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When you get to this level of engineering, there's a lot more you have to look at. While all of these components can access a bus at a given speed, they can only read and write finite amounts of data at that speed. After this, generally, they are much slower.
The challenge in building your own SAN is end-to-end engineering. The PC with highest CPU number isn't necessarily the fastest for your application. You have to consider all the factors.
In general, I would say for eight high-performance drives you would benefit more than one controller unless you have a custom controller. But without knowing the specifics of the hardware and your block sizes, read/write mix, etc., we couldn't get to the best answer.
However, while you've probably already done this, I would like to point out that matching your block size to your application -- which most people never do -- has a much larger impact on end-to-end performance than what we're doing here.
The kinds of numbers we would get -- the specs we would create -- by doing this low-level engineering are the numbers the vendors use to sell SANs and NAS devices. As the independant reviews point out time after time, these numbers have little relationship to the performance end users actually experience.
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