Run a Linux server farm for nix

David M Williams
Monday, 06 August 2007 11:54

Business IT - Open Source

One great thing about Linux is its rock-solid nature even when you load it up with as many daemons and services as you like. Yet, often “best practice” dictates you separate out some apps across a couple of servers or at the very least provide a safe development environment which is distinct from your production environment.

Or you might be about to make a serious change in your environment and need a safe way of making a complete backup with instantaneous restore if need be. Or maybe you want to try out different Linux distros without reformatting, but Live CDs don’t let you customise enough and USB sticks are too slow.

The solution to all these problems and more is virtualisation and the key players we check out here are VMWare, XenSource and Microsoft Virtual PC.

What is virtualisation?

The key concept behind virtualisation is that a pseudo-computer runs as an application just like any other application. Its file system is completely self-contained in distinct disk-based files. Any operating system installed on the virtual computer thinks itself to be running on a real machine, with a real hard disk. Other devices – monitor, keyboard, mouse, CD/DVD drives, sound, etc – are similarly handled.

Virtualisation differs from emulators like WINE in that the computer system itself is the focus. The virtual machine effectively runs a personal computer. An emulator strives to reproduce the functionality of a specific environment – like Microsoft Windows – and its coders thus have to cater for every system function that environment provides. This is why emulators will handle some programs well but not others.

The two downsides of a virtual machine are you need a legal copy of the operating system you want to run (which is a no-brainer for Linux) and also it will run slower than the same operating system on the actual hardware itself largely because the processor is working on other tasks at the same time.

However, the upsides are many. In particular a virtual machine can be ‘suspended’ and brought back alive at any time with all the memory contents immediately restored. And the fact that a virtual machine’s hard disk is just a disk-based file gives huge potential. For instance, a clean system can be set up. The disk-based hard disk can be copied just like any other file to allow a good, clean system to be brought back at any time. These copies can be used to seed further virtual machines. You can have two otherwise identical setups where one uses the latest version of a software package and the other has the second-latest release. Or where one has 256Mb RAM and the other has 512Mb RAM. Or where one uses Fedora Linux and the other Ubuntu. Or any other variation you can conceive of. Really, virtualisation is the ultimate testing platform and disaster recovery environment.

Where this comes into its own for Linux users like us is in four key ways.