OpenShift Container Platform

Spreading node pool VMs by using topologySpreadConstraint

In some scenarios, node pool virtual machines (VMs) might run on the same node, which can cause availability issues. To avoid distribution of VMs on a single node, use the descheduler to continuously honor the topologySpreadConstraint constraint to spread VMs on multiple nodes.

By default, KubeVirt VMs created by a node pool are scheduled on any available nodes that have the capacity to run the VMs. The topologySpreadConstraint constraint is set to schedule VMs on multiple nodes.

Prerequisites

You installed the Kube Descheduler Operator. For more information, see "Installing the descheduler".

Procedure

Open the KubeDescheduler custom resource (CR) by entering the following command, and then modify the KubeDescheduler CR to use the SoftTopologyAndDuplicates and KubeVirtRelieveAndMigrate profiles so that you maintain the topologySpreadConstraint constraint settings.

The KubeDescheduler CR named cluster runs in the openshift-kube-descheduler-operator namespace.
```
$ oc edit kubedescheduler cluster -n openshift-kube-descheduler-operator
```
Example KubeDescheduler configuration
```
apiVersion: operator.openshift.io/v1
kind: KubeDescheduler
metadata:
  name: cluster
  namespace: openshift-kube-descheduler-operator
spec:
  mode: Automatic
  managementState: Managed
  deschedulingIntervalSeconds: 30
  profiles:
  - SoftTopologyAndDuplicates
  - KubeVirtRelieveAndMigrate
  profileCustomizations:
    devDeviationThresholds: AsymmetricLow
    devActualUtilizationProfile: PrometheusCPUCombined
# ...
```
where:

spec.deschedulingIntervalSeconds

Sets the number of seconds between the descheduler running cycles.

spec.profiles

The SoftTopologyAndDuplicates profile evicts pods that follow the whenUnsatisfiable: ScheduleAnyway soft topology constraint. The KubeVirtRelieveAndMigrate profile balances resource usage between nodes and enables strategies, such as RemovePodsHavingTooManyRestarts and LowNodeUtilization.