A NAS appliance is a single-purpose device designed to be easy to purchase, implement and use. It combines the file server with the storage array in a single, integrated appliance. Like all NAS devices, a NAS appliance presents data in the form of shared files, which allow consolidation of both file servers and storage.

Although easy to use, NAS appliances have their drawbacks. NAS appliances don't pool their storage, making high utilization difficult to achieve. While designed to be cost-effective, they have difficulty in scaling to the levels required for high-end consolidation and application support. Furthermore, they often support only a single type of storage, making it difficult to tier different service levels for different business requirements.

A NAS gateway is fundamentally different. It uses the existing SAN environment to overcome many of the limitations of an appliance-based approach to NAS. In addition, because a gateway separates the file server from the storage device, it costs less than traditional NAS appliances. For customers with existing SANs, purchasing a NAS gateway along with the desired additional drive capacity is a more cost-effective approach than purchasing an appliance that includes the NAS head, storage array and drive capacity.

In a gateway environment, NAS capacity is pooled with SAN capacity, allowing storage administrators to drive utilization rates far higher than with an appliance-based approach. SAN capacity can be reconfigured as NAS capacity and back again, giving administrators more flexibility in responding to storage growth.

In addition, a single NAS gateway can address different types of storage from the lowest-cost Advanced Technology Attachment (ATA) to the highest-performing fiber channel drives, providing the flexibility to match the right storage to the right requirement at the right cost.

In demanding environments, NAS gateways can also leverage the power of multiple back-end SAN storage arrays for increased performance. NAS gateways can also take advantage of high-end local and remote replication, allowing greater degrees of consolidation than with stand-alone NAS appliances. Finally, NAS gateways allow a single administrator to manage and monitor the combined NAS and SAN storage resources, enabling even higher productivity from storage administrators.

It's not surprising that industry analysts expect the market for NAS gateways to grow at a compound annual growth rate of 47 percent through 2007.

All the above benefits may come with other considerations. NAS gateway environments are inherently more complex to design and implement than their appliance counterparts, so the ability to support them may become a concern.

Although NAS gateways may be considered for some environments in the future, organizations can implement NAS appliances that can be converted to a gateway environment later. These 'gateway-ready' NAS appliances offer all the simplicity and ease-of-deployment advantages associated with appliances, yet they can easily migrate to gateway environments by uncoupling the NAS server from its associated storage.

The NAS server becomes a gateway in the SAN environment, and its associated storage becomes part of the overall SAN storage pool.