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Specifying nodes for OpenShift Virtualization components

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The default scheduling for virtual machines (VMs) on bare metal nodes is appropriate. Optionally, you can specify the nodes where you want to deploy OpenShift Virtualization Operators, workloads, and controllers by configuring node placement rules.

You can configure node placement rules for some components after installing OpenShift Virtualization, but virtual machines cannot be present if you want to configure node placement rules for workloads.
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About node placement rules for OpenShift Virtualization components

You can use node placement rules for the following tasks:

  • Deploy virtual machines only on nodes intended for virtualization workloads.

  • Deploy Operators only on infrastructure nodes.

  • Maintain separation between workloads.

Depending on the object, you can use one or more of the following rule types:

nodeSelector

Allows pods to be scheduled on nodes that are labeled with the key-value pair or pairs that you specify in this field. The node must have labels that exactly match all listed pairs.

affinity

Enables you to use more expressive syntax to set rules that match nodes with pods. Affinity also allows for more nuance in how the rules are applied. For example, you can specify that a rule is a preference, not a requirement. If a rule is a preference, pods are still scheduled when the rule is not satisfied.

tolerations

Allows pods to be scheduled on nodes that have matching taints. If a taint is applied to a node, that node only accepts pods that tolerate the taint.

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Applying node placement rules

You can apply node placement rules by editing a HyperConverged or HostPathProvisioner object using the command line.

Prerequisites

  • The oc CLI tool is installed.

  • You are logged in with cluster administrator permissions.

Procedure

  1. Edit the object in your default editor by running the following command:

    $ oc edit <resource_type> <resource_name> -n {CNVNamespace}

  2. Save the file to apply the changes.

Node placement rule examples

You can specify node placement rules for a OpenShift Virtualization component by editing a HyperConverged or HostPathProvisioner object.

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HyperConverged object node placement rule example

To specify the nodes where OpenShift Virtualization deploys its components, you can edit the nodePlacement object in the HyperConverged custom resource (CR) file that you create during OpenShift Virtualization installation.

Example HyperConverged object with nodeSelector rule
apiVersion: hco.kubevirt.io/v1beta1
kind: HyperConverged
metadata:
  name: kubevirt-hyperconverged
  namespace: openshift-cnv
spec:
  infra:
    nodePlacement:
      nodeSelector:
        example.io/example-infra-key: example-infra-value (1)
  workloads:
    nodePlacement:
      nodeSelector:
        example.io/example-workloads-key: example-workloads-value (2)
1 Infrastructure resources are placed on nodes labeled example.io/example-infra-key = example-infra-value.
2 workloads are placed on nodes labeled example.io/example-workloads-key = example-workloads-value.
Example HyperConverged object with affinity rule
apiVersion: hco.kubevirt.io/v1beta1
kind: HyperConverged
metadata:
  name: kubevirt-hyperconverged
  namespace: openshift-cnv
spec:
  infra:
    nodePlacement:
      affinity:
        nodeAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            nodeSelectorTerms:
            - matchExpressions:
              - key: example.io/example-infra-key
                operator: In
                values:
                - example-infra-value (1)
  workloads:
    nodePlacement:
      affinity:
        nodeAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            nodeSelectorTerms:
            - matchExpressions:
              - key: example.io/example-workloads-key (2)
                operator: In
                values:
                - example-workloads-value
          preferredDuringSchedulingIgnoredDuringExecution:
          - weight: 1
            preference:
              matchExpressions:
              - key: example.io/num-cpus
                operator: Gt
                values:
                - 8 (3)
1 Infrastructure resources are placed on nodes labeled example.io/example-infra-key = example-value.
2 workloads are placed on nodes labeled example.io/example-workloads-key = example-workloads-value.
3 Nodes that have more than eight CPUs are preferred for workloads, but if they are not available, pods are still scheduled.
Example HyperConverged object with tolerations rule
apiVersion: hco.kubevirt.io/v1beta1
kind: HyperConverged
metadata:
  name: kubevirt-hyperconverged
  namespace: openshift-cnv
spec:
  workloads:
    nodePlacement:
      tolerations: (1)
      - key: "key"
        operator: "Equal"
        value: "virtualization"
        effect: "NoSchedule"
1 Nodes reserved for OpenShift Virtualization components are labeled with the key = virtualization:NoSchedule taint. Only pods with matching tolerations are scheduled on reserved nodes.

HostPathProvisioner object node placement rule example

You can edit the HostPathProvisioner object directly or by using the web console.

You must schedule the hostpath provisioner and the OpenShift Virtualization components on the same nodes. Otherwise, virtualization pods that use the hostpath provisioner cannot run. You cannot run virtual machines.

After you deploy a virtual machine (VM) with the hostpath provisioner (HPP) storage class, you can remove the hostpath provisioner pod from the same node by using the node selector. However, you must first revert that change, at least for that specific node, and wait for the pod to run before trying to delete the VM.

You can configure node placement rules by specifying nodeSelector, affinity, or tolerations for the spec.workload field of the HostPathProvisioner object that you create when you install the hostpath provisioner.

Example HostPathProvisioner object with nodeSelector rule
apiVersion: hostpathprovisioner.kubevirt.io/v1beta1
kind: HostPathProvisioner
metadata:
  name: hostpath-provisioner
spec:
  imagePullPolicy: IfNotPresent
  pathConfig:
    path: "</path/to/backing/directory>"
    useNamingPrefix: false
  workload:
    nodeSelector:
      example.io/example-workloads-key: example-workloads-value (1)
1 Workloads are placed on nodes labeled example.io/example-workloads-key = example-workloads-value.
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Additional resources