Do you think Hard Disk Drives are Slow? Think again.（原声文稿）

Inside every hard drive is engineering so incredible, it sounds impossible.

It starts with a stack of four discs, each around 3 .5 inches across, spinning at a constant 7200 RPM.

Hard drives are relatively inexpensive in large, especially when compared with SSDs, so you'd probably think this isn't all that impressive.

But after some quick calculations, we find that the outer edge of the disc, where data is stored, is moving at a linear speed of 75 miles per hour. That's incredibly fast.

But here's the seemingly impossible engineering. And it comes in three parts.

First, the read -write head passes over the surface, yet it never touches it.

Instead, it uses a small buffer of high -speed air created by the fast -moving surface and precision -engineered airfoil -like structures called air bearings to glide only three nanometers above the disk's surface.

That leaves fewer than 15 air molecules between the read -write head and a surface moving at 75 miles an hour.

For scale, a single strand of hair is 20 ,000 times larger than this air gap. Okay, that's number one.

The second seemingly impossible feat is that to store and read data, the read -write head manipulates and measures magnetic domains inside the disk.

However, to reach a terabyte of storage capacity across a single platter, these domains are incredibly small, measuring only 30 by 30 nanometers in size.

And for scale, here's that strand of hair again.

So while the disk zooms by at 75 miles an hour, the read -write head manipulates and measures regions only 30 nanometers long. That's two.

The third incredible feat is that the read right head itself isn't stationary, rather it sweeps in and out across the disc and is driven by a voice coil motor at the opposite end of its arm.

It also moves rather quickly, jumping from the outer edge towards the center and back again.

Data is stored across 400 ,000 concentric rings called tracks, and as we've established the outer track speed is 75 miles per hour.

In contrast, the innermost tracks clock in at only 35 miles an hour, and therefore, as the read -write head jumps in and out, it must continuously adjust for the difference in speed between the tracks. Additionally, 400 ,000 tracks are a lot.

If we straightened out every one of those rings and laid them end -to -end, just one side of a single platter would stretch almost 50 miles.

In an 8 -terabyte hard drive, There are four platters with data recorded on both sides, and thus, there are around 400 miles of data.

That's equivalent to half the width of the state of Texas.

So let's put these three feet of engineering into perspective using a visualization.

Imagine a futuristic hover car racing at 75 miles an hour, down a highway spanning half the length of Texas, while floating only 15 air molecules above the pavement.

Then, as the car zooms by, it paints dots on the road that are only 30 nanometers long, all while taking pictures and reading the data from the pavement.

At this incredible resolution, a single grain of sand would have 6 ,000 dots across it, and a banana's length of pavement would have 6 million dots along it.

Again, this is a single banana with 6 million dots, and all the data spans half the length of Texas.

Finally, this hover car slows to 35 miles per hour, then speeds back up to 75 within a fraction of a second.

That's essentially the typical engineering hiding inside your computer's hard drive.

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