Everything you need to know about hard drive vibration (2016)

Summary

Hard Disk Drives (HDD’s) are one of the most impressive and important electromechanical devices ever created. When I think about it, it is really amazing that these things actually work, let alone work so well! These disks must quickly and precisely position heads slightly above very narrow tracks on rapidly spinning platters. If there is an error in the angular position of the head, it will not be positioned above the correct sector and therefore cannot correctly read or write data. One thing that can interfere with this amazingly fine and fast positioning is vibration. Vibration can cause small variations in the position of the head with respect to the track, so it can be an enemy of the proper functioning of HDD’s. But the mechanics of HDD vibration is an obscure subject, and as a result, there is an aura of mystery surrounding vibration. As part of my job, I’ve spent a lot of time and effort to understand vibration and to engineer our systems to deal with it. I’m hoping to share some of that with you to help demystify the subject. HDD structure and scale The figure below shows the structure of a typical HDD, showing platters, tracks, heads, sectors, and cylinders (sets of corresponding tracks on platter surfaces). To get a sense of how small all of this really is, let’s do some back of a napkin calculations. * A WD Purple 8TB HDD has approximately 248 million tracks cut up into just north of 15 billion sectors across all platters. * Let’s assume there are 8 platters in there and all are double-sided. That means there are ~15.5 million tracks on each surface of a platter. * Assuming 3.5” outer diameter and 1.75” inside diameter, the actual usable space of the platter has a radius of 0.875”. * That leaves us with 17.7 Million tracks per inch. * Considering the thickness of a piece of standard letter paper is 0.004” that means you could fit 70,800 tracks on the edge of a piece of paper. That really is some tiny dimensions we’re working with here. Once you see how t...