Why Don’t HDDs Spin Faster than 15K RPM?
Some folks who look at the speed difference between HDDs and SSDs ask themselves why HDD makers don’t defend their market against the SSD menace by cranking the speed beyond 15,000 RPM. An unfortunately popular answer is that the edge of the HDD would have to break the sound barrier and the HDD would shatter. Is this true?
With a little back-of-the-envelope calculation The SSD Guy can rapidly disprove this. (Forgive me, but I am going to mix metric and English measure here.)
The speed of sound is 340.29 meters/second. HDD diameters are measured in inches: 2.5″ and 3.5″ and speeds are in revolutions per minute, so let’s convert this to inches per minute. To do this multiply by 60 seconds/minute and multiply again by 39.37 inches per meter. That gives us 803,833 inches per minute.
A 3.5″ HDD has a circumference of 3.5 x π or 11 inches. That’s the distance traveled in one revolution. Divide this into 803,833 and you get 73,105 RPM. That’s pretty fast!
Naturally, when you go to a 2.5″ HDD the speed gets even faster. Its circumference is 7.85″, and 803,833/7.85 = 102,347 RPM.
So why don’t we see any 20,000 or 30,000 RPM HDDs on the market? I asked a number of HDD designers and got a very simple reply: It would take too much power.
As the speed of an HDD increases the power it takes to run it increases disproportionately. These designers told me that increasing the speed significantly beyond today’s 15K RPM level would cause undue headaches in power management and could even threaten the lifetime of the HDD.
Although the “speed of sound” argument is catchy, it’s just wrong. The power argument, however accurate, is very dull by comparison. But consider this – SSDs already trounce HDDs in measures of IOPS/Watt. Imagine a comparison with a 30K RPM HDD with perhaps three times the power consumption of a 15K model but only twice (or less) the IOPS performance. How would that compare in IOPS/Watt to an SSD? Th HDD would look far worse than even its 15K RPM counterpart!
Objective Analysis has published a report on enterprise SSDs: The Enterprise SSD: Technologies & Markets that can be ordered for immediate download from our website. This report discusses issues of IOPS/Watt and IOPS/$ and how these compare to enterprise HDDs.
Oddly enough, the photo accompanying this post is often said to be of an aircraft breaking the sound barrier. According to Snopes, this is also a myth, and the phenomenon really relates to a sudden pressure change that creates a condensation cloud surrounding the plane.