Another issue is choosing the right form of storage virtualization, network-based or array-based. The network-based virtualization technology is delivered via server-based software, a network appliance, or an intelligent Fibre Channel switch, and it comes in two flavors: block-level and file-level. Array-based virtualization is typically provided as part of the storage management software that comes with an array.

Array-based virtualization is mature, says Burton Group's Simpson. But it's limited to storage attached directly to the array or allocated just to that array via a SAN; IT usually must buy array storage from the array vendor, creating expensive vendor lock-in.

Network-based storage virtualization has been in existence just a few years and so has largely been offered by startups. It's the most flexible form of storage virtualization, says Forrester's Andrew Reichman, and lets you manage almost all your storage resources, even offsite, as long as they are available via the SAN. Although these tools can theoretically act as a choke point on your SAN, in practice the vendors are good at preventing that problem, he notes.

Most network-based storage virtualization products work at the block level, meaning they deal with groups of bits rather than whole files. While block-level network-based storage virtualization is the most flexible option, the technology typically requires that an enterprise change its storage network switches and other network devices to ones that are compatible, Nadkarni notes. "But no one wants to shut down their SAN to do so," he says. Although you can add the technology incrementally, that just raises the complexity, since you now have some virtualized storage and some nonvirtualized storage, all of which need to be managed in parallel.

Thus, most organizations should consider adopting network-based storage virtualization as part of a greater storage reengineering effort, he advises.

That's exactly what both Champion's Etcheverry and PHNS's Walls did. Etcheverry brought virtualization in as part of an enterprisewide storage redesign, while Walls brought it in as part of adding a new data center and disaster recovery site.

In both cases, all the setup work happened in a nonproduction environment and could be tested thoroughly without affecting users. Once the two IT leaders were happy with their new systems, they then transferred the data over and brought them online. That meant there was only a single disruption to the storage environment that users noticed. "This was a one-time event," Walls notes.