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Virtualisation for Beginners

It's not just for server jockeys, you know

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Today, products like VirtualBox, Parallels Desktop and VMWare will make use of these hardware features if they're available, but still perform well on processors without them.

Parallels, for example, uses these new Intel and AMD hardware virtualisation extensions because they are almost always available on modern hardware, especially on the Mac. But Parallels can also deliver full functionality without Pacifica or Vanderpool by running in 'software mode', using techniques similar to VMWare's segment faulting

Virtualisation for Beginners

A VM is a useful testing ground for a new operating system -- in this case Google's ChromeOS under VMWare

It's tempting to think that on a multicore processor, virtualisation might allocate different cores to each of the running operating systems, but this isn't how it works. Cores are shared in much the same way as in a uniprocessor system, although the virtualiser will typically allow the user to set up 'core affinity', which associates a particular virtual machine with one or more specific cores.

But What Can I Do With It?

The most obvious use for virtualisation is running legacy apps which are perhaps no longer supported on current versions of the operating system. A tried and trusted old DOS accounting system, for example, might be carried over to Linux inside a VirtualBox virtual machine running a readily available open source version of DOS like FreeDOS.

A valuable side effect is that the application will be running in an environment isolated from other applications on the same machine, and so will probably behave more reliably than in its original habitat. And whereas in the past a failure of the application and/or the DOS environment might entail a complete reinstallation of one or both of these, now DOS and the app it's hosting can simply be reinstated as a whole, should the need arise, by restoring a snapshot of the virtual machine.

When you 'revert to snapshot' like this you'll discover immediately - or preferably have had the nous to realise well in advance - that any data used or created in connection with the application should be stored outside the virtual machine. It's usually easy enough to set up the virtual machine so that drive D: (for instance) is actually mapped to a data directory owned by the host operating system, or off on a network drive somewhere. This ensures that your set of up-to-date data hasn't been discarded with the now overwritten old version of the virtual machine.

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