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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 {product-title} 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.

Creating Daemonsets

Important

Before creating daemonsets, ensure you have been given the required role by your {product-title} 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
Important

Currently, updating a daemonset’s pod template does not affect existing pod replicas. Moreover, if you delete a daemonset and then create a new daemonset with a different template but the same label selector, it will recognize any existing pod replicas as having matching labels and thus will not update them or create new replicas despite a mismatch in the pod template.

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