Merge pull request #258107 from jotavar/add-alert-to-coredns-scaling-configmap-configuration

Added alert regarding recommended minimum number of CoreDNS pod replicas

Author: prmerger-automator[bot]

articles/aks/coredns-custom.md

@@ -224,6 +224,9 @@ Sudden spikes in DNS traffic within AKS clusters are a common occurrence due to
 
 CoreDNS uses [horizontal cluster proportional autoscaler][cluster-proportional-autoscaler] for pod auto scaling. The `coredns-autoscaler` ConfigMap can be edited to configure the scaling logic for the number of CoreDNS pods. The `coredns-autoscaler` ConfigMap currently supports two different ConfigMap key values: `linear` and `ladder` which correspond to two supported control modes. The `linear` controller yields a number of replicas in [min,max] range equivalent to `max( ceil( cores * 1/coresPerReplica ) , ceil( nodes * 1/nodesPerReplica ) )`. The `ladder` controller calculates the number of replicas by consulting two different step functions, one for core scaling and another for node scaling, yielding the max of the two replica values. For more information on the control modes and ConfigMap format, please consult the [upstream documentation][cluster-proportional-autoscaler-control-patterns].
 
+> [!IMPORTANT]
+> A minimum of 2 CoreDNS pod replicas per cluster is recommended. Configuring a minimum of 1 CoreDNS pod replica may result in failures during operations which require node draining, such as cluster upgrade operations.
+
 To retrieve the `coredns-autoscaler` ConfigMap, you can run the `kubectl get configmap coredns-autoscaler -n kube-system -o yaml` command which will return the following:
 
 ```yaml