Understanding Windows container workloads

Red Hat OpenShift support for Windows Containers provides built-in support for running Microsoft Windows Server containers on OKD. For those that administer heterogeneous environments with a mix of Linux and Windows workloads, OKD allows you to deploy Windows workloads running on Windows Server containers while also providing traditional Linux workloads hosted on Fedora CoreOS (FCOS) or Fedora.

Windows Machine Config Operator prerequisites

The following information details the supported platform versions, Windows Server versions, and networking configurations for the Windows Machine Config Operator. See the vSphere documentation for any information that is relevant to only that platform.

WMCO 8.0.0 supported platforms and Windows Server versions

The following table lists the Windows Server versions that are supported by WMCO 8.0.0, based on the applicable platform. Windows Server versions not listed are not supported and attempting to use them will cause errors. To prevent these errors, use only an appropriate version for your platform.

Supported networking

Hybrid networking with OVN-Kubernetes is the only supported networking configuration. See the additional resources below for more information on this functionality. The following tables outline the type of networking configuration and Windows Server versions to use based on your platform. You must specify the network configuration when you install the cluster.

Additional resources

• See Configuring hybrid networking with OVN-Kubernetes

Windows workload management

To run Windows workloads in your cluster, you must first install the Windows Machine Config Operator (WMCO). The WMCO is a Linux-based Operator that runs on Linux-based control plane and compute nodes. The WMCO orchestrates the process of deploying and managing Windows workloads on a cluster.

Figure 1. WMCO design

Before deploying Windows workloads, you must create a Windows compute node and have it join the cluster. The Windows node hosts the Windows workloads in a cluster, and can run alongside other Linux-based compute nodes. You can create a Windows compute node by creating a Windows compute machine set to host Windows Server compute machines. You must apply a Windows-specific label to the compute machine set that specifies a Windows OS image.

The WMCO watches for machines with the Windows label. After a Windows compute machine set is detected and its respective machines are provisioned, the WMCO configures the underlying Windows virtual machine (VM) so that it can join the cluster as a compute node.

Figure 2. Mixed Windows and Linux workloads

The WMCO expects a predetermined secret in its namespace containing a private key that is used to interact with the Windows instance. WMCO checks for this secret during boot up time and creates a user data secret which you must reference in the Windows MachineSet object that you created. Then the WMCO populates the user data secret with a public key that corresponds to the private key. With this data in place, the cluster can connect to the Windows VM using an SSH connection.

After the cluster establishes a connection with the Windows VM, you can manage the Windows node using similar practices as you would a Linux-based node.

Scheduling Windows workloads to a Windows node can be done with typical pod scheduling practices like taints, tolerations, and node selectors; alternatively, you can differentiate your Windows workloads from Linux workloads and other Windows-versioned workloads by using a RuntimeClass object.

Windows node services

The following Windows-specific services are installed on each Windows node:

Known limitations

Note the following limitations when working with Windows nodes managed by the WMCO (Windows nodes):

• The following OKD features are not supported on Windows nodes:

  • Image builds

  • OpenShift Pipelines

  • OpenShift Service Mesh

  • OpenShift monitoring of user-defined projects

  • OpenShift Serverless

  • Horizontal Pod Autoscaling

  • Vertical Pod Autoscaling

• The following Red Hat features are not supported on Windows nodes:

  • Red Hat cost management

  • Red Hat OpenShift Local

• Windows nodes do not support pulling container images from private registries. You can use images from public registries or pre-pull the images.

• Windows nodes do not support workloads created by using deployment configs. You can use a deployment or other method to deploy workloads.

• Windows nodes are not supported in clusters that use a cluster-wide proxy. This is because the WMCO is not able to route traffic through the proxy connection for the workloads.

• Windows nodes are not supported in clusters that are in a disconnected environment.

• Red Hat OpenShift support for Windows Containers does not support adding Windows nodes to a cluster through a trunk port. The only supported networking configuration for adding Windows nodes is through an access port that carries traffic for the VLAN.

• Red Hat OpenShift support for Windows Containers supports only in-tree storage drivers for all cloud providers.

• Kubernetes has identified the following node feature limitations :

  • Huge pages are not supported for Windows containers.

  • Privileged containers are not supported for Windows containers.

• Kubernetes has identified several API compatibility issues.