Over the past several months, I've written about the benefits that Windows Powered Network Attached Storage (WPNAS) confers over general purpose Windows file servers. If all you need is optimized file serving, a WPNAS beats a Windows server in price, performance, and ease of use. But what happens when your storage needs outgrow your WPNAS devices? Although the newest WPNAS devices support terabytes (TBs) of storage, you might eventually outgrow them. Or perhaps you've purchased smaller WPNAS devices that have reached their storage limit. You might even have file servers and WPNAS devices that are maxed out on space today.

NAS storage virtualization is a software-based solution that lets you manage multiple NAS devices and file servers as one storage pool. When one NAS device reaches its limit, the NAS storage virtualization software automatically starts allocating storage from that device to another NAS device or file server in the same pool. Several vendors offer NAS storage virtualization solutions. Let's look at the capabilities of a few of these solutions.

1Vision Software ( http://www.1vision.com ) offers the vSERV and vNAS storage virtualization solutions, which let you combine Windows-based file servers or WPNAS devices into one storage pool or aggregation group. Once a WPNAS device is part of an aggregation group, you can set a storage threshold (i.e., high water mark) rule. For example, a threshold rule might specify that when a device reaches 75 percent of its storage capacity, data that would have been stored on that device will instead be storedon another device in the aggregation group. This threshold rule lets you balance storage resources across storage devices, Windows servers, or WPNAS devices in the aggregation group.

1Vision's vNAS solution incorporates an advanced file system that sits on top of NTFS. The file system handles the virtualization of NAS and file servers, and NTFS handles all other file system functions. You must load the vNAS or vSERV software on each WPNAS or Windows file server in the aggregation pool, but you don't need to make changes at the client level. The clients see only NTFS or whatever file system that the file server or WPNAS device supports. Pricing of vNAS or vSERV is $699 per NAS or server.

Other utilities, such as Microsoft Dfs or VERITAS Volume Manager, provide a level of storage virtualization, but they don't enable the automatic allocation of storage based on storage thresholds--a key factor in the practical use of storage virtualization. In the event that you run low on NAS resources, you don't want your NAS devices to shut down or require a lot of manual intervention. Having the ability to use a pool of NAS storage is convenient, easier to manage than separate storage islands, and provides a true "hands off" environment.

Spinnaker Networks ( http://www.spinnakernet.com ) also make NAS devices that support virtualization. Spinnaker bundles proprietary software with its hardware to achieve such functionality. A downside of this configuration is that you can't simply add Spinnaker software to a WPNAS device to achieve NAS virtualization.

Several other products provide virtualization of Storage Area Network (SAN) devices, and I'll detail them in a future commentary. The SAN virtualization products enable the aggregation of disparate SAN devices into one large storage pool. Pricing for these devices typically starts at more than $100,000.

Claude Lorenson, Microsoft Enterprise Storage Division's product manager, says, "The need for NAS virtualization features for the mainstream audience for NAS devices will grow as the need for more data maxes out smaller NAS devices. If you've filled up your 500GB captive NAS device and you need more NAS space, your only choice is to buy another NAS device. NAS virtualization, then, becomes very useful."

A NAS virtualization is to use a SAN to provide a virtualized growth path. In this scenario, you use a NAS head or gateway as a front end to a SAN device. The SAN device provides the entire storage growth path in this SAN/NAS combination environment. To learn more about WPNAS heads integrating with existing SANs, read the article at the following link: http://www.storageadmin.com/articles/index.cfm?articleid=37894

NAS virtualization is an additional tool now available to users of WPNAS and Windows Server-based file servers. The technology provides administrators who want one point of administration for WPNAS devices an easy way to create storage pools and the ability to avoid maxing out storage.