Hard drive technology is often an underappreciated wonder. Chip technology deserves more credit than it gets for creating the modern world, but semiconductor manufacturing gets far more attention than hard drive technology. Yet hard drives have continued to give us more and more capacity in the same space for decades, following about the same general trend as Moore's Law, but not as smoothly—hard drive density tends to grow very quickly when a new technology is introduced, and slow down until the next big innovation comes along.

Right now, we're just entering the transition phase. The current technology, known as perpendicular magnetic recording (PMR) underpinning virtually all of the hard drives made today, is beginning to run out of steam. New techniques such as heat-assisted magnetic recording (HAMR) are on the way but still a few years off.

As a result, we're seeing some specialized drives reach new capacities—for instance, Seagate's new 8TB business-class drive, and HGST's 10TB version—but the basic consumer hard drives are not as quick to get that much more density. It's been a couple of years since I really looked deep at this technology, so I took the opportunity recently to talk to the drive makers about the technology and where it is heading.

For the past several years, drives have been using the PMR process, and today mainstream drives have an aerial density of 650 Gbit/sq. inch, allowing 500GB per platter on a 2.5-inch drive and 1TB per platter on a 3.5-inch drive. (Most hard drives have multiple platters, which are written on both sides.)

A few drives have taken this a bit further, moving up to 1.2TB per platter, allowing for 6TB on a five-platter, 3.5-inch drive; or even archival 2TB drives using three 2.5-inch platters, according to William Cain, vice president of technology for Western Digital. And Mark Re, Seagate's senior vice president and Chief Technology Officer, says he believes "there's still a lot of mileage in the current technology," using tighter tolerances to improve density.

Beyond this, to push density in the near term, a number of drive makers are turning to new technologies.

Shingled Magnetic Recording (SMR)

Seagate has pushed a technique called Shingled Magnetic Recording (SMR) in which the tracks that the drive heads follow overlap, sort of like shingles on a roof. According to Re, this technology can allow for a 25 percent boost in aerial density.

SMR uses conventional read/write heads, which work just like a conventional drive for reading the data. But for writing, it requires actually writing to multiple tracks, and this requires the drive to be grouped into different bands.

Re says Seagate has now shipped "many millions of drives" using SMR technology, including branded retail drives and near-line business-critical storage drives. This began with the company's 5TB desktop drive aimed at near-line enterprise storage, but has now moved to other products as well. The 8TB drive the company recently announced has a variant that will use SMR technology.

He says the future of SMR should see notebook drives introduced within the year, and he sees this moving from 750GB per platter to 1TB per platter and maybe eventually as much as 2TB per platter.

One issue with SMR, Cain pointed out, is that the drive has to write information differently, in a more sequential way, and to do that requires manipulating the data size to make it efficient. Re said he agreed that there were issues in some workloads, but said that in 99.9 percent of cases there was no notable performance difference. Generally, he said, typical amounts of cache on the drive eliminate the impact. Cain noted that there are some new standards—zone block commands (ZBC) for SAS drive and zone ATA commands (ZAC) for SATA drives designed to standardize the use of SMR drives.

Scott Wright, Toshiba's enterprise HDD product marketing manager, said Toshiba is participating in the subcommittees working on the standardization of commands for SMR drives and expected a ratified standard in the next few months and believes it is a good fit for applications with lots of sequential writing, such as object storage. He expects to see all of the vendors offering drives aimed at early adopters over the next year or so, with large-scale adoption in the second half of 2015.

Sealed Drives

Another option we're beginning to see involves sealed drives with helium replacing air inside an airtight drive.

Last year, HGST began shipping a 6TB drive that allows for more platters in a sealed, single-height drive. This uses a technology it calls HelioSeal, in which the drive platters are enclosed in a sealed drive filled with helium. Cain points out that helium, which is lighter than air, reduces air turbulence and drag between the platters and, as a result, can reduce active power requirements significantly. Thus, Cain says, it is ideal for environments that prize power usage and the number of spindles in place. (Note that while HGST is a subsidiary of WDC, it is run separately from the Western Digital division. Cain says that while Western Digital has looked at helium and shingled magnetic recording, it hasn't yet shipped drives with either technology, though he said "both technologies have value in certain market segments.")

HGST recently announced an 8TB version of this drive called the Ultrastar He8 using the current PMR drives, as well as the Ultrastar He10, which will use the helium-filled techniques as well as the shingled (SMR) technique. It also offers a more standard 6TB drive, which uses five 1.2TB platters in a traditional (non-sealed) drive enclosure.

Seagate has chosen not to use helium at this point with Re saying that while it does have drives that use the technology, it isn't convinced it is the most effective way to increase density.

Toshiba's Wright had similar comments, saying helium may be necessary in the long-term but that it believes it can get to the next "several generations of technology without it." He said the industry has a road map advancing to six or more platters, and Toshiba expects to do that.

Two-Dimensional Magnetic Recording(TDMR)

Over the next couple of years, WD is interested in a technique called two-dimensional magnetic recording (TDMR), in which you have two read heads and can thus have more data in the same area with adjacent bits being examined and compared, which Cain compared to the way a noise-cancelling headset deals with ambient noise. He said this did add complexity but might make sense for some specific projects in some markets, as it extends conventional recording technology.

Heat-Assisted Magnetic Recording (HAMR)

But almost everyone I talked to agrees the next big jump in density is likely to come from a technique known as heat-assisted magnetic recording (HAMR), which involves a laser-generated beam heating up a small portion of the magnetic media allowing bits to be written and then to be stable when they cool off. Such drives could be much more densely packed than any of today's technologies.

The concept isn't new—Seagate demonstrated it back in 2002—but it does seem to be getting closer.

For instance, Seagate's Re said HAMR should be ready for some commercial introductions in 2016, probably initially with strategic partners, and is likely to become a more general part of the hard drive industry by 2018. He said the promise of HAMR should put the hard drive industry on the "next S-curve" (for improvements in density) for the next decade or so. Seagate has said it hopes to have a 20TB drive using HAMR technology by 2020.

Seagate's implementation uses a near-field transducer as a write head with a laser shining 830nm light on the "surface plasmons," which is then focused on a smaller location to heat the material up to 600 degrees Kelvin, at which point a bit can be switched from a 1 to a 0 or vice-versa. Once the location cools down, the bit is stable. The whole heating and cooling cycle takes place in a nanosecond, Re said.

Western Digital's Cain says that HAMR offers the potential to increase areal density three to five times but will add cost. He said the company has tests with thousands of hours of live heads in drives and said the technology is getting feasible, but said 2016 "may be a little aggressive," though he too thought the technology could enter the mainstream by 2018.

Toshiba's Wright was a bit more skeptical, saying the future of HAMR was "still somewhat unclear," and saying that while everyone is investing in "energy-assisted" recording, the jury is still out as to when it will be deployed. He predicted it was at least three or four years away.

Bit-patterned Media

Another topic that has gotten some attention is bit-patterned media, but the companies I talked to all believe this is much further off. Re said this technology is "not ready for primetime" and that the infrastructure for it is just not available. Cain agreed that it was a "much longer-term" solution, although he said the company did have techniques like nano-imprinting and self-assembly in the labs. And Wright said that while the "science is being done," Toshiba doesn't yet see a "specific intercept" when it could enter mass production.

Flash Memory

Some people outside of the hard drive industry have suggested that flash memory could replace hard drive technology altogether, but that seems unlikely. While flash drives are gaining in popularity, especially in notebooks and as a part of a tiered storage solution on the enterprise, flash remains much more expensive than magnetic media, especially for storing lots of data that isn't accessed frequently. Besides, the total capacity of the flash chips manufactured, though growing, is not nearly enough to replace spinning media.

Even Toshiba, which is one of the two largest flash memory producers, agreed with that perspective, with Wright noting that "nothing will touch magnetic media for a decade" from a cost perspective and that there isn't enough NAND flash manufactured to take over even 15 percent of marketplace.

Instead, all of the makers of enterprise storage have systems that combine some amount of flash with hard drives; and on the client side, the hard drive vendors are pushing hybrid drives that combine a bit of flash for speed with magnetic media for more capacity.

Re said Seagate has been offering notebook drives which such features (which it calls SSHDs for solid state hard drives) with desktop drives now following. Western Digital has a similar line with its WD Black2 line, with Cain saying hybrid drives offer "real value."

One thing that stands out is that there may not be one technology that takes over and that the future may well have room for all sorts of storage solutions—from pure flash, either connected directly over a bus or attached as an SSD; to conventional, shingled, and HAMR—all on the market at the same time.

In general, the hard drive technology has moved from one technology to another with the new technology replacing the previous one, just as the current perpendicular magnetic recording (PMR) replaced traditional longitudinal recording over the past decade. But this time may be different, Cain says, with multiple different techniques providing solutions for different markets because of big differences in cost and speed. "The future doesn't necessary look like the past," he said.

Overall, Cain said that by 2020 we could have 5TB or 6TB 3.5-inch drives as standard mainstream drives with up to 20TB drives (with six 3.3TB platters) possible for some highly specialized applications, and that could grow to 50TB drives when the HAMR technology gets fully mature. That's simply an amazing amount of storage.

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