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Lee Schlesinger
What happens when hard-drive technology hits its size limit?

Lee Schlesinger
Senior Technology Editor, ZDNet
Tuesday, August 7, 2001
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Nowadays, new desktop PCs typically come with 20GB to 40GB of hard drive space. That's plenty for most people today. But the increasing popularity of digital sound and video is likely to make storage needs grow.

Most desktop PCs use IDE (Integrated Drive Electronics) disks that follow the ANSI T13 committee's AT Attachment (ATA) spec. The latest version of ATA specifies a maximum of 28 bits for addressing the logical blocks of the cylinders, heads, and sectors that make up the organizational hierarchy of a hard drive. That's 228 sector addresses. Multiply that large number by the 512 bytes in a sector, and you discover that the maximum size for an ATA hard drive is 137GB.

A DRIVE OF 137GB may seem large, but historically, the largest drives have been about six times the size of the average drives sold. That means we're about to come up against the limits of ATA technology.

What will happen when manufacturers hit the limit? Not much. Workarounds have been around for a long time in the form of redundant arrays of inexpensive disks (RAID). If you prefer a single drive, you can already purchase SCSI drives that exceed 137GB.

ATA drive manufacturers are not unaware of the impending capacity ceiling. They've pushed the T13 committee into drafting a new version of the ATA spec, ATA-6, which raises the number of address bits from 28 to 48. That would increase the maximum capacity of an ATA drive to about 144 petabytes.

WHAT'S A PETABYTE? Just as a gigabyte is 1,000 megabytes or 1,000,000 kilobytes, a petabyte is 1,000 terabytes, or 1,000,000 gigabytes. That's a lot of bytes. By the way, after petabytes come exabytes (and you wondered where the tape drive company got its name), zettabytes, and yottabytes. A yottabyte is a million billion gigabytes.

Though the standard is not quite final, disk manufacturers are poised to turn out products that comply with it when the time comes. You can expect to see ATA-6 hard drives on the market by the end of the year.

A capacity limit of 144 petabytes ought to be enough to satisfy hard drive makers for a good long time. Unfortunately, it won't be the last disk bottleneck. Because most modern operating systems, including all flavors of Windows through Windows XP, use 32-bit addressing, they won't be able to address more than 232 bytes, or 2.2 terabytes, of storage on a single disk, even though a disk itself may be capable of storing more than that. While that number sounds outrageously high now, remember that just three years ago an 8GB drive was a massive unit. It should be interesting to see the state of storage in 2004.

Do you think the new ATA spec will keep drive capacity from maxing out? TalkBack to me.

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