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Use node taints in an AKS enabled by Azure Arc cluster

  • Article

Applies to: Azure Stack HCI, version 23H2

This article describes how to use node taints in an AKS cluster.

Overview

The AKS scheduling mechanism is responsible for placing pods onto nodes and is based on the upstream Kubernetes scheduler, kube-scheduler. You can constrain a pod to run on particular nodes by instructing the node to reject a set of pods using node taints, which interact with the AKS scheduler.

Node taints work by marking a node so that the scheduler avoids placing certain pods on the marked nodes. You can place tolerations on a pod to allow the scheduler to schedule that pod on a node with a matching taint. Taints and tolerations work together to help you control how the scheduler places pods onto nodes. For more information, see example use cases of taints and tolerations.

Taints are key-value pairs with an effect. There are three values for the effect field when using node taints: NoExecute, NoSchedule, and PreferNoSchedule.

  • NoExecute: Pods already running on the node are immediately evicted if they don't have a matching toleration. If a pod has a matching toleration, it might be evicted if tolerationSeconds are specified.
  • NoSchedule: Only pods with a matching toleration are placed on this node. Existing pods aren't evicted.
  • PreferNoSchedule: The scheduler avoids placing any pods that don't have a matching toleration.

Before you begin

  • This article assumes you have an existing AKS cluster. If you need an AKS cluster, you can create one using Azure CLI, Azure PowerShell, or the Azure portal.
  • When you create a node pool, you can add taints to it. When you add a taint, all nodes within that node pool also get that taint.

Important

You should add taints or labels to nodes for the entire node pool using az aksarc nodepool. We don't recommend using kubectl to apply taints or labels to individual nodes in a node pool.

Set node pool taints

Create a node pool with a taint using the az aksarc nodepool add command. Specify the name taintnp and use the --node-taints parameter to specify sku=gpu:NoSchedule for the taint:

az aksarc nodepool add \
    --resource-group myResourceGroup \
    --cluster-name myAKSCluster \
    --name taintnp \
    --node-count 1 \
    --node-taints sku=gpu:NoSchedule \
    --no-wait

Check the status of the node pool using the az aksarc nodepool list command:

az aksarc nodepool list -g myResourceGroup --cluster-name myAKSCluster

The following example output shows that the taintnp node pool creates nodes with the specified nodeTaints:

[
  {
    ...
    "count": 1,
    ...
    "name": "taintnp",
    ...
    "provisioningState": "Succeeded",
    ...
    "nodeTaints":  [
      "sku=gpu:NoSchedule"
    ],
    ...
  },
 ...
]

The taint information is visible in Kubernetes for handling scheduling rules for nodes. The Kubernetes scheduler can use taints and tolerations to restrict which workloads can run on nodes.

  • A taint is applied to a node that indicates only specific pods can be scheduled on them.
  • A toleration is then applied to a pod that allows them to "tolerate" a node's taint.

Set node pool tolerations

In the previous step, you applied the sku=gpu:NoSchedule taint when you created the node pool. The following example YAML manifest uses a toleration to allow the Kubernetes scheduler to run an NGINX pod on a node in that node pool:

Create a file named nginx-toleration.yaml and copy/paste the following example YAML:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
  - image: mcr.microsoft.com/oss/nginx/nginx:1.15.9-alpine
    name: mypod
    resources:
      requests:
        cpu: 100m
        memory: 128Mi
      limits:
        cpu: 1
        memory: 2G
  tolerations:
  - key: "sku"
    operator: "Equal"
    value: "gpu"
    effect: "NoSchedule"

Schedule the pod using the kubectl apply command:

kubectl apply -f nginx-toleration.yaml

It takes a few seconds to schedule the pod and pull the NGINX image.

Check the status using the kubectl describe pod command:

kubectl describe pod mypod

The following condensed example output shows that the sku=gpu:NoSchedule toleration is applied. In the Events section, the scheduler assigned the pod to the moc-lbeof1gn6x3 node:

[...]
Tolerations:     node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
                 node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
                 sku=gpu:NoSchedule
Events:
  Type    Reason     Age    From                Message
  ----    ------     ----   ----                -------
  Normal  Scheduled  54s  default-scheduler   Successfully assigned default/mypod to moc-lbeof1gn6x3
  Normal  Pulling    53s  kubelet             Pulling image "mcr.microsoft.com/oss/nginx/nginx:1.15.9-alpine"
  Normal  Pulled     48s  kubelet             Successfully pulled image "mcr.microsoft.com/oss/nginx/nginx:1.15.9-alpine" in 3.025148695s (3.025157609s including waiting)
  Normal  Created    48s  kubelet             Created container
  Normal  Started    48s  kubelet             Started container

Only pods that have this toleration applied can be scheduled on nodes in taintnp. Any other pods are scheduled in the nodepool1 node pool. If you create more node pools, you can use taints and tolerations to limit what pods can be scheduled on those node resources.

Update a cluster node pool to add a node taint

Update a cluster to add a node taint using the az aksarc update command and the --node-taints parameter to specify sku=gpu:NoSchedule for the taint. All existing taints are replaced with the new values. The old taints are deleted:

az aksarc update -g myResourceGroup --cluster-name myAKSCluster --name taintnp --node-taints "sku=gpu:NoSchedule"

Next steps