NAS was primarily used for file sharing initially and it does that very well. Recently the usage of NAS has expanded for a variety of purposes (including backup now) all based upon capacity and performance improvements that have been made. NAS does file I/O while SAN does block I/O. Some applications may have certain requirements that make block I/O preferable. It all depends on your requirements for performance, security, etc. There isn't a simple answer and it is getting somewhat confused with varying "SAN/NAS convergence" solutions. This area is still in its early stages so there will be many new products and degrees of maturity.
NAS does connect over a local area network. Most are implemented using Ethernet of some speed but there are other technologies such as ATM supported by a few vendors.
Evaluator Group, Inc.
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This was first published in June 2002