Stripping down Longhorn virtualization

Shortly after The Virtualization Procrastinators appeared in Computerworld last week I was contacted by Jim Ni, Microsoft’s group manager for server virtualization. He wanted to clarify a point in the story that he said might be confusing to readers. After our e-mail exchange I can see why. (The key takeaways appear in bold below).

In the Virtual Competitors sidebar to that story I had paraphrased Ni's description of the virtualization hypervisor in Longhorn as a "stripped-down version of Windows." While a full or stripped down version of Longhorn server is required to make virtualization happen, technically the hypervisor is a separate software layer.

Microsoft's current virtualization offering, Virtual Server 2005, does not run directly on the hardware but on top of a host Windows operating system. This is similar to what V Mware offers with its free Virtual Server product. In contrast, a hypervisor is a software layer that sits on the "bare metal," providing virtualization services without the need for an underlying host operating system (the approach taken by VMware's ESX Server). A hypervisor approach has less overhead and is therefore more stable and efficient (read: faster). Windows Server Longhorn will use a hypervisor but preserve the requirement for a dedicated instance of Windows Server to orchestrate it.

Microsoft's model is different from that pursued by VMware with its ESX Server hypervisor. While there is no host OS under the hypervisor, you still need a dedicated copy of Longhorn running on top of it.

Says Ni:

"The Windows hypervisor is not a stripped down version of Windows Server “Longhorn”. It is a thin layer of software written by our virtualization team that sits between the hardware and the operating system and allows multiple operating systems to run on a host computer at the same time. It provides simple partitioning functionality and is responsible for maintaining strong isolation between partitions. Windows Server “Longhorn” needs to be the running operating system in the parent partition to enable virtualization.

When Windows Server “Longhorn” is installed as the parent partition, it is possible to opt for a minimal installation, called “Server Core”, which installs only the services and subsystems necessary for the server to operate in a virtualization role. This is the “leaner” Windows Server I was referring to. Some benefits of this include a smaller footprint and attack surface, resulting in less management and servicing, as well as a highly optimized partition that is well-suited for virtualization. The Server Core installation option is available in all editions of Windows Server and also supports some other server roles, such as file server, domain controller and DNS server."

Bottom line:

In Windows Server Longhorn, the hypervisor is a software layer that sits between the Windows Server OS and the hardware. However, to operate, it requires that the Windows Server Longhorn OS be running on top of it in what's called a "parent partition." Other, parallel, child partitions also sit on top of the hypervisor layer and run Windows and whatever applications are desired.

So you need both, the hypervisor and a dedicated copy of Longhorn server to run VMs. You still have an extra copy of the OS in the loop. It just runs on top of the hypervisor rather than underneath it. One benefit of this approach, Ni says, is the way in which device drivers are handled.

"Windows Server virtualization is leveraging the device driver model from Windows Server. This means a considerably broader set of hardware devices will be supported by the solution out of the gate. We are leveraging all the years and processes we’ve built with our ecosystem of hardware partners to develop, test, and certify that the OS and HW devices are compatible. This is going to be a great customer benefit as it offers them greater flexibility in choice of HW for virtualization."

In lieu of a full OS install, administrators can deploy a stripped down, "server core" version of Longhorn that installs only what's needed to facilitate virtualization.

All of this brings up two questions. First, what are the licensing implications of having to keep an extra copy of Windows Server Longhorn online to assist the hypervisor with virtualization? The answer is that, as with Virtual Server 2005 today, you'll have to by an extra Windows Server license on each physical machine to support virtualization in the Longhorn environment. However, Enterprise Edition users will be able to run guest instances of Windows in up to four virtual machine partitions on the same physical server without added licensing fees.

Secondly, if the parent partition's job is to manage VMs, why would you ever install a full version rather than a stripped down, server core version in the parent partition? That's just more overhead, right? Says Ni: "You might want to take advantage of graphical utilities and programs that will only be available in the full installation, such as the Event Viewer and Windows Server Backup."