At the tape library level, virtualization is used for making online storage appear as tape to the backup software.

2. Do you want host-, network- or array-based virtualization?

When deciding what type of virtualization is best, "it really comes down to what problem you're trying to solve and what kind of vendor affinity you have," Nadkarni says.

For most IT units, having host-based virtualization is a given, since volume managers run on the host. More often than not, you'll see host-based virtualization in a storage-area network (SAN) environment.

"Array-based virtualization is more of a function of which vendor you're going with for your primary storage," Nadkarni says. For example, with some Hitachi Data Systems storage products, virtualization can be deployed by enabling an existing software key within HDS's Universal Storage Platform or its Network Storage Controller. "So you'll go with array-based if you plan to buy a Hitachi frame for your Tier 1 storage," he says.

Network-based virtualization is typically used if you plan to make your SAN a multiprotocol storage network and in doing so are porting the network intelligence -- which also includes virtualization. Cisco Systems Inc. and Brocade Communications Systems Inc. are now coming out with products that let you virtualize within the network, without the need for an array or a host to do anything.

Some products blur the lines between host-, array- and network-based virtualization. "Products like Seanodes' would be considered host-based virtualization because you're virtualizing over the nodes," Hacopians explains. "You could also think of it as network-based, because you're virtualizing and spreading it across and letting the network take care of itself."

Industry watchers agree that virtualization might be easier to implement and cost less if IT groups stick with their vendors. "If you're primarily in a Hitachi environment, for example, array-based virtualization is probably going to make the most sense," Nadkarni says. "If you're a Cisco SAN, and you already have the infrastructure to implement Cisco virtualization, then network-based makes more sense."

3. How much complexity can you handle?

Host- and array-based virtualization are usually the easiest to implement, experts say. Network-based systems are often the trickiest because there is no direct way of virtualizing in a network. Most IT shops use third-party appliances. Cisco's system usually requires users to buy enabler software or an appliance or other third-party tool that sits alongside it, Nadkarni says.

"Then you have to figure out whether it's going to be asymmetric or symmetric," he says. "Where are you going to store your depository? What services do you want to provide? What arrays are you going to virtualize?"