Talk to any expert about virtualisation and somewhere along the line they'll mention open source technology from developer XenSource. Getting hold of Xen virtualisation software and putting it to practical use, however, has always been problematic, especially if you want to run Windows-based virtual machines. Until now, that is, with the release of XenServer for Windows which, like VMWare and Microsoft alternatives, supports virtual machines running both Windows and Linux.

The big difference with XenServer compared to most other products is that it's based around a so-called virtualisation hypervisor. That means that it's loaded and run directly onto a bare server rather than as an application beneath an existing host O/S. The benefits are, supposedly, improved performance and security, but hardware compatibility can be an issue and setup is a lot more involved.

Another major difference on this particular implementation is the need for processors that support Intel's VT or AMD-V virtualisation technologies, which means fairly recent hardware. You can get by without, but only if you want to run Linux. For Windows it's an absolute prerequisite.

For our tests we used a Dell PowerEdge server with the latest Intel Xeon processors fitted to provide us with the necessary VT support. Getting started then involves downloading a CD-ROM image from the XenSource site, which we used to create a bootable installation CD, although the contents can also be copied to a TFTP server for network deployment.

We then booted the server from the CD and followed the Linux-like installation routine which proved - remarkably - no harder than loading a typical Linux distro. Which isn't really surprising as, in addition to the Xen Hypervisor itself, what you're actually doing is installing a modified Debian Linux distro, employed here mainly for its driver support. As a result most industry standard hardware options, such as video controllers, storage interfaces and so on, will be identified and configured automatically.

We certainly had no problems, but compatibility isn't guaranteed and it's important to understand that problems can arise which may not be easy to fix.

There are strict limits, too, when it comes to the versions of Windows that can be run by XenServer. To this end it's 32-bit only with a choice of either Windows XP with SP2 or Windows Server 2003, although this can be either Standard or Enterprise edition with support for SP1 and R2 implementations. Support for Windows 2000 is expected later this year.

It's also worth noting that although virtual SMP is available in a recently released beta of the next version, the current product only supports one virtual processor and 2GB of memory per VM. Added to which you're limited to eight concurrent virtual machines on each host system.

Management is via a remotely-hosted Java console through which we were easily able to create new virtual machines. This also provides tools to start and stop virtual machines, monitor performance and perform some manual load balancing. An integrated VNC client is used to access the operator console on each VM with, on Windows, the option of using Remote Desktop if preferred.

Tools to convert physical servers into virtual machines are another option and the package as whole worked as expected. However, the XenServer software is nowhere near as polished or easy to manage as more established products such as VMWare Server or Microsoft's Virtual Server 2004. Moreover, it's hard to see any benefit in terms of performance and you may want to wait for the XenSource technology to be further developed before considering it for anything other than testing.

The XenSource hypervisor-based virtualisation technology behind XenServer makes for an interesting alternative to hosted solutions from VMWare and Microsoft and in this implementation it can support virtual machines running Windows. However, hardware requirements and software restrictions limit its usefulness and it needs further development to catch up with its rivals.