Yesterday Microsoft shipped the Release Candidate 0 (RC0, and no I don't understand the naming convention either) version of Windows Server 2008, allowing customers to get their hands on near-final code of the upcoming OS. Windows 2008 RC0 doesn't differ much from a functional perspective from the previous release, a June 2007 Community Technical Preview (CTP) release. But it does come with one major new feature: the first public prerelease version of Windows Server Virtualization, code-named Viridian.

I discussed Viridian a bit two weeks ago. This week, I'm excited to finally be able to provide more detail, as this is a technology that we've all been anxious to get some hands-on time with. Here's what I know so far.

Bryon Surace, a program manager in Microsoft's virtualization team told me in a briefing earlier this month that the overall schedule for Windows Server Virtualization hasn't changed. The company still intends to ship a beta version of Windows Server Virtualization when Windows 2008 is completed in the first quarter of 2008. What shipped this week is a pre-beta, or alpha, version of the technology that's essentially feature-complete. It is not, however, extensively tested, so you shouldn't expect to use Windows Server Virtualization anywhere near production servers. Even the branding for Windows Server Virtualization is still in play: Although Microsoft might use the Windows Server Virtualization name for the final version of the product, the company is considering other options.

"The big issue here is that Viridian is a component of Windows Server 2008 and not a separate product like Virtual Server," Surace told me. "It's a brand new architecture based on the Windows hypervisor. From a management perspective, Viridian is installed and managed as a role under Windows 2008, just like DHCP, file and print services, and so on."

Contrary to some reports, Windows Server Virtualization can be installed on either a Server Core installation of Windows 2008 or a more traditional installation of the product. That said, Microsoft strongly recommends that any future live deployments of the technology should occur on Server Core, because that type of near-bare-metal installation will provide a smaller attack surface and better performance. So although the company will support Windows Server Virtualization installations on traditional Windows 2008 servers, it's providing that option mostly for testing purposes.

Another issue to consider is the underlying hardware. Although Windows 2008 will ship in both 32-bit (x86) and 64-bit (x64) variants (as well as some versions for the Itanium platform), Windows Server Virtualization will be available only on x64 versions of the product. (And although the specific product versions haven't yet been locked down, Surace tells me that Windows Server Virtualization will be an option on all x64 versions of Windows 2008, and not limited to certain versions.) The reason for this limitation is that Windows Server Virtualization relies on hardware virtualization features that are available only in the latest AMD and Intel chipsets.

Windows Server Virtualization will support both 32-bit and 64-bit guest OSs, which in Windows Server Virtualization parlance run in child partitions. (The host OS runs in the so-called parent partition.) It will support up to 32GB of RAM in each guest OS, a huge improvement over the 3.6GB limit in Virtual Server 2005. Windows Server Virtualization will also support allocating up to four CPU cores to each guest OS.

Aside from the underlying architectural differences, the big difference between Windows Server Virtualization and Virtual Server, and indeed, other virtual machine (VM) platforms like those provided by VMware, is that Windows Server Virtualization supports the notion of virtualized, or synthesized, devices. In other systems, the hardware that's "seen" by each guest OS is emulated, which makes for decent compatibility but poor performance, Surace said. Windows Server Virtualization's synthetic device drivers improve performance by dramatically reducing the number of traversals the system needs to make between kernel mode and user mode.

"This is a new approach that removes the performance bottleneck," Surace told me. "We call it OS enlightenment: The OS knows it's using synthesized device drivers and knows that it's being virtualized. It's similar to the para-virtualization scheme in Xen \[an open-source virtualization engine\]." Enlightened OSs include Windows Server 2003 and Windows 2008, and Microsoft is partnering with Xensource to ship drivers for Linux so that Linux, too, can be "enlightened."

I'm out of space, but I have more information about this software, along with instructions on how to install it in the RC0 release, in my Viridian Preview on the SuperSite for Windows. http://www.winsupersite.com/article/showcase/windows-server-virtualization-preview.aspx

If you're interested in Windows 2008 RC0 or the Viridian preview, you can download both from the Microsoft Web site. http://www.microsoft.com/windowsserver2008/audsel.mspx