A couple of weeks ago, during the Virtualization for Developers event in Redmond, we had a short presentation on Viridian by Arno Mihm, Program Manager for Windows Virtualization. He explained the features that will be available on that platform, the differences with the current Virtual Server architecture, and also talked a little bit about the hypervisor.
Windows Virtualization is coming up with some pretty impressive features. The Windows Hypervisor is a tiny piece of code (right now it is around 160KB… it has grown, though, since it was 140KB when we first heard about it on the Longhorn Developer Review back in April), that manages the different partitions running on the physical computer. Microsoft has taken an intelligent approach at this level. The hypervisor only manages context switches between the VMs and protects access to the different VM’s resources. All device drivers and any other logic are managed by the parent OS. This way the hypervisor code can remain really small and extremely fast, and provide the type of reliability that is necessary for this type of environment.
Another nice feature of Windows Virtualization is the device driver architecture. “Enlightened” operating systems will route all device requests through Virtualization Service Clients, that through a very efficient communication mechanism (called VMBus) will communicate directly with Virtualization Service Providers on the parent partition of the server, and then call the hardware directly. This is more efficient than current implementations, in which calls to virtual devices are trapped and handled through the virtual machine worker processes, requiring several context switches in the process.
The best part of the presentation, however, was to finally get to see Windows Virtualization in action. He did a short demo on his laptop, that had a preliminary build of Longhorn server with the Hypervisor enabled. One of the virtual machines was also running Longhorn server, and he showed us how you can dynamically add memory to a virtual machine, WHILE THE VM IS RUNNING, and the client operating system (Longhorn server in this case) will pick it up immediately. This is very useful for those times when you need to give an extra boost to a virtual machine so it can complete a certain task. And my understanding is that all new server products from Microsoft (starting with Exchange 2007) will be able to dynamically pick up these changes as well.