Virtualisation is hardly a new topic. But, since the advent of high performance, low cost hardware -- memory, storage and especially computing power are of course immensely more capable and cheaper than they were during the early days of experimentation with virtual machines on the PC platform -- the technique has grown in both popularity and sophistication.

The result is that the market has grown to the point where we are now seeing a divergence in the techniques for achieving similar ends. Leaving aside the virtual DOS box in today's Windows platforms, the majority approach encompasses products such as Microsoft's Virtual Server and Virtual PC, along with EMC VMware's range of products, such as VMware ACE, VMware Workstation and its two server products, ESX and GSX Server.

Others include Xen, the open source virtualisation technology which Novell is shipping with its own Linix distribution, SuSE, and which also appears in the latest development of Red Hat's Fedora, Core 4. However, Xen is still work-in-progress, rather than a finished and polished product.

All the above work in similar ways. They virtualise the hardware -- and will, incidentally, find it easier to do so now that Intel's hardware-assisted virtualisation-aware processors are rolling out of the foundries. This means that the virtualisation system presents each virtualised OS with access to the underlying hardware, the advantage of which is that you can run pretty much any OS you choose on top of it. It also guarantees isolation between VMs.

The downside is that there is a cost involved, namely software licensing. If you need to run an OS, it needs a licence, irrespective of whether it's running on a VM or on the hardware directly. If that is not to incur massive costs -- although the project may of course will be worth it -- then that might lead you in the direction of an open source solution.

A virtual alternative
There is an alternative though. Initially Linux-only, SWsoft recently launched its virtualisation technology for Windows. Called Virtuozzo, it virtualises the OS so that multiple virtual private servers can run on a single physical server.

Its benefits are that it offers more advanced hardware support than rivals because of its links with the operating system. The way it works, according to a company spokeswoman, is that it builds on top of the operating system and therefore can support any piece of hardware underneath. The VM does not need pre-allocated memory, as it is a process within the host OS, rather than being encapsulated within a virtualisation wrapper. The upside to this approach is that only one OS licence is required to support multiple virtual private servers. The company reckons it still gets the benefits of the more isolated technologies, though, and says that you can manage your VMs easily.

However, it also limits your choices, as each pseudo-VM is locked to the underlying OS, and Virtuozzo only guarantees support for Windows and RH Linux. In mitigation, SWsoft execs say end users can expect to see updates to Virtuozzo for Windows over the coming months, including support for additional operating system environments, enhanced management capabilities and high availability clustering and replication.

So two approaches, two economic models. And plenty of others are working on more innovative approaches, as we reported earlier this year, including one company that plans to build a management platform for VMs.

Which you plump for depends on usage -- the key thing being to decide what the end goal is, before leaping in.