Rotational Vibration Safeguard: Selling the Wet in Water?

by Robin Harris on Monday, 24 April, 2006

Maybe I’m late to the party but I just noticed that Hitachi is touting RVS, Rotational Vibration Safeguard (pdf). Since all disks in disk drives rotate, all drives have had rotational vibration from day one. So what’s new? Is this the equivalent of slapping “New: Now with Wet” on a bottle of water?

Rotational vibration is part of a larger problem for disk drives: keeping read/write heads on track. In the old days (pre-mid 80’s) most disk drives had one side of one of several platters dedicated to servo information. One head would track on the servo and all the other heads would read and write data. Worked fine, as long as all the heads were precisely lined up. As recording densities increased this got harder to do.

So in the mid-80’s drives went to embedded servo, where the head-positioning information was placed on the same track as the data, so the head could check its position as it read data. This is what all drives (AFAIK) still do today. The problem is that if the head gets knocked off track it takes an entire rotation of the disk — about 8.3 milliseconds, an eternity to GHz chips — to get back on track.

Disk drives are incredibly intricate machines, one of the wonders of mass production, and the disks are no exception. They are finely balanced despite the stresses of high rotational speeds. At 15,000 RPM for example, 3.5″ aluminum disks flex too much, so all the 3.5″ 15k disk drives actually use platters as small as 2.5″, one reason these drives are much lower capacity than slower drives (and much more expensive). A single drive handles its own rotational vibration just fine.

The problems start when you have a few drives in close proximity, as in a storage array. Each drive is vibrating and those vibrations move in and out of phase with those of the other drives, sometimes creating harmonics that create sharply higher vibration forces. For many years proper mounting of disks could limit these forces sufficiently. But as density continues to increase, ever smaller vibrations can knock a read/write head off track.

So Hitachi’s RVS takes vibration response one step further. Sensors on the edge of the drive sense incoming vibration and adjust the head position to, essentially, vibrate in tune with the external waves. This keeps the head on track, reducing those 8ms penalties for getting off track. As Hitachi reports on their shake test on two servers:

. . . we shook the high-end machine so hard that, without RVS, it could only score 15% in our performance tests. However, the moment RVS was switched on, the score shot up to almost 70%. In the business machine, the improvement was just as significant, boosting performance from 30% to almost 90%.

Since it doesn’t appear to cost much, if anything, extra, I’d recommend that anyone buying an ATA-based storage array, even a four drive shoebox, look for this feature. Another cool technology that really does something to keep disk drives the dominant force in data storage technology. More importantly, another reason that low-cost, high-capacity SATA drives will take an ever increasing share of the enterprise storage market.

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