Node maintenance

Before removing a kubernetes node from the cluster, users will want to ensure that VirtualMachineInstances have been gracefully terminated before powering down the node. Since all VirtualMachineInstances are backed by a Pod, the recommended method of evicting VirtualMachineInstances is to use the kubectl drain command, or in the case of OKD the oc adm drain command.

Evict all VMs from a Node

Select the node you’d like to evict VirtualMachineInstances from by identifying the node from the list of cluster nodes.

kubectl get nodes

The following command will gracefully terminate all VMs on a specific node. Replace <node-name> with the name of the node where the eviction should occur.

kubectl drain <node-name> --delete-local-data --ignore-daemonsets=true --force --pod-selector=kubevirt.io=virt-launcher

Below is a break down of why each argument passed to the drain command is required.

  • kubectl drain <node-name> is selecting a specific node as a target for the eviction

  • --delete-local-data is a required flag that is necessary for removing any pod that utilizes an emptyDir volume. The VirtualMachineInstance Pod does use emptyDir volumes, however the data in those volumes are ephemeral which means it is safe to delete after termination.

  • --ignore-daemonsets=true is a required flag because every node running a VirtualMachineInstance will also be running our helper DaemonSet called virt-handler. DaemonSets are not allowed to be evicted using kubectl drain. By default, if this command encounters a DaemonSet on the target node, the command will fail. This flag tells the command it is safe to proceed with the eviction and to just ignore DaemonSets.

  • --force is a required flag because VirtualMachineInstance pods are not owned by a ReplicaSet or DaemonSet controller. This means kubectl can’t guarantee that the pods being terminated on the target node will get re-scheduled replacements placed else where in the cluster after the pods are evicted. KubeVirt has its own controllers which manage the underlying VirtualMachineInstance pods. Each controller behaves differently to a VirtualMachineInstance being evicted. That behavior is outlined further down in this document.

  • --pod-selector=kubevirt.io=virt-launcher means only VirtualMachineInstance pods managed by KubeVirt will be removed from the node.

Evict all VMs and Pods from a Node

By removing the -pod-selector argument from the previous command, we can issue the eviction of all Pods on a node. This command ensures Pods associated with VMs as well as all other Pods are evicted from the target node.

kubectl drain <node name> --delete-local-data --ignore-daemonsets=true --force

Evacuate VMIs via Live Migration from a Node

If the LiveMigration feature gate is enabled, it is possible to specify an evictionStrategy on VMIs which will react with live-migrations on specific taints on nodes. The following snippet on a VMI or the VMI templates in a VM ensures that the VMI is migrated during node eviction:

  1. spec:
  2.   evictionStrategy: LiveMigrate

Here a full VMI:

  1. apiVersion: kubevirt.io/v1alpha3
  2. kind: VirtualMachineInstance
  3. metadata:
  4.   name: testvmi-nocloud
  5. spec:
  6.   terminationGracePeriodSeconds: 30
  7.   evictionStrategy: LiveMigrate
  8.   domain:
  9.     resources:
  10.       requests:
  11.         memory: 1024M
  12.     devices:
  13.       disks:
  14.       - name: containerdisk
  15.         disk:
  16.           bus: virtio
  17.       - disk:
  18.           bus: virtio
  19.         name: cloudinitdisk
  20.   volumes:
  21.   - name: containerdisk
  22.     containerDisk:
  23.       image: kubevirt/fedora-cloud-container-disk-demo:latest
  24.   - name: cloudinitdisk
  25.     cloudInitNoCloud:
  26.       userData: |-
  27.         #cloud-config
  28.         password: fedora
  29.         chpasswd: { expire: False }

Behind the scenes a PodDisruptionBudget is created for each VMI which has an evictionStrategy defined. This ensures that evictions are be blocked on these VMIs and that we can guarantee that a VMI will be migrated instead of shut off.

Note Prior to v0.34 the drain process with live migrations was detached from the kubectl drain itself and required in addition specifying a special taint on the nodes: kubectl taint nodes foo kubevirt.io/drain=draining:NoSchedule. This is no longer needed. The taint will still be respected if provided but is obsolete.

Re-enabling a Node after Eviction

The kubectl drain will result in the target node being marked as unschedulable. This means the node will not be eligible for running new VirtualMachineInstances or Pods.

If it is decided that the target node should become schedulable again, the following command must be run.

kubectl uncordon <node name>

or in the case of OKD.

oc adm uncordon <node name>

Shutting down a Node after Eviction

From KubeVirt’s perspective, a node is safe to shutdown once all VirtualMachineInstances have been evicted from the node. In a multi-use cluster where VirtualMachineInstances are being scheduled alongside other containerized workloads, it is up to the cluster admin to ensure all other pods have been safely evicted before powering down the node.

VirtualMachine Evictions

The eviction of any VirtualMachineInstance that is owned by a VirtualMachine set to running=true will result in the VirtualMachineInstance being re-scheduled to another node.

The VirtualMachineInstance in this case will be forced to power down and restart on another node. In the future once KubeVirt introduces live migration support, the VM will be able to seamlessly migrate to another node during eviction.

VirtualMachineInstanceReplicaSet Eviction Behavior

The eviction of VirtualMachineInstances owned by a VirtualMachineInstanceReplicaSet will result in the VirtualMachineInstanceReplicaSet scheduling replacements for the evicted VirtualMachineInstances on other nodes in the cluster.

VirtualMachineInstance Eviction Behavior

VirtualMachineInstances not backed by either a VirtualMachineInstanceReplicaSet or an VirtualMachine object will not be re-scheduled after eviction.