One choice stands out for IT directors adding storage capacity to their IT networks. It is whether to opt for networked attached storage (NAS) or a storage area network (SAN).

The choice between these two emerging shared storage architectures, both of which exploit the availability of high-speed networking, is no mere technical nicety. Each has its own particular application strengths and weaknesses. So which works best where, and will that always be the case in the future?

First, some definitions. A SAN establishes a dedicated "fibre channel" connection between storage devices and application servers while keeping the storage and tape subsystems independent of the server environment. It is designed to support nationwide or globally distributed data centres.

NAS products, on the other hand, are not treated as an independently managed resource. They plug into existing local area networks (LANs) to provide convenient, low-cost additional storage for local or regional data centres. "Network attached storage is gaining ground in organisations where the current servers on the LAN are running out of capacity and the IT manager needs to add more storage quickly without bringing down the servers in production," explains Noel May, managing director and chief technology officer of Atlanta-based Raidtec, which sells both types of storage.

But therein lies the weakness of the NAS approach too. "Because NAS [products] are attached to the network, they will impose a load on the network when such activities as backing up and mirroring with other NAS devices takes place," says Stephen Filbin, UK head of business intelligence and data warehousing at Cap Gemini Ernst & Young.

And this is where SANs step in. "The most obvious advantages are that greater capacity and data transfer rates are possible," says Alastair McAulay, senior consultant at PA Consulting Group. "Organisations can store all their data in one place and get easy access to it. However, many of the real benefits of SAN are much more subtle."

For example, he explains how SANs also allow the organisation to better manage resilience. Rather than achieving resilience by duplicating all data, if there is a component failure the SAN system can be rapidly reconfigured to route around the problem area.

But SANs have their own inherent weaknesses too. "The SAN implementations do require significant investment by the customer in fibre, host bus adaptors, and the SAN management tools," says Peter Jopling, business unit manager at Tivoli Security Solutions.

And further, NAS technology has in part overcome its former weaknesses. For example, the decline in LAN performance during backup is reduced by directly connecting a tape library, explains David Liff, regional vice president for storage at Computer Associates.

So what of these trade-offs in practice? Commerzbank, the big German bank, is installing NAS as a back-up solution at the branch level. It is an economical and effective way of providing an extra local level of protection of data in addition to centralised storage in Frankfurt.

The bank chose Quantum's Snap Server solution. "The practicality and dependability of the Snap Server fits in perfectly with Commerzbank's strategy of reducing costs in branch banking," says Gerhard Fay, head of national client/server services in the bank's IT department. "Snap Server ensures data integrity even with high storage needs."

Alternatively, US-owned Chase Global Private Banking has opted for a SAN in the shape of DataCore's SANsymphony product for day to day office storage requirements. "We use SANsymphony to share physical storage among Microsoft Exchange, Lotus Notes, and Netware servers," says Dick Boyle, vice president at the bank. "We needed the flexibility to allocate storage quickly and easily as our needs grow and change, and to do LAN-free backups."

But if the SAN solutions show their lead in certain situations, why are organisations still holding back to some degree in implementing them? The problem is identified by Chris Atkins, storage product manager for Sun Microsystems.

"[SAN] standards are not yet sufficiently complete to ensure interoperability between different vendors' products, let alone the management of different vendors' products through a single management station," he says. "This is the single issue that is limiting widespread SAN deployment. Today people are usually rolling out small scale SANs where the short term benefits justify the extra cost, complexity and risk."

However, SAN standards are appearing. In fact, some advocates say they are already extensive enough, as witnessed by the growing SAN market. "The introduction of new SAN standards has resulted in increased competition in the market, forcing prices down," says Peter Coleman of Gadzoox Networks, the California-based SAN specialists.

"Customers can now mix and match products from different vendors rather than being locked into proprietary technologies," says Mr Coleman, who is also president of one of the leading storage industry bodies in Europe, the Fibre Channel Industry Association.

The differences between the two approaches are, in any case, becoming narrower and less distinct, according to PricewaterhouseCoopers.

"NAS subsystems are becoming more powerful and functional, and adding features previously found only in high-end enterprise storage systems, such as data snapshot, remote copy and high availability," says PwC in its latest Technology Forecast.

"At the same time, SAN solutions are becoming much less expensive and a lot easier to install and manage."

Other commentators believe that to set up the relationship between NAS and SAN as competitive is a distraction from the key issue that is data management. What is most likely is that companies will have a hybrid storage strategy with, say, a SAN for the company wide requirements and a NAS for individual departmental requirements.

"No single storage networking approach solves all problems or optimises all variables," says Tim Wilkinson, technical director of InTechnology, the data management company. "The best of both worlds can be achieved when NAS gateways and non-gateway users can actually share the SAN capacity."

"It is unlikely that any one technology will 'win', it is most likely to come down to appropriate use," continues Wayne Naya, senior consultant for storage products at BMC Software. "Most organisations will most likely end up with a hybrid of all types of technology. There's an element of 'if it ain't broke, don't fix it'."