There's a format war going on, and given the increasing popularity of virtualization in the enterprise, purchasing virtualization solutions will get more complicated for Microsoft customers. Some believe that standards will help accelerate the development of virtualization solutions and add-on products. Vendors are waiting for the prominent market players to fork over their virtualization APIs to industry standards organizations so those organizations can generate some virtual machine standards to foster interoperability between products. Here's what you need to know about VMware's and Microsoft's virtualization products and the two companies' different strategies.

What VMware Offers
As the true innovator in this space, VMware jump-started the virtualization market for PC-based servers and has been steadily advancing the technology for the past decade. VMware has a variety of server-and client-based virtualization solutions that are continually updated and improved and offers versions that support both Windows and Linux, making it the preferred supplier for corporations that mix and match platforms according to their needs.

In addition to VMware Workstation on the client side, VMware offers VMware Player—free software that lets you run prebuilt virtual machine (VM) environments without having to use the VMware Workstation product. On the server side, VMware offers the free VMware Server as well as VMware ESX Server and VMware Virtual Center, which is a management front-end for the other virtualization products. The server products are also available in a suite called VMware Infrastructure 3.

What Microsoft Offers
Microsoft offers two major virtualization products: Virtual PC 2004 Service Pack 1 (SP1) and Virtual Server 2005 Release 2 (R2). Virtual PC is a client-side solution that hasn't been updated in over a year, but Microsoft recently made it available for free to users and will ship a free Virtual PC 2007 product for Windows Vista next year.

Virtual Server 2005 R2, which Windows Server 2003 and Windows XP users can download from the Microsoft Web site for free, is a server-based version of Virtual PC with many additional enterprise-oriented features. Microsoft is in the process of engineering its various management tools to be virtualization savvy by recognizing which installations are virtualized and offering different options where appropriate.

Standardization Efforts
In a laudable first step toward standardization, both VMware and Microsoft participate in the Distributed Management Task Force (DMTF) interoperability standards group. To get the ball rolling, VMware has offered its experience, APIs, and some of its code to the DMTF. Microsoft also brings experience to the DMTF, and although it hasn't openly released source code or APIs, it is willing to work with customers and partners to develop Windows-based virtualization products. As an example, Microsoft provided a preliminary version of its hypercall interface for Windows hypervisor to Windows Hardware Engineering Conference (WinHEC) attendees a few months ago.

VMware has also started the VMware Community Source initiative. This program provides a way for VMware partners to look at the ESX Server source code, understand how VMware is implementing virtualization, and make suggestions.

Bones of Contention
Microsoft licenses its Virtual Hard Disk (VHD) image format for free in the hope that other management-tool vendors and third-party developers will support it and make it a de facto standard. However, Microsoft retains control of the format and could revoke its free license at any time.

Looking ahead, Microsoft is talking up its hypervisor technology, a virtualization layer that will be added to Windows Server after the release of Longhorn Server. Microsoft describes hypervisor as amazingly small, and because it will sit at a low level in the OS, it should offer superior performance to today's application-level virtualization solutions. But VMware cautions that Microsoft's hypervisor technology is a closed system with arbitrary licensing and ties customers to a Windows platform instead of allowing them to think in terms of the solutions they need.

Given Microsoft's historic antitrust troubles, you might think that VMware could make a strong case that Microsoft is bundling its virtualization technologies with Windows in order to defeat the market leader. But VMware President Diane Greene tells me that VMware will compete with Microsoft in the market, not in the courts. Instead, she says, she wants the market to standardize on a virtualization technology—whether it's VMware's or Microsoft's—so that virtualization can move to the next level.

So why would Microsoft fight such a customer-centric move? "If everyone uses Microsoft's format, they can lock customers into their management tools," Greene told me. "But the actual formats don't matter." VMware, tellingly, can convert VHD format into its own formats using VMware Virtual Machine Importer.

Recommendations
Virtualization is important for enterprises now, and they don't have the luxury of waiting several years for the standardization process to run its course. Corporations that want to virtualize parts of their environment should try to encourage vendors to open up their products to the true standardization processes and cede control of VM formats to independent groups outside their sphere of influence. Until this happens, virtualization will never gain the industry acceptance it needs to become a full-fledged member of every business's toolbox.

What does all this mean? For now, diehard Microsoft shops should seek out the Microsoft solutions, but others might prefer VMware's deeper, more functional product line.