How many terabytes can you fit on a 2.5-inch hard drive?

Fun with areal densities

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Next gen security for virtualised datacentres

Hofemann thinks HDD and media suppliers will engage in pilot production this year and next year, then move on to volume production in the 2010/11 period. This agrees with the Xyratex view.

Beyond DTR and BPM is Heat-Assisted Magnetic Recording (HAMR) with Seagate thinking it might attain 10Tbit/sq in with that technology. Seagate sources have indicated that it might use HAMR before BPR. HAMR will involve large changes in head technology to provide the heat needed for writing data.

Hofemann has said: "Most present technology extensions are running out of steam, and going to 1Tb/in.2 won’t be easy," Hofemann said. "We’re at about 700-800Gb/sq in, and these extensions are hitting limits. Discrete track media will get us past 1.5Tb/sq in, and then it’ll be necessary to go to bit patterning. Heat-assisted recording theoretically could work at a 10-30Tb/sq in level, taking us to the end of the decade."

Head density improvements

Hitachi GST has said it hoped to achieve a 2.5 TB 3.5-inch drive in the 2010 period with existing tunnelling magneto-resistive (TMR) head technology. A TMR head will be given a Wrap-Around Shield (WAS) to prevent the head's own magnetic forces corrupting perpendicularly recorded data on adjacent tracks to the read track. Then it expects to move to CPP-GMR (Current-Perpendicular-to-Plane Giant Magneto-Resistance), a more advanced technology, able to deliver recording densities of 500Gbit/sq in to 1Tbit/sq in.

This means, Hitachi GST avers, a 625Gbit/sq in areal density would be practicable instead of today's 250Gbit/sq in, giving us a 2.5TB, 3-platter, 3.5-inch drive by 2010/11. The next-generation CPP-GMR heads would take Hitachi GST through to terabit areal densities, still using PMR media, Hitachi GST says, and a 5TB, 3-platter, 3.5-inch drive by 2013.

These numbers are against a base line of a supposed 250Gbit/sq in areal density today. As we have seen, advanced notebook drives are already way beyond this, at 380GBit/sq in or higher. But they rotate more slowly than enterprise 3.5-inch drives and thus can pack their data bits closer together. They would meet the PMR limit sooner and so move to DTR sooner. They are also facing burgeoning competition from solid-state drives (SSD) - Toshiba has just introduced a 250GB SSD in the 2.5-inch form factor.

Applying the same 2.5 times increase to their areal densities gets us to a 1.25TB 2.5-inch drive in 2010. What happens if we apply a 50 per cent annual increase in areal density to today's highest-capacity 1.8-inch, 2.50inch, and 3.5-inch drives?

  • 1.8-inch - 560GB with 1 platter in 2010, 1.26TB in 2012.
  • 2.5-inch - 1.1TB with 2 platters in 2010, 2.5TB in 2012.
  • 3.5-inch - 2.2TB with 3 platters in 2010, 5TB in 2012.

The probability seems to be that we will get a fifth PMR generation next year, a sixth in 2010, the first DTR drives in 2011/12 with BPR appearing possibly in 2013/14. All this is very speculative and individual manufacturer's mileage may vary, but a kind of consensus is emerging that both and head technology developments will enable us to bypass the areal density roadblock approaching us as HDD suppliers push PMR densities higher. Whether the 2010 and 2012 HDD capacity points above are realistic only time will tell. ®

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