Fast, cheap, and out of control: That's an accurate description for many enterprise storage environments, which are stuffed full of file servers and network-attached storage appliances. As the cost of disk comes down and the amount of data to store goes up, it's tempting to simply throw more NAS appliances into the mix to keep up with growing storage demands.

But that's a poor excuse for a strategy. For one, file servers and NAS appliances can only serve the clients and applications attached to them. This can lead to unbalanced storage utilization. Workgroup A's appliance may be near to bursting while workgroup B's sits half empty. Reconfiguring workgroup A's clients to tap into workgroup B's storage device is often more trouble than simply adding another device. For another, the sticker price on a storage appliance isn't a full measure of its overall cost. The proliferation of low-cost storage devices can increase costs elsewhere, such as management and power and cooling.

Nonetheless, IT can't stop adding storage--in particular storage for unstructured data. Word documents, spreadsheets, PowerPoint presentations, and e-mail expand like ballplayers on steroids. File-based storage increases by 50% to 120% every year, according to IDC.

Enter file virtualization. This technology inserts itself--whether logically or physically--between clients and applications and storage devices. Clients and applications are configured to connect to the virtualization layer, which appears to the clients as one giant storage system known as the global namespace. The virtualization system then manages the actual connections to the storage devices.

By creating a global namespace, several things become possible. First, storage devices can be used by any client or application, which enables storage administrators to make better use of available capacity and to easily distribute new capacity. Second, the file virtualization layer simplifies common tasks such as migration and mirroring. Some file virtualization technologies let administrators move data even while users are making changes to the files. Third, administrators can get better control over data disposition, such as moving lower-value data to a less expensive disk.

File virtualization has yet to make significant inroads to the enterprise. A rough estimate from the Taneja Group, a storage consulting firm, puts total current revenue from file virtualization products at just $100 million. But analysts expect those revenue numbers to soar. That's because file virtualization provides immediate relief from a chronic IT pain: data migration.

Enterprises migrate data--typically files such as Office documents and CAD/CAM files--for two main reasons. A NAS appliance may be reaching its capacity, both in terms of available storage and I/O. Lots of users shipping information back and forth will affect the NAS appliance's performance. Slow response times lead to slower apps and unproductive employees. In this case, a storage administrator may migrate some data to another NAS device, often one that's faster and has more storage capacity. The admin may also split the data among multiple NAS appliances.

Enterprises also migrate data to reclaim unused storage. "We found a lot of our projects had space allocated to them that wasn't being used, like 30% to 35%," says John James, officer in charge for the U.S. IT unit of SPi, a global business process outsourcing provider. That unused storage was essentially wasted money, he says.

So James copied information to tape, deleted it off the original system, and then restored it to another location. But there's always a risk that this process could've been interrupted by a business opportunity that required users to get access to the data. "We'd have to stop and allow production to do their thing, and then restart again," says James.

Indeed, migrating data is like trying to move office furniture--you can only do it when your employees aren't using it. That means storage administrators have to set up a time with business units in which the data becomes unavailable to workers, such as a weekend.

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