While VMware holds the lion’s share of the server virtualisation market, several competing products are available -- or are in the works. Here’s how a few of the key players stack up.

Virtual Server 2005: Unlike VMware’s ESX Server, which runs directly on the system hardware, Microsoft Virtual Server 2005 R2 runs virtual machines, or guest operating systems, on top of another instance of Windows. Architecturally, Virtual Server is similar to VMware Server (formerly GSX Server). Both products are free. Virtual Server 2005 R2 SP2, which is currently in beta, supports hardware-assisted virtualisation in the latest processors from AMD and Intel. Volume Shadow Copy Service has been enhanced so that a physical server and all apps on it can be captured in a single snapshot, rather than requiring individual backups of each guest operating system.

Windows Server Longhorn: The hypervisor software in the Longhorn release of Windows Server will directly compete with ESX Server. Longhorn will ship sometime in 2007, and the virtualisation capabilities will roll out about six months later. Jim Ni, Microsoft’s group manager for server virtualisation, describes the hypervisor as a stripped-down version of Windows. Longhorn will support both Windows and Xen-enabled versions of Linux as virtual servers. While users still have to pay for a Windows Server licence for the hypervisor, Enterprise Edition users will be able to run guest instance of Windows in up to four virtual machine sessions without additional licensing fees.

VMware: VMware, a unit of EMC, offers both Virtual Server and ESX Server, part of its VMware Infrastructure suite, which supports 32 processors. VMware has the most advanced tools. The recently released Version 3 added the Consolidated Backup and Distributed Resource Scheduler tools for dynamic load balancing across virtual machines. Another new tool, VMware HA, automatically restarts virtual machines running on one physical server on a backup server in the event of hardware failure.

Xen: Both Novell and Red Hat are including the open-source Xen hypervisor into their Linux distributions. XenSource Inc. has released Xen Enterprise, which supports virtual machines running many Unix and Linux distributions. It will also support Windows virtual machines by year’s end, says Simon Crosby, chief technology officer at XenSource. Like VMware's ESX Server, Xen Enterprise can run on a 32-way machine. “We’re a little lacking in terms of spit and polish,” Crosby acknowledges. But, he adds, “we’re about one-tenth the cost.”