In an array-based setup, "you take your second-tier arrays and just virtualize them behind your existing arrays. It's one view to the whole world -- like having one entrance to the office," Nadkarni adds.

Gene Ruth, an analyst at Burton Group, says the simplest approach is to choose an all-inclusive system, add appliances and then link them. But beware of diminishing returns. "At some point, it just gets complicated, and it may not be worth it when you aggregate too many appliances," Ruth says. "Then you have to ask yourself, is it better or are you getting this lowest common denominator?"

The hardest part is the planning phase, says Roman Perez, systems engineer at Business Technology Partners Inc. in New York. "If you have a big company with thousands of servers, you have to do it little by little, and that's a big project," he says.

4. What's your budget?

"It's always cheaper when it's well thought-out and part of a bigger project," Nadkarni says.

Your budget will depend on the type of virtualization you need. Block-level virtualization is cheaper if you implement it as part of your upgrade. If you're buying or implementing a new SAN, then incorporating storage virtualization within the SAN is more prudent than buying off the shelf. "It tends to be pricey because you're now trying to 'a la carte' it. Do it as part of a larger upgrade so you can bundle some costs into the upgrade itself," says Ruth.

He also recommends that IT managers compile a spreadsheet to compare those scenarios. Adding appliances could be cheaper in tough economic times, but be sure to amortize the cost over three to four years, since older systems will eventually need to be replaced.

For virtual tape libraries, it's important to correctly estimate virtualization needs -- or risk buying much more capacity than you need.

5. Do you have an exit strategy?

Keep in mind that network-based virtualization can get tricky, Nadkarni says. "It can get a little complex over time, so you have to make sure that whatever architecture you're implementing [can be withdrawn from]," he says. "You shouldn't be stuck with it."

Most storage virtualization products create metadata from your data. That's how the storage objects they virtualize are managed. "Unvirtualizing means figuring out how to reappoint your metadata back to original data," Nadkarni explains. "The second problem is, your data could be across multiple storage areas or multiple objects. In that case, you now have a challenge of trying to present the same data, in a committed manner, back to the host again."