Tightly coupled NAS clusters, on the other hand, are typically bought as a preconfigured solution from the supplier. While still likely to be cost effective, they may not be as much so as loosely coupled clusters. That said, they may have greater reliability in the field since the manufacturer is in control of the entire configuration. In a loosely coupled cluster, the weak link is what is introduced into it. If you use an inexpensive node or dirt cheap storage and it fails, that could compromise the solution.
Performance and scale should be the key differentiator. As I mentioned above, with a loosely coupled cluster you are limited to the capabilities of the individual node, which in most customer's situations shouldn't be an issue. Tightly coupled clusters essentially scale as you add nodes; the more nodes you add, the more storage I/O performance you get and, in many cases, the more network I/O performance you achieve.
The truth is, we don't know yet. In NAS clusters there are few cases that I have heard of customers hitting the wall. And once again, you should make the selection on what makes sense for you. If both methods can hit your performance and capacity needs as far into the future as you can predict, then look for the service, support, and features you think will make the project successful.
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George Crump is founder of Storage Switzerland, an analyst firm focused on the virtualization and storage marketplaces. It provides strategic consulting and analysis to storage users, suppliers, and integrators. An industry veteran of more than 25 years, Crump has held engineering and sales positions at various IT industry manufacturers and integrators. Prior to Storage Switzerland, he was CTO at one of the nation's largest integrators.