Here is a white paper from Quantum, justifying BigFoot at 3600 rpm vs. 5400 rpm (BigFoot actually spins at 4500, I'm not sure why they used 3600, unless it was for dramatic effect):
quantum.com
Of course, they are talking about two different form factors as well, but you may find it useful. The article is longer, you may want to go to the original and read more of it, or some of their other white papers.
An excerpt:
Rotational speed
Rotational speed (RPM) is often cited as a type of shorthand for two aspects of performance, data rates and latency. (Data rates indicate how fast data can be transferred on or off the disk, while latency is the average time read/write heads wait for data to rotate into position after a particular data track is located.) Using RPM as an indicator of performance is accurate as long as form factors are identical. But, as demonstrated below, a 3,600 RPM 5.25-inch drive can offer data rates equivalent to a 5,400 RPM 3.5-inch drive.
To begin, it's important to understand that there is a close relationship between peak data channel rates and rotational speed ó especially as drive performance and capacities grow. In fact, the selection of a particular data channel integrated circuit (IC) and the data rate it is capable of handling actually determines the maximum rotational speed of the drive mechanics. That's because the data rate is directly determined by the speed of the bits passing by the recording heads; the faster the drive spins, the more quickly data goes by the heads.
For the Quantum Bigfoot product, the design criteria were centered around balancing maximum capacity and attractive costs. For maximum capacity, a 5.25-inch drive was chosen because it provided 90 percent additional recording area over a 3.5-inch disk. To minimize cost, a field-proven Partial Response Maximum Likelihood (PRML) read channel IC was chosen. The chip's design could reliably support internal data rates up to 89 megabits (Mb) per second. These two design choices combined to determine an "appropriate" rotational speed: 3,600 RPM.
It is interesting to note that the read channel used in the new Quantum Bigfoot family was first used in a family of 3.5-inch form factor Quantum drives called Quantum Fireball™. Since the circumference of the outer track on the 3.5-inch disk is smaller, the rotational speed of the drive could be increased to 5,400 RPM and still be supported by the same read channel.
How can a 5,400 RPM 3.5-inch drive (Quantum Fireball) and a 3,600 RPM 5.25-inch drive (Quantum Bigfoot) have similar data rates? Because of the larger diameter of a 5.25-inch drive (spinning at 3,600 RPM), head velocity over the media at its outside diameter is the same as a 5,400 RPM 3.5-inch drive. This factor, coupled with the advanced read channel, results in a data rate equivalent to that of the smaller 3.5-inch drive.
As is evident, RPM by itself is not an accurate measurement of performance across different disk drive form factors. A true performance comparison should look at all the factors that affect a drive's ability to transfer data.
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