Working with nodes

As an administrator, you can perform several tasks to make your clusters more efficient.

Understanding how to evacuate pods on nodes

Evacuating pods allows you to migrate all or selected pods from a given node or nodes.

You can only evacuate pods backed by a replication controller. The replication controller creates new pods on other nodes and removes the existing pods from the specified node(s).

Bare pods, meaning those not backed by a replication controller, are unaffected by default. You can evacuate a subset of pods by specifying a pod-selector. Pod selectors are based on labels, so all the pods with the specified label will be evacuated.

Procedure

  1. Mark the nodes unschedulable before performing the pod evacuation.

    1. Mark the node as unschedulable:

      1. $ oc adm cordon <node1>

      Example output

      1. node/<node1> cordoned

    2. Check that the node status is Ready,SchedulingDisabled:

      1. $ oc get node <node1>

      Example output

      1. NAME        STATUS                     ROLES     AGE       VERSION
      2. <node1>     Ready,SchedulingDisabled   worker    1d        v1.27.3

  2. Evacuate the pods using one of the following methods:

    • Evacuate all or selected pods on one or more nodes:

      1. $ oc adm drain <node1> <node2> [--pod-selector=<pod_selector>]

    • Force the deletion of bare pods using the --force option. When set to true, deletion continues even if there are pods not managed by a replication controller, replica set, job, daemon set, or stateful set:

      1. $ oc adm drain <node1> <node2> --force=true

    • Set a period of time in seconds for each pod to terminate gracefully, use --grace-period. If negative, the default value specified in the pod will be used:

      1. $ oc adm drain <node1> <node2> --grace-period=-1

    • Ignore pods managed by daemon sets using the --ignore-daemonsets flag set to true:

      1. $ oc adm drain <node1> <node2> --ignore-daemonsets=true

    • Set the length of time to wait before giving up using the --timeout flag. A value of 0 sets an infinite length of time:

      1. $ oc adm drain <node1> <node2> --timeout=5s

    • Delete pods even if there are pods using emptyDir volumes by setting the --delete-emptydir-data flag to true. Local data is deleted when the node is drained:

      1. $ oc adm drain <node1> <node2> --delete-emptydir-data=true

    • List objects that will be migrated without actually performing the evacuation, using the --dry-run option set to true:

      1. $ oc adm drain <node1> <node2>  --dry-run=true

      Instead of specifying specific node names (for example, <node1> <node2>), you can use the --selector=<node_selector> option to evacuate pods on selected nodes.

  3. Mark the node as schedulable when done.

    1. $ oc adm uncordon <node1>

Understanding how to update labels on nodes

You can update any label on a node.

Node labels are not persisted after a node is deleted even if the node is backed up by a Machine.

Any change to a MachineSet object is not applied to existing machines owned by the compute machine set. For example, labels edited or added to an existing MachineSet object are not propagated to existing machines and nodes associated with the compute machine set.

  • The following command adds or updates labels on a node:

    1. $ oc label node <node> <key_1>=<value_1> ... <key_n>=<value_n>

    For example:

    1. $ oc label nodes webconsole-7f7f6 unhealthy=true

    You can alternatively apply the following YAML to apply the label:

    1. kind: Node
    2. apiVersion: v1
    3. metadata:
    4.   name: webconsole-7f7f6
    5.   labels:
    6.     unhealthy: ‘true’
    7. #…

  • The following command updates all pods in the namespace:

    1. $ oc label pods --all <key_1>=<value_1>

    For example:

    1. $ oc label pods --all status=unhealthy

Understanding how to mark nodes as unschedulable or schedulable

By default, healthy nodes with a Ready status are marked as schedulable, which means that you can place new pods on the node. Manually marking a node as unschedulable blocks any new pods from being scheduled on the node. Existing pods on the node are not affected.

  • The following command marks a node or nodes as unschedulable:

    Example output

    1. $ oc adm cordon <node>

    For example:

    1. $ oc adm cordon node1.example.com

    Example output

    1. node/node1.example.com cordoned
    3. NAME                 LABELS                                        STATUS
    4. node1.example.com    kubernetes.io/hostname=node1.example.com      Ready,SchedulingDisabled

  • The following command marks a currently unschedulable node or nodes as schedulable:

    1. $ oc adm uncordon <node1>

    Alternatively, instead of specifying specific node names (for example, <node>), you can use the --selector=<node_selector> option to mark selected nodes as schedulable or unschedulable.

Handling errors in single-node OpenShift clusters when the node reboots without draining application pods

In single-node OpenShift clusters and in OKD clusters in general, a situation can arise where a node reboot occurs without first draining the node. This can occur where an application pod requesting devices fails with the UnexpectedAdmissionError error. Deployment, ReplicaSet, or DaemonSet errors are reported because the application pods that require those devices start before the pod serving those devices. You cannot control the order of pod restarts.

While this behavior is to be expected, it can cause a pod to remain on the cluster even though it has failed to deploy successfully. The pod continues to report UnexpectedAdmissionError. This issue is mitigated by the fact that application pods are typically included in a Deployment, ReplicaSet, or DaemonSet. If a pod is in this error state, it is of little concern because another instance should be running. Belonging to a Deployment, ReplicaSet, or DaemonSet guarantees the successful creation and execution of subsequent pods and ensures the successful deployment of the application.

There is ongoing work upstream to ensure that such pods are gracefully terminated. Until that work is resolved, run the following command for a single-node OpenShift cluster to remove the failed pods:

  1. $ oc delete pods --field-selector status.phase=Failed -n <POD_NAMESPACE>

The option to drain the node is unavailable for single-node OpenShift clusters.

Additional resources

Deleting nodes

Deleting nodes from a cluster

When you delete a node using the CLI, the node object is deleted in Kubernetes, but the pods that exist on the node are not deleted. Any bare pods not backed by a replication controller become inaccessible to OKD. Pods backed by replication controllers are rescheduled to other available nodes. You must delete local manifest pods.

Procedure

To delete a node from the OKD cluster, edit the appropriate MachineSet object:

If you are running cluster on bare metal, you cannot delete a node by editing MachineSet objects. Compute machine sets are only available when a cluster is integrated with a cloud provider. Instead you must unschedule and drain the node before manually deleting it.

  1. View the compute machine sets that are in the cluster:

    1. $ oc get machinesets -n openshift-machine-api

    The compute machine sets are listed in the form of <clusterid>-worker-<aws-region-az>.

  2. Scale the compute machine set:

    1. $ oc scale --replicas=2 machineset <machineset> -n openshift-machine-api

    Or:

    1. $ oc edit machineset <machineset> -n openshift-machine-api

    You can alternatively apply the following YAML to scale the compute machine set:

    1. apiVersion: machine.openshift.io/v1beta1
    2. kind: MachineSet
    3. metadata:
    4.   name: <machineset>
    5.   namespace: openshift-machine-api
    6. spec:
    7.   replicas: 2
    8. #…

Additional resources

Deleting nodes from a bare metal cluster

When you delete a node using the CLI, the node object is deleted in Kubernetes, but the pods that exist on the node are not deleted. Any bare pods not backed by a replication controller become inaccessible to OKD. Pods backed by replication controllers are rescheduled to other available nodes. You must delete local manifest pods.

Procedure

Delete a node from an OKD cluster running on bare metal by completing the following steps:

  1. Mark the node as unschedulable:

    1. $ oc adm cordon <node_name>

  2. Drain all pods on the node:

    1. $ oc adm drain <node_name> --force=true

    This step might fail if the node is offline or unresponsive. Even if the node does not respond, it might still be running a workload that writes to shared storage. To avoid data corruption, power down the physical hardware before you proceed.

  3. Delete the node from the cluster:

    1. $ oc delete node <node_name>

    Although the node object is now deleted from the cluster, it can still rejoin the cluster after reboot or if the kubelet service is restarted. To permanently delete the node and all its data, you must decommission the node.

  4. If you powered down the physical hardware, turn it back on so that the node can rejoin the cluster.