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KubeVirt Updates

Overview

Starting with KubeVirt release v0.17.0 zero downtime updates are supported through the use of the KubeVirt operator.

Zero Downtime in the context of KubeVirt means the following will hold true.

  • The KubeVirt control plane components will remain available throughout the update process allowing for uninterrupted VM/VMI creations, deletions, and modifications.

  • Existing VMI workloads will not be interrupted during the update process.

There are some known disruptions that will occur during updates though. This is limited to anything that involves persistent connections to a control plane component.

  1. In-flight migrations will fail. The VMI will remain available, but the migration itself will fail due to a severed TLS connection occurring when virt-handler updates.

  2. virtctl console and virtctl vnc connections will get dropped. This is due to the virt-api instance that is proxying the connection getting shut down in favor of a an updated virt-api instance. The new virt-api instance will be available before the old one is terminated which is why creation, deletions, and modifications are not impacted.

Methods to Trigger KubeVirt Update

There are two ways to trigger an update of KubeVirt.

By Patching KubeVirt CR's imageTag value

If an imageTag is specified in the KubeVirt CR, then KubeVirt can be updated by patching the imageTag value with a new release tag.

Example:

The cluster has the following KubeVirt CR deployed.

apiVersion: kubevirt.io/v1alpha3
kind: KubeVirt
metadata:
  name: kubevirt
  namespace: kubevirt
spec:
  imageTag: v0.17.0
  imagePullPolicy: IfNotPresent

To update the KubeVirt release to a new version, we'd patch the KubeVirt object.

kubectl patch kv kubevirt -n kubevirt --type=json -p '[{ "op": "add", "path": "/spec/imageTag", "value": "v0.18.0" }]'

By Updating KubeVirt Operator

If neither the imageTag nor the imageRegistry values are specified, the system assumes that the KubeVirt CR's version is locked to the KubeVirt operator's version. This means that updating the operator will automatically trigger updating KubeVirt as well.

Example:

The cluster has the following KubeVirt CR deployed.

apiVersion: kubevirt.io/v1alpha3
kind: KubeVirt
metadata:
  name: kubevirt
  namespace: kubevirt
spec:
  imagePullPolicy: IfNotPresent

Since the CR does not have the imageTag or imageRegistry values set, the KubeVirt and KubeVirt Operator versions are locked.

Updating the KubeVirt operator to a new version will result in the underlying KubeVirt installation to also be updated.

export RELEASE=v0.17.0
kubectl apply -f https://github.com/kubevirt/kubevirt/releases/download/${RELEASE}/kubevirt-operator.yaml

Implementation Details

Component Update Ordering

Controllers (virt-controller and virt-handler) are updated before virt-api.

The old virt-api acts as a feature gate ensuring that new behavior can not be utilized until all the new controllers are available to support the new features.

Once all the controllers are updated, virt-api is updated which will allow usage of new functionality.

RBAC

Since during the update our control plane will be briefly running both old and new versions, we have to ensure that RBAC permissions are available that allow both the old and new versions of components to operate at the same time.

This means that during the update we are using the RBAC rules defined by both the old and new KubeVirt versions. Once the update completes, only the new RBAC rules will remain.

New APIs

New APIs are not available until after the entire update process has completed. This ensures that a new API object can't be posted to the cluster until every component within the cluster is updated to learn of the object.

API version field changes

Changes to the CRD's version field are not currently supported. We do not intend to change the version field of any of our APIs until we have the ability to support multiple versions in parallel.