What has driven the market for virtual servers more than the potential to squeeze several servers worth of performance out of just one physical server? It's the relative ease with which most applications can move from a physical infrastructure to a virtual one.

But make the move without planning, run into a few of the major performance "gotchas", and your apps sitting on a brand-new virtual infrastructure will run like they're locked in an old box that's sitting around because it's too much trouble to throw out. Here are five virtualisation performance points to keep in mind.

1. Skimpy Hardware

Sure, one main purpose of virtualisation is to make physical servers disappear, but that doesn't mean the hardware itself doesn't matter, says Ian Scanlon, IS operations manager for Computacenter, a datacentre and IT services company based in London but covering most of Europe.

Computacenter migrated the 700-plus servers running its internal IT operations to VMware beginning in 2007, and has had very few performance problems, even with applications very demanding of either I/O or computing resources, Scanlon says. All the VMs run on relatively high-end HP blade servers, with 48 GB of RAM and plenty of SAN space each. Without that elbow room, the data warehouse, SQL Server-based applications and other demanding systems might not pass muster with the business units, he says.

2. Weak Supporting Characters

Virtual servers definitely save money, but if you focus too much on cost savings when you're setting them up, you're not going to save money or get decent performance, Scanlon says. High-bandwidth network connections, fiber rather than copper, for example, and powerful SAN gear make far more sense than saving a few bucks in capital costs upfront, he says.

"We went into this [migration] with the goal of saving costs, initially, which we haven't really done," he says. "The benefits really are in agility, the amount of time it takes to stand up a new server or expand something, and making management easier and issues like that. We're not really saving much, but I don't think anyone here would say it was the wrong decision."

According to CIO research, plenty of companies do see substantial savings from virtualization efforts, but they value agility gains just as much.

3. Downsized Support Plans

Everyone likes getting "free" servers by running several VMs on one physical box, but that doesn't mean you've actually gotten rid of any of the load on your infrastructure, says Chris Wolf, analyst at the Burton Group. In fact, you've probably increased the load on your power, network and storage resources.