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Shingled Magnetic Recording Paves the Way for 5TB Hard Drives

By Wesley Fenlon

Seagate's new hard drive technology promises 25 percent denser platters in 2014, with even higher capacity HDDs coming in the future.

Don't count the trusty old hard drive out just yet. While the price-to-gigabyte ratio for SSDs creeps ever downward, making larger SSDs more affordable, hard drives are still the only way to go for mass storage. And they're about to get even larger. Seagate has broken through the limitations that constrained physical spinning disks to one terabyte of data per platter, meaning hard drives are about to jump past that 3TB barrier in 2014.

Seagate's breakthrough comes from a technology called shingled magnetic recording. In case you don't know your hard drive tech, here are the basics: Data is recorded onto thin strips of magnetized film in concentric tracks around a thin, non-magnetic platter. Tiny sites along that film represent bits of storage. The closer together the tracks, the greater the storage density.

Photo credit: Flickr user dtrimarchi via Creative Commons

Put the tracks too close together, however, and you have a problem. They have to be spaced enough to avoid magnetic interference between bits. This is tied to the size of the physical read and write heads that move across the platter. These are harder to shrink, and the write head is always larger. If the tracks are too close together, the write head may read data from more than one track at a time, hence the gap between them.

Thanks to a process called perpendicular magnetic recording, hard drive manufacturers have gotten tracks down to a width of only 75 nanometers. Shingled magnetic recording changes things up by overlapping those tracks--like shingles!--and increasing capacity 25 percent. The trick is in how Seagate is preserving data, since normally the gap between tracks is necessary to prevent the magnetism from getting all screwy. This doesn't affect reading data, because read heads are smaller than write heads. But with writing data, the write heads are now wider than individual tracks.

Anandtech explains what this means: "The obvious downside of SMR is actually very NAND flash-like. When writing data sequentially to an empty platter, SMR is full of advantages. When you're writing to a series of tracks that already contains data, the SMR writing process is actually destructive. Since the writer remains full width and tracks now overlap, overwriting one track will actually harm the next track; those subsequent tracks will need to be overwritten as a result.

"Seagate's SMR groups tracks into bands, with the end of each band breaking the shingled track layout. Breaking the shingled layout regularly reduces max attainable density, but it makes it so that overwriting a portion of one track doesn't force a re-write of the complete disk. At worst, overwriting some sectors will force a re-write of an entire band, not an entire platter."

In 2014, Seagate plans to ship four platter drives that hold 5TB of data, with 1.25TB now fitting onto a single platter. They claim that the 25 percent boost is just the beginning, and hope to use SMR to increase density even more. Empty 5TB drives using SMR tech should perform excellently, but the real test of the technology is how it performs when those tracks fill up. Re-writing those bands could lead to a performance hit on a near-full drive.

(And we can't talk about magnetic storage technology innovations without recalling Hitachi's Get Perpendicular video from 2005)