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For years, Microsoft has been trumpeting its upcoming release of Windows Server Virtualization, a feature of Windows Server 2008 (formerly code-named Longhorn Server) that will ultimately ship separately from that product. However, 2007 hasn't been kind to Virtualization: With development of Windows 2008 winding down, Microsoft has scaled back dramatically its plans for this technology. The result will be a more barebones experience than the company originally announced, with less functionality and less parity with VMware's established ESX Server Infrastructure 3 product line. Here's what you need to know about changes to Windows Server Virtualization.

Inside Viridian
Windows Server Virtualization, code-named Viridian, is Microsoft's hypervisor-based virtualization solution for Windows Server 2008, the next major revision of Windows Server. Windows Server Virtualization will effectively replace Microsoft Virtual Server 2005 R2 in the market and provides dramatic performance and reliability improvements over that product, thanks to its hypervisor-based implementation. That is, unlike host-based virtualization solutions like Virtual Server, Windows Server Virtualization is so-called "full" virtualization software that runs almost directly on the hardware, and not as an application or service under a host OS.

Architecturally, Windows Server Virtualization differs a bit from other full virtualization solutions like ESX Server. Virtualization is typically installed on the Server Core implementation of Windows 2008 though it can also be installed on the mainstream versions of that OS. On a Windows Server Virtualization installation, there's an instance of Windows 2008 Server Core running in a parent partition, with one or more virtualized OS environments running in child partitions. With ESX, there's no parent and child partition. Instead, each OS installation is virtualized and logically runs side-by-side. Although technically inferior, the advantage of Microsoft's approach is that Server Core benefits from the wellspring of device drivers that have been created for Windows. Thus, it should offer superior compatibility and reliability. Likewise, as a hypervisor-based solution, Windows Server Virtualization will offer dramatic performance improvements over host-based virtualization environments.

As a version 1.0 product of sorts, Windows Server Virtualization has gone through a number of permutations. Originally, it was to have shipped as an integrated part of Windows Server 2008. Then, in 2006, Microsoft announced that it would deliver Windows Server Virtualization within 180 days of Windows 2008 as a free add-on, though the company was vague about how that technology would be delivered. It promised a first external beta of Windows Server Virtualization in the first half of 2007. Then, everything changed.

What's Changing?
In early- to mid-2007, Microsoft announced two changes to Virtualization. The first of these announcements came in April 2007: Microsoft would no longer be able to deliver a public beta of Virtualization before mid-year and would instead ship a public beta late in 2007. The company reiterated its commitment to ship Virtualization to customers within 180 days of the completion of Windows 2008, which was still expected by late 2007.

A month later, Microsoft delivered far more damaging news. In addition to the beta delays, the company would be cutting several important Virtualization features to meet its shipping deadlines. So while this technology would still ship within 180 days of the completion of Windows 2008, it would be stripped of some expected core functionality, dramatically reducing its usefulness to enterprises. Here's what's been cut:
Live migration. Originally, Virtualization was to have included a live migration feature that would have enabled customers to seamlessly move a running instance of a virtual machine (VM) from one physical machine to another without any noticeable loss of service.
Hot-add hardware. Virtualization drops its ability to hot-add RAM, storage, microprocessors, and network cards, significantly reducing its ability to scale on the fly to increased demands. Now, Virtualization-based servers will need to be taken offline to be upgraded with these hardware components.
Processor core support. Virtualization was to have originally supported up to 32 processor cores per server. (For example, a server with eight physical processors, each with four processor cores.) Now, it will support just 16 processor cores per server, impairing its ability to efficiently serve the largest enterprises.

Microsoft defended the decision to drop functionality by noting, "shipping is a feature too," a somewhat flip assessment given the company's repeated promises for this technology. On the other hand, I can now report how Microsoft plans to ship Virtualization to customers: It will be installed automatically via Microsoft Update (or whatever Microsoft-oriented update mechanism you're using). On supported Windows 2008 systems, Virtualization will appear as a standard server role alongside other Server Core roles.

Microsoft vs. VMware
Microsoft's detuning of Windows Server Virtualization leaves a lot of unanswered questions. Compared with competing solutions—especially mature and full-featured ESX Server— Virtualization will come up short, both in overall functionality and management capabilities. Given the staggered release schedule for Windows Server products, it's unlikely that we'll see the missing Virtualization features appear before the expected 2009 release of Windows Server 2008 R2. But who can say how much ESX will improve by that point?

On the management front, Microsoft will ship a separately licensed product called System Center Virtual Machine Manager 2007 by the end of 2007. This product should be considered a necessary part of any Virtualization rollout, given its capabilities: System Center Virtual Machine Manager will provide a centralized management console for all Microsoft-oriented VMs, physical-to-virtual (P2V) and virtual-to-virtual (V2V) conversion utilities, and automated facilities for provisioning server hardware for the deployed virtual environments. However, we don't know at the time of this writing how much Microsoft will charge for this product. Arguably, it should be simply included free with Windows 2008 if Microsoft is serious about promoting its virtualization technologies.

Recommendations
So what's an enterprise to do? VMware has told me that it thinks the market for virtualization is big enough to support two major players, and I believe this to be the case. However, Microsoft's decision to delay and detune Virtualization is going to cause headaches for anyone who had expected to standardize on Microsoft's technology. My advice is simple: Though disappointing, Microsoft's plans for Virtualization aren't problematic enough to offset the advantages for all but the most demanding environments. Microsoft is slowly working toward integrating virtualization wherever it makes sense in its product lines, and with technologies like Microsoft SoftGrid and even Terminal Services RemoteApp filling the virtualization gaps, Microsoft should have solutions in place for (ahem) virtually any virtualization need by early 2008. What's missing, of course, is the depth of functionality and maturity of ESX Server, but then that's something that Microsoft was never going to achieve with its 1.0 release anyway. Microsoft's decision to integrate virtualization capabilities into the core OS is the right one for customers, and the architecture of the system should eventually provide reliability and compatibility advantages over ESX systems, especially for Microsoft shops. As for those moving most aggressively into virtualization, your decision is a bit more difficult. But then it's likely that such companies were already evaluating or using ESX by this point anyway.

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