Run Strategies

Overview

VirtualMachines have a Running setting that determines whether or not there should be a guest running or not. Because KubeVirt will always immediately restart a VirtualMachineInstance for VirtualMachines with spec.running: true, a simple boolean is not always enough to fully describe desired behavior. For instance, there are cases when a user would like the ability to shut down a guest from inside the virtual machine. With spec.running: true, KubeVirt would immediately restart the VirtualMachineInstance.

RunStrategy

To allow for greater variation of user states, the RunStrategy field has been introduced. This is mutually exclusive with Running as they have somewhat overlapping conditions. There are currently four RunStrategies defined:

• Always: The system is tasked with keeping the VM in a running state. This is achieved by respawning a VirtualMachineInstance whenever the current one terminated in a controlled (e.g. shutdown from inside the guest) or uncontrolled (e.g. crash) way. This behavior is equal to spec.running: true.

• RerunOnFailure: Similar to Always, except that the VM is only restarted if it terminated in an uncontrolled way (e.g. crash) and due to an infrastructure reason (i.e. the node crashed, the KVM related process OOMed). This allows a user to determine when the VM should be shut down by initiating the shut down inside the guest. Note: Guest sided crashes (i.e. BSOD) are not covered by this. In such cases liveness checks or the use of a watchdog can help.

• Once: The VM will run once and not be restarted upon completion regardless if the completion is of phase Failure or Success.

• Manual: The system will not automatically turn the VM on or off, instead the user manually controlls the VM status by issuing start, stop, and restart commands on the VirtualMachine subresource endpoints.

• Halted: The system is asked to ensure that no VM is running. This is achieved by stopping any VirtualMachineInstance that is associated ith the VM. If a guest is already running, it will be stopped. This behavior is equal to spec.running: false.

Note: RunStrategy and running are mutually exclusive, because they can be contradictory. The API server will reject VirtualMachine resources that define both.

Virtctl

The start, stop and restart methods of virtctl will invoke their respective subresources of VirtualMachines. This can have an effect on the runStrategy of the VirtualMachine as below:

Table entries marked with - don’t make sense, so won’t have an effect on RunStrategy.

RunStrategy Examples

Always

An example usage of the Always RunStrategy.

apiVersion: kubevirt.io/v1
kind: VirtualMachine
metadata:
  labels:
    kubevirt.io/vm: vm-cirros
  name: vm-cirros
spec:
  runStrategy: Always
  template:
    metadata:
      labels:
        kubevirt.io/vm: vm-cirros
    spec:
      domain:
        devices:
          disks:
          - disk:
              bus: virtio
            name: containerdisk
      terminationGracePeriodSeconds: 0
      volumes:
      - containerDisk:
          image: kubevirt/cirros-container-disk-demo:latest
        name: containerdisk