applications in AKS:

Volumes

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.

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. Data volumes can use Azure Disks or Azure Files.

Azure Disks

Use Azure Disks to create a Kubernetes DataDisk resource. Disks can use:

• Azure Premium storage, backed by high-performance SSDs, or
• Azure Standard storage, backed by regular HDDs.

Since Azure Disks are mounted as ReadWriteOnce, they're only available to a single pod. For storage volumes that can be accessed by multiple pods simultaneously, use Azure Files.

Azure Files

Use Azure Files to mount an SMB 3.0 share backed by an Azure Storage account to pods. Files let you share data across multiple nodes and pods and can use:

• Azure Premium storage, backed by high-performance SSDs, or
• Azure Standard storage backed by regular HDDs.

Volume types

Kubernetes volumes represent more than just a traditional disk for storing and retrieving information. Kubernetes volumes can also be used as a way to inject data into a pod for use by the containers.

Common volume types in Kubernetes include:

emptyDir

Commonly used as temporary space for a pod. All containers within a pod can access the data on the volume. Data written to this volume type persists only for the lifespan of the pod. Once you delete the pod, the volume is deleted. This volume typically uses the underlying local node disk storage, though it can also exist only in the node's memory.

secret

You can use secret volumes to inject sensitive data into pods, such as passwords.

1. Create a Secret using the Kubernetes API.
2. Define your pod or deployment and request a specific Secret.
   • Secrets are only provided to nodes with a scheduled pod that requires them.
   • The Secret is stored in tmpfs, not written to disk.
3. When you delete the last pod on a node requiring a Secret, the Secret is deleted from the node's tmpfs.
   • Secrets are stored within a given namespace and can only be accessed by pods within the same namespace.

configMap

You can use configMap to inject key-value pair properties into pods, such as application configuration information. Define application configuration information as a Kubernetes resource, easily updated and applied to new instances of pods as they're deployed.

Like using a Secret:

1. Create a ConfigMap using the Kubernetes API.
2. Request the ConfigMap when you define a pod or deployment.
   • ConfigMaps are stored within a given namespace and can only be accessed by pods within the same namespace.

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. As noted in the Volumes section, the choice of Disks or Files is often determined by the need for concurrent access to the data or the performance tier.

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 create a StorageClass.

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.

In AKS, four initial StorageClasses are created for cluster using the in-tree storage plugins:

For clusters using the new Container Storage Interface (CSI) external plugins (preview) the following extra StorageClasses are created:

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.

You can create a StorageClass for additional needs using kubectl. The following example uses Premium Managed Disks and specifies that the underlying Azure Disk should be retained when you delete the pod:

kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: managed-premium-retain
provisioner: kubernetes.io/azure-disk
reclaimPolicy: Retain
parameters:
  storageaccounttype: Premium_LRS
  kind: Managed

Persistent volume claims

A PersistentVolumeClaim requests either Disk or File 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.

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 managed-premium StorageClass and requests a Disk 5Gi in size:

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

When you create a pod definition, you also specify:

• The persistent volume claim to request the desired storage.
• The volumeMount for your applications to read and write data.

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: myfrontend
      image: mcr.microsoft.com/oss/nginx/nginx:1.15.5-alpine
      volumeMounts:
      - mountPath: "/mnt/azure"
        name: volume
  volumes:
    - name: volume
      persistentVolumeClaim:
        claimName: azure-managed-disk

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
...

Next steps

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

To see how to create dynamic and static volumes that use Azure Disks or Azure Files, see the following how-to articles:

For more information on core Kubernetes and AKS concepts, see the following articles:

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