Microsoft on Monday 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 Server 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 pre-release version of Windows Server Virtualization (WSV), codenamed Viridian.

We discussed Viridian a bit last week. This week, I'm excited to finally be able to provide more detail, as this is a technology that we've all been waiting 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 WSV hasn't changed. The company still intends to ship a beta version of WSV when Windows Server 2008 is completed in the first quarter of 2008. What they've shipped this week is a pre-beta, or alpha, version of the technology that is essentially feature-complete. It is not, however, extensively tested, so you shouldn't expect to use WSV anywhere near production servers. Even the branding for WSV is still in play: While Microsoft might use the WSV name for the final version of the product, it 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, WSV can be installed on either a Server Core install of Windows 2008 or a more traditional install 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 install type will provide a smaller attack surface and better performance. So while the company will support WSV installs on traditional Windows 2008 servers, it is essentially providing that option mostly for testing purposes.

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

WSV will support both 32-bit and 64-bit guest operating systems, which in WSV parlance run in child partitions. (The host OS runs in the so-called parent partition.) It will support up to 32 GB of RAM in each guest OS, a huge improvement over the 3.6 GB limit in Virtual Server 2005. WSV will also support allocating up to 4 CPU cores to each guest OS.

Aside from the underlying architectural differences, the big difference between WSV and Virtual Server, and indeed, other virtual machine platforms like those provided by VMWare, is that WSV 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. WSV'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 is 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 OSes include Windows Server 2003 and 2008, and Microsoft is partnering with Xensource to ship drivers for Linux so that Linux, too, can be "enlightened."

For more information about this software, along with instructions on how to install it in the RC0 release, in my Viridian Preview.

This article originally appeared in the September 25, 2007 issue of Windows IT Pro UPDATE.