Why Don’t HDDs Spin Faster

I think the super fast hdd in the new age of mechanical hard drives should be titanium. I am not a materials scientist, just speculating. But today’s most common RPM rates, in both laptop and desktop PCs, are between 5400 and 7200 RPM. Given two identically designed hard drives with the same areal densities, a 7200 RPM drive will deliver data about 33% faster than the 5400 RPM drive.

RPM hard drives, which can make your OS or programs run faster. Note: 7200 RPM hard drives can’t perform as well as SSDs.Given two identically designed hard drives with the same areal densities, a 7200 RPM drive will deliver data about 33% faster than the 5400 RPM drive. Consequently, this specification is important when evaluating the expected performance of a hard drive or when comparing different HDD models.

Over the past few weeks, I?ve heard folks promoting solid-state storage solutions and repeating the myth that spinning disks haven?t gotten faster than 15K RPM because it would mean that the airflow over the heads would become supersonic. While it?s a good story, and the thought of little tiny sonic booms inside a disk drive is amusing, there?s very little truth to the story.

For anyone who remembers his or her high school math, it?s pretty simple to calculate the linear velocity of the outer edge of a spinning disk. The formula is pretty simple, where LV is the linear velocity and RV is the disk?s rotational velocity:

LV=D*?*RV

That problem is power. The power required to spin any given disk faster increases at the square of the rotational velocity. So while a 30,000 RPM disk would have half the rotational latency than a 15K disk, it would take four times the power to spin the disk. Since spindle motor power is generally 75% or more of total disk drive power, the drive would draw at least three times as much.

Even then the performance benefit would be minimal. Sure, doubling the rotation speed will cut rotational latency from 2ms to 1ms. However random I/Os not only wait for the right sector to come around under the heads, but also move those heads to a different track. On Seagate?s fastest disks, the head motion takes an average of 2.6ms, so total random I/O latency on a 30K RPM drive would be 3.6ms compared to 4.6ms for the 15K drive, or just 22% faster.

So, no children, you don?t have to be afraid of supersonic disk drives. The SSDs made them irrelevant.

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