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Using Daemonsets

Overview

A daemonset can be used to run replicas of a pod on specific or all nodes in an OpenShift Container Platform cluster.

Use daemonsets to create shared storage, run a logging pod on every node in your cluster, or deploy a monitoring agent on every node.

For more information on daemonsets, see the Kubernetes documentation.

Daemonset scheduling is incompatible with project’s default node selector. If you fail to disable it, the daemonset gets restricted by merging with the default node selector. This results in frequent pod recreates on the nodes that got unselected by the merged node selector, which in turn puts unwanted load on the cluster.

Therefore,

  • Before you start using daemonsets, disable the default project-wide node selector in your namespace, by setting the namespace annotation openshift.io/node-selector to an empty string:

    # oc patch namespace myproject -p \
    '{"metadata": {"annotations": {"openshift.io/node-selector": ""}}}'

  • If you are creating a new project, overwrite the default node selector using oc adm new-project --node-selector="".

Creating Daemonsets

Before creating daemonsets, ensure you have been given the required role by your OpenShift Container Platform administrator.

When creating daemonsets, the nodeSelector field is used to indicate the nodes on which the daemonset should deploy replicas.

  1. Define the daemonset yaml file:

    apiVersion: extensions/v1beta1
kind: DaemonSet
metadata:
  name: hello-daemonset
spec:
  selector:
      matchLabels:
        name: hello-daemonset (1)
  template:
    metadata:
      labels:
        name: hello-daemonset (2)
    spec:
      nodeSelector: (3)
        type: infra
      containers:
      - image: openshift/hello-openshift
        imagePullPolicy: Always
        name: registry
        ports:
        - containerPort: 80
          protocol: TCP
        resources: {}
        terminationMessagePath: /dev/termination-log
      serviceAccount: default
      terminationGracePeriodSeconds: 10
    1 The label selector that determines which pods belong to the daemonset.
    2 The pod template’s label selector. Must match the label selector above.
    3 The node selector that determines on which nodes pod replicas should be deployed.

  2. Create the daemonset object:

    oc create -f daemonset.yaml

  3. To verify that the pods were created, and that each node has a pod replica:

    1. Find the daemonset pods:

      $ oc get pods
hello-daemonset-cx6md   1/1       Running   0          2m
hello-daemonset-e3md9   1/1       Running   0          2m

    2. View the pods to verify the pod has been placed onto the node:

      $ oc describe pod/hello-daemonset-cx6md|grep Node
Node:        openshift-node01.hostname.com/10.14.20.134
$ oc describe pod/hello-daemonset-e3md9|grep Node
Node:        openshift-node02.hostname.com/10.14.20.137

  • If you update a DaemonSet’s pod template, the existing pod replicas are not affected.

  • If you delete a DaemonSet and then create a new DaemonSet with a different template but the same label selector, it recognizes any existing pod replicas as having matching labels and thus does not update them or create new replicas despite a mismatch in the pod template.

  • If you change node labels, the DaemonSet adds pods to nodes that match the new labels and deletes pods from nodes that do not match the new labels.

To update a DaemonSet, force new pod replicas to be created by deleting the old replicas or nodes.