There are two common methods of measuring performance in a storage network: Throughput in megabytes per second (MB/s) and in I/O operations per second (IO/s). But it is important to understand what your needs are when using these metrics.
Typically MB/s and IO/s are more-or-less inversely related because the amount of overhead required for each block of I/O detracts from the total throughput speed. Which metric is a better measure of real-world performance depends critically on the nature of the data flowing through the SAN. In a transaction-oriented environment IO/s is likely to be more important because transactions typically involve many small blocks of data. At the other extreme, something like a video editing system will stress raw throughput and high MB/s values.
It's important to remember the real test of SAN performance is the ability to support the relevant applications. Ultimately conclusions about the level of SAN performance have to be drawn based on the mix of applications, and you have to measure the characteristic that best characterizes your infrastructure, application mix and network.
Imperial Technology discusses this issue in a technical note available at: http://www.imperialtech.com/technology_whitepapers_Good_Performance.htm.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.