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Taints and tolerations allow the node to control which pods should (or should not) be scheduled on them.

Understanding taints and tolerations

A taint allows a node to refuse a pod to be scheduled unless that pod has a matching toleration.

You apply taints to a node through the Node specification (NodeSpec) and apply tolerations to a pod through the Pod specification (PodSpec). When you apply a taint a node, the scheduler cannot place a pod on that node unless the pod can tolerate the taint.

Example taint in a node specification
apiVersion: v1
kind: Node
metadata:
  name: my-node
#...
spec:
  taints:
  - effect: NoExecute
    key: key1
    value: value1
#...
Example toleration in a Pod spec
apiVersion: v1
kind: Pod
metadata:
  name: my-pod
#...
spec:
  tolerations:
  - key: "key1"
    operator: "Equal"
    value: "value1"
    effect: "NoExecute"
    tolerationSeconds: 3600
#...

Taints and tolerations consist of a key, value, and effect.

Table 1. Taint and toleration components
Parameter Description

key

The key is any string, up to 253 characters. The key must begin with a letter or number, and may contain letters, numbers, hyphens, dots, and underscores.

value

The value is any string, up to 63 characters. The value must begin with a letter or number, and may contain letters, numbers, hyphens, dots, and underscores.

effect

The effect is one of the following:

NoSchedule [1]

  • New pods that do not match the taint are not scheduled onto that node.

  • Existing pods on the node remain.

PreferNoSchedule

  • New pods that do not match the taint might be scheduled onto that node, but the scheduler tries not to.

  • Existing pods on the node remain.

NoExecute

  • New pods that do not match the taint cannot be scheduled onto that node.

  • Existing pods on the node that do not have a matching toleration are removed.

operator

Equal

The key/value/effect parameters must match. This is the default.

Exists

The key/effect parameters must match. You must leave a blank value parameter, which matches any.

  1. If you add a NoSchedule taint to a control plane node, the node must have the node-role.kubernetes.io/master=:NoSchedule taint, which is added by default.

    For example:

    apiVersion: v1
    kind: Node
    metadata:
      annotations:
        machine.openshift.io/machine: openshift-machine-api/ci-ln-62s7gtb-f76d1-v8jxv-master-0
        machineconfiguration.openshift.io/currentConfig: rendered-master-cdc1ab7da414629332cc4c3926e6e59c
      name: my-node
    #...
    spec:
      taints:
      - effect: NoSchedule
        key: node-role.kubernetes.io/master
    #...

A toleration matches a taint:

  • If the operator parameter is set to Equal:

    • the key parameters are the same;

    • the value parameters are the same;

    • the effect parameters are the same.

  • If the operator parameter is set to Exists:

    • the key parameters are the same;

    • the effect parameters are the same.

The following taints are built into Red Hat OpenShift Service on AWS:

  • node.kubernetes.io/not-ready: The node is not ready. This corresponds to the node condition Ready=False.

  • node.kubernetes.io/unreachable: The node is unreachable from the node controller. This corresponds to the node condition Ready=Unknown.

  • node.kubernetes.io/memory-pressure: The node has memory pressure issues. This corresponds to the node condition MemoryPressure=True.

  • node.kubernetes.io/disk-pressure: The node has disk pressure issues. This corresponds to the node condition DiskPressure=True.

  • node.kubernetes.io/network-unavailable: The node network is unavailable.

  • node.kubernetes.io/unschedulable: The node is unschedulable.

  • node.cloudprovider.kubernetes.io/uninitialized: When the node controller is started with an external cloud provider, this taint is set on a node to mark it as unusable. After a controller from the cloud-controller-manager initializes this node, the kubelet removes this taint.

  • node.kubernetes.io/pid-pressure: The node has pid pressure. This corresponds to the node condition PIDPressure=True.

    Red Hat OpenShift Service on AWS does not set a default pid.available evictionHard.

Using tolerations to control log store pod placement

By default, log store pods have the following toleration configurations:

Elasticsearch log store pods default tolerations
apiVersion: v1
kind: Pod
metadata:
  name: elasticsearch-example
  namespace: openshift-logging
spec:
# ...
  tolerations:
  - effect: NoSchedule
    key: node.kubernetes.io/disk-pressure
    operator: Exists
  - effect: NoExecute
    key: node.kubernetes.io/not-ready
    operator: Exists
    tolerationSeconds: 300
  - effect: NoExecute
    key: node.kubernetes.io/unreachable
    operator: Exists
    tolerationSeconds: 300
  - effect: NoSchedule
    key: node.kubernetes.io/memory-pressure
    operator: Exists
# ...
LokiStack log store pods default tolerations
apiVersion: v1
kind: Pod
metadata:
  name: lokistack-example
  namespace: openshift-logging
spec:
# ...
  tolerations:
  - effect: NoExecute
    key: node.kubernetes.io/not-ready
    operator: Exists
    tolerationSeconds: 300
  - effect: NoExecute
    key: node.kubernetes.io/unreachable
    operator: Exists
    tolerationSeconds: 300
  - effect: NoSchedule
    key: node.kubernetes.io/memory-pressure
    operator: Exists
# ...

You can configure a toleration for log store pods by adding a taint and then modifying the tolerations syntax in the ClusterLogging custom resource (CR).

Prerequisites
  • You have installed the Red Hat OpenShift Logging Operator.

  • You have installed the OpenShift CLI (oc).

  • You have deployed an internal log store that is either Elasticsearch or LokiStack.

Procedure
  1. Add a taint to a node where you want to schedule the logging pods, by running the following command:

    $ oc adm taint nodes <node_name> <key>=<value>:<effect>
    Example command
    $ oc adm taint nodes node1 lokistack=node:NoExecute

    This example places a taint on node1 that has key lokistack, value node, and taint effect NoExecute. Nodes with the NoExecute effect schedule only pods that match the taint and remove existing pods that do not match.

  2. Edit the logstore section of the ClusterLogging CR to configure a toleration for the log store pods:

    Example ClusterLogging CR
    apiVersion: logging.openshift.io/v1
    kind: ClusterLogging
    metadata:
    # ...
    spec:
    # ...
      logStore:
        type: lokistack
        elasticsearch:
          nodeCount: 1
          tolerations:
          - key: lokistack (1)
            operator: Exists (2)
            effect: NoExecute (3)
            tolerationSeconds: 6000 (4)
    # ...
    1 Specify the key that you added to the node.
    2 Specify the Exists operator to require a taint with the key lokistack to be present on the node.
    3 Specify the NoExecute effect.
    4 Optional: Specify the tolerationSeconds parameter to set how long a pod can remain bound to a node before being evicted.

This toleration matches the taint created by the oc adm taint command. A pod with this toleration can be scheduled onto node1.

Using tolerations to control the log visualizer pod placement

You can use a specific key/value pair that is not on other pods to ensure that only the Kibana pod can run on the specified node.

Prerequisites
  • You have installed the Red Hat OpenShift Logging Operator, the OpenShift Elasticsearch Operator, and the OpenShift CLI (oc).

Procedure
  1. Add a taint to a node where you want to schedule the log visualizer pod by running the following command:

    $ oc adm taint nodes <node_name> <key>=<value>:<effect>
    Example command
    $ oc adm taint nodes node1 kibana=node:NoExecute

    This example places a taint on node1 that has key kibana, value node, and taint effect NoExecute. You must use the NoExecute taint effect. NoExecute schedules only pods that match the taint and remove existing pods that do not match.

  2. Edit the visualization section of the ClusterLogging CR to configure a toleration for the Kibana pod:

    apiVersion: logging.openshift.io/v1
    kind: ClusterLogging
    metadata:
    # ...
    spec:
    # ...
      visualization:
        type: kibana
        kibana:
          tolerations:
          - key: kibana  (1)
            operator: Exists (2)
            effect: NoExecute (3)
            tolerationSeconds: 6000 (4)
          resources:
            limits:
              memory: 2Gi
            requests:
              cpu: 100m
              memory: 1Gi
          replicas: 1
    # ...
    1 Specify the key that you added to the node.
    2 Specify the Exists operator to require the key, value, and effect parameters to match.
    3 Specify the NoExecute effect.
    4 Optionally, specify the tolerationSeconds parameter to set how long a pod can remain bound to a node before being evicted.

This toleration matches the taint created by the oc adm taint command. A pod with this toleration would be able to schedule onto node1.

Using tolerations to control log collector pod placement

By default, log collector pods have the following tolerations configuration:

apiVersion: v1
kind: Pod
metadata:
  name: collector-example
  namespace: openshift-logging
spec:
# ...
  collection:
    type: vector
    tolerations:
    - effect: NoSchedule
      key: node-role.kubernetes.io/master
      operator: Exists
    - effect: NoSchedule
      key: node.kubernetes.io/disk-pressure
      operator: Exists
    - effect: NoExecute
      key: node.kubernetes.io/not-ready
      operator: Exists
    - effect: NoExecute
      key: node.kubernetes.io/unreachable
      operator: Exists
    - effect: NoSchedule
      key: node.kubernetes.io/memory-pressure
      operator: Exists
    - effect: NoSchedule
      key: node.kubernetes.io/pid-pressure
      operator: Exists
    - effect: NoSchedule
      key: node.kubernetes.io/unschedulable
      operator: Exists
# ...
Prerequisites
  • You have installed the Red Hat OpenShift Logging Operator and OpenShift CLI (oc).

Procedure
  1. Add a taint to a node where you want logging collector pods to schedule logging collector pods by running the following command:

    $ oc adm taint nodes <node_name> <key>=<value>:<effect>
    Example command
    $ oc adm taint nodes node1 collector=node:NoExecute

    This example places a taint on node1 that has key collector, value node, and taint effect NoExecute. You must use the NoExecute taint effect. NoExecute schedules only pods that match the taint and removes existing pods that do not match.

  2. Edit the collection stanza of the ClusterLogging custom resource (CR) to configure a toleration for the logging collector pods:

    apiVersion: logging.openshift.io/v1
    kind: ClusterLogging
    metadata:
    # ...
    spec:
    # ...
      collection:
        type: vector
        tolerations:
        - key: collector (1)
          operator: Exists (2)
          effect: NoExecute (3)
          tolerationSeconds: 6000 (4)
        resources:
          limits:
            memory: 2Gi
          requests:
            cpu: 100m
            memory: 1Gi
    # ...
    1 Specify the key that you added to the node.
    2 Specify the Exists operator to require the key/value/effect parameters to match.
    3 Specify the NoExecute effect.
    4 Optionally, specify the tolerationSeconds parameter to set how long a pod can remain bound to a node before being evicted.

This toleration matches the taint created by the oc adm taint command. A pod with this toleration can be scheduled onto node1.