VirtualMachineInstance Node Eviction
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 OpenShift the oc adm drain command.
How to Evict all VMs on 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.
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-datais a required flag that is necessary for removing any pod that utilizes an emptyDir volume. The VirtualMachineInstance Pod does use emptryDir volumes, however the data in those volumes are ephemeral which means it is safe to delete after termination.
--ignore-daemonsets=trueis 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.
--forceis 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 futher down in this document.
--pod-selector=kubevirt.io=virt-launchermeans only VirtualMachineInstance pods managed by KubeVirt will be removed from the node.
How to Evict all VMs and Pods on 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
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 OpenShift
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 along side other containerized workloads, it is up to the cluster admin to ensure all other pods have been safely evicted before powering down the node.
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.