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Placing pods on specific nodes using node selectors

A node selector specifies a map of key-value pairs. The rules are defined using custom labels on nodes and selectors specified in pods.

For the pod to be eligible to run on a node, the pod must have the indicated key-value pairs as the label on the node.

If you are using node affinity and node selectors in the same pod configuration, see the important considerations below.

Using node selectors to control pod placement

You can use node selector labels on pods to control where the pod is scheduled.

With node selectors, OpenShift Container Platform schedules the pods on nodes that contain matching labels.

You can add labels to a node or MachineConfig, but the labels will not persist if the node or machine goes down. Adding the label to the MachineSet ensures that new nodes or machines will have the label.

To add node selectors to an existing pod, add a node selector to the controlling object for that node, such as a ReplicaSet, Daemonset, or StatefulSet. Any existing pods under that controlling object are recreated on a node with a matching label. If you are creating a new pod, you can add the node selector directly to the pod spec.

You cannot add a node selector to an existing scheduled pod.
Prerequisites

If you want to add a node selector to existing pods, determine the controlling object for that pod. For exeample, the router-default-66d5cf9464-m2g75 pod is controlled by the router-default-66d5cf9464 ReplicaSet:

$ oc describe pod router-default-66d5cf9464-7pwkc

Name:               router-default-66d5cf9464-7pwkc
Namespace:          openshift-ingress

....

Controlled By:      ReplicaSet/router-default-66d5cf9464

The web console lists the controlling object under ownerReferences in the pod YAML:

  ownerReferences:
    - apiVersion: apps/v1
      kind: ReplicaSet
      name: router-default-66d5cf9464
      uid: d81dd094-da26-11e9-a48a-128e7edf0312
      controller: true
      blockOwnerDeletion: true
Procedure

  1. Add the desired label to your nodes:

    $ oc label <resource> <name> <key>=<value>

    For example, to label a node:

    $ oc label nodes ip-10-0-142-25.ec2.internal type=user-node region=east

    The label is applied to the node:

    kind: Node
apiVersion: v1
metadata:
  name: ip-10-0-131-14.ec2.internal
  selfLink: /api/v1/nodes/ip-10-0-131-14.ec2.internal
  uid: 7bc2580a-8b8e-11e9-8e01-021ab4174c74
  resourceVersion: '478704'
  creationTimestamp: '2019-06-10T14:46:08Z'
  labels:
    beta.kubernetes.io/os: linux
    failure-domain.beta.kubernetes.io/zone: us-east-1a
    node.openshift.io/os_version: '4.2'
    node-role.kubernetes.io/worker: ''
    failure-domain.beta.kubernetes.io/region: us-east-1
    node.openshift.io/os_id: rhcos
    beta.kubernetes.io/instance-type: m4.large
    kubernetes.io/hostname: ip-10-0-131-14
    region: east (1)
    beta.kubernetes.io/arch: amd64
    type: user-node (1)
....
    1 Specify the label(s) you will add to the node.

    Alternatively, you can add the label to a MachineSet:

    $ oc edit MachineSet abc612-msrtw-worker-us-east-1c
    apiVersion: machine.openshift.io/v1beta1
kind: MachineSet

....

spec:
  replicas: 2
  selector:
    matchLabels:
      machine.openshift.io/cluster-api-cluster: ci-ln-89dz2y2-d5d6b-4995x
      machine.openshift.io/cluster-api-machine-role: worker
      machine.openshift.io/cluster-api-machine-type: worker
      machine.openshift.io/cluster-api-machineset: ci-ln-89dz2y2-d5d6b-4995x-worker-us-east-1a
  template:
    metadata:
      creationTimestamp: null
      labels:
        machine.openshift.io/cluster-api-cluster: ci-ln-89dz2y2-d5d6b-4995x
        machine.openshift.io/cluster-api-machine-role: worker
        machine.openshift.io/cluster-api-machine-type: worker
        machine.openshift.io/cluster-api-machineset: ci-ln-89dz2y2-d5d6b-4995x-worker-us-east-1a
    spec:
      metadata:
        creationTimestamp: null
        labels:
          region: east (1)
          type: user-node (1)
....
    1 Specify the label(s) you will add to the node.

  2. Add the desired node selector a pod:

    • To add a node selector to existing and furture pods, add a node selector to the controlling object for the pods:

      For example:

      kind: ReplicaSet

....

spec:

....

  template:
    metadata:
      creationTimestamp: null
      labels:
        ingresscontroller.operator.openshift.io/deployment-ingresscontroller: default
        pod-template-hash: 66d5cf9464
    spec:
      nodeSelector:
        beta.kubernetes.io/os: linux
        node-role.kubernetes.io/worker: ''
        type: user-node (1)
      1 Add the desired node selector.

    • For a new pod, you can add the selector to the pod specification directly:

      apiVersion: v1
kind: Pod

...

spec:
  nodeSelector:
    <key>: <value>

...

      For example:

      apiVersion: v1
kind: Pod

....

spec:
  nodeSelector:
    region: east
    type: user-node

If you are using node selectors and node affinity in the same pod configuration, note the following:

  • If you configure both nodeSelector and nodeAffinity, both conditions must be satisfied for the pod to be scheduled onto a candidate node.

  • If you specify multiple nodeSelectorTerms associated with nodeAffinity types, then the pod can be scheduled onto a node if one of the nodeSelectorTerms is satisfied.

  • If you specify multiple matchExpressions associated with nodeSelectorTerms, then the pod can be scheduled onto a node only if all matchExpressions are satisfied.