Gallery

Virtualise your servers: pitfalls and planning

Posted on 7 Dec 2009 at 14:59

Jon Honeyball emphasises the need for careful planning before attempting to virtualise a server

In the first part of our guide to virtualising your servers, we’d arrived at the stage of being ready to create our first virtual machine on our shiny server.

At this point it’s tempting to fumble for the installation DVDs and try to do everything in an afternoon. The sober reality is that it’s rarely a good idea.

We need more planning, however boring that might seem, because the fact is that what we’re trying to virtualise has no idea what we’re trying to do to it: our trusty operating systems Windows Server 2003 and 2008 really don’t understand what it means to be virtualised.

Let me give you a few examples of the sort of “gotchas” you can run into. First up is resources. With a modern virtualisation solution you can add resources as necessary, so if a particular VM might benefit from more RAM, it isn’t difficult to give it some from the global heap of RAM on that machine.

Need more processing grunt? Add another CPU core (or three). More disk storage? No problem, just extend the virtual machine partition size. All of which sounds great, until you realise that most of your system software isn’t capable of hot-adding anything at all.

If a company the size of Adobe can screw up over printer drivers, just how many of your other applications do you think will cope with extra CPUs magically appearing?

Another concerns increasing the size of your disk partition. Windows can certainly enlarge an NTFS partition into newly created space, except if it’s on the boot drive. There’s no way to do that other than to download some hack-o-matic utility to stretch the partition into the newly available space, a utility that probably won’t run on server OS installations anyway.

Worse still, think about the applications. In an earlier column I wrote about how Adobe Photoshop Essentials couldn’t cope with me removing a printer driver while it was running – try it; the app exploded.

Try adding a printer driver while Photoshop Essentials is running and it, too, blows up. Why? Because its authors made all sorts of assumptions when the application starts up, and didn’t check that those assumptions would remain correct throughout the lifetime of running the application.

Now, if a company the size of Adobe can screw up so badly over printer drivers, just how many of your other applications do you think will cope with extra CPUs magically appearing while they’re running? Or more RAM?

You might expect, with some justification, that any app that follows the guidelines and lets Windows handle all such matters should have no problem, and in Alice’s Wonderland you’d be right. However, applications make all sorts of assumptions about the amount of space they’re occupying, and many of them go behind, around and under Windows in the process.

Download a year of Jon Honeyball's Advanced Windows columns by heading to our Free Downloads site