Small disks, big specs


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Areal density
Areal density refers to how tightly data bits can be packed onto the surface of a spinning platter inside the hard drive and is the key to HDD capacity. Because the size of the platter (2.5 inches or 3.5 inches) is finite, drive makers need to make the bits smaller to increase capacity and pack them more tightly into increasingly narrower tracks that can still be written and read by the magnetic disk drive head.

Storage managers have enjoyed a seemingly endless ride up the areal density curve. As areal density has increased, HDD vendors have been able to reduce the cost per gigabyte of storage to pennies and improve performance. But there's a theoretical limit to this joyride, the superparamagnetic limit, which is the point at which a magnetic bit is so small it can't hold its charge and becomes unstable. The industry appeared to reach this limit a number of times, but at various junctures researchers expanded the amount of data that can be packed on a disk by miraculously pulling another rabbit out of their bag of technical tricks.

The current HDD capacity gains result from the advent of perpendicular storage, by which bits are laid down perpendicular to the surface (vertical) rather than horizontally. "Perpendicular recording technology is good for another three to four more years," says Jim Porter, an industry observer and commentator at Mountain View, CA-based Disk/Trend. Porter has watched the industry overcome the superparamagnetic limit numerous times.

However, it's not clear what the next technical trick will be. The one mentioned most often is multilevel magnetic recording, in which bits are stacked on top of each other and the magnetic heads read and write bits at different depths. "There are probably four things in the hopper. Two may come to pass; it may not be just one thing," says Seagate's Black coyly, reluctant to tip his organization's hand. Others think the solution will entail a change to new materials, such as ceramics.

This was first published in September 2008

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