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Drive suppliers hit capacity increase difficulties

PMR or HAMR for your platter?

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Hard disk drive suppliers are looking to add platters to increase capacity because of the expensive and difficult transition to next-generation recording technology.

There are two candidates to replace the current Perpendicular Magnetic Recording (PMR) technology. The first is Heat-Assisted Magnetic Recording (HAMR) and the second Bit-Patterned Media. PMR becomes unstable once the magnetised areas become very small - they are subject to influence from neighbouring bits, among other interference. HAMR uses a media formulation in which heat is needed to write data, thus reducing such influence. BPM has a ring of insulating material around each bit to deal with the same problem.

Conversations with various industry-people, including Xyratex CEO Steve Barber, create a picture in which PMR has to be kept in use for four to five years because of the difficulty and expense of moving to either BPM or HAMR.

We have heard from Isilon and Buffalo spokespeople that hard disk drive (HDD) manufacturers have been talking about 4TB 3.5-inch drives arriving in the middle of 2011. We have also heard that there are 3TB evaluation drives out amongst OEMs.

Currently we have 2TB 3.5-inch SATA drives which have four double-sided platters inside them, with a read/write head per platter surface. Seagate has announced a 3TB FreeAGent GoFlex external drive but has not said what sort of disk drive is inside it. Our understanding is that it is a 5-platter 3.5-inch unit.

There are two ways to increase HDD capacity: increase the areal density per platter or increase the number of platters. We rule out increasing the size of the platters as that would increase the vibration at their edge and make the maintenance of the correct read/write head height more difficult. A platter size increase is also against the grain of HDD evolution and we are currently seeing a transition to 2.5-inch units. These employ a glass platter which is stiffer than the aluminium ones seen in 3.5-inch format drive.

Xyratex supplies drive array subsystems to IBM, Dell, EMC and NetApp. It buys an large number of disk drives and knows what is going on in the HDD industry. Barber says one HDD manufacturer is telling Xyratex that 3.5-inch drives are dead, with 2.5-inch the future, due to rebuild times: "2TB drive rebuild times are heading towards a week."

Technology transition costs

Barber says the fact the that HDD manufacturers are discussing 3TB drives with their customers indicates that they can't double densities as they have in the past. The costs and technical difficulties of a move to BPM or HAMR technology are huge: "The next-gen technologies work in the labs but are extraordinarily difficult to move into manufacturing. BPM manufacturing costs are off the scale; they'll need 12 to 14 per cent of [HDD manufacturer's] CAPEX [capital expenditure]." The current CAPEX percentage devoted to PMR manufacturing is rather less.

What will drive manufacturers do? End-users are growing data at a 50 per cent compound annual rate and the world needs disk drives to store it. "HDD suppliers will put more heads and platters into drives or ship more drives."

But datacentre floor space and power supplies are both limited. You can't simply double the number of storage arrays. The conclusion is that we're going to see more platters and heads inside disk drives. He thinks we could see three or four platters inside 2.5-inch drives, possibly even five. The laptop 2.5-inch drive bay slot needn't constrain enterprise arrays. We could see one, two or three platters with 3.5-inch drives.

That means that drive slots inside storage arrays will have to become larger. It's conceivable that you could add just one more platter and maybe shrink the overall height of the plaster stack to keep the drive enclosure depth unaltered, but add another two or three platters and a deeper clamshell case is needed. Increased platter-count drives will also draw more power.

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