Earlier this month, Western Digital announced the retail availability of its Gold 16TB and 18TB CMR units, as well as an upcoming 20TB Ultrastar SMR unit. These nine platter drives are the largest single hard drives available today.
Earlier this year, rival unit provider Seagate promised to deliver 18TB and 20TB units in 2020, but they have yet to materialize on retail channels. Seagate’s larger units, such as Western Digital, needed new technology to overcome the trilemma of magnetic recording, but Western Digital’s EAMR is considerably less exotic than the HAMR (heat-assisted magnetic recording) used by Seagate. That more conservative approach likely helped Western Digital beat its rival in the market.
Understanding the trilemma of magnetic recording
The maximum usable data density on a magnetic recording device is limited by three competitive factors. Magnetic coercivity, the intensity of the magnetic field required to demagnetize a domain, must be high enough to prevent the separately recorded grains from influencing each other and corrupting the data. The field strength of the write head must be high enough to overcome the coercivity of the medium. Finally, the size of the field generated by the write head must be small enough not to overwrite adjacent areas.
To increase the amount of data stored on a single plate of a fixed size, you must use narrower fields, but this negatively influences the intensity of the potential field, which in turn means that the smallest and weakest field cannot exceed means that are sufficiently resistant to “crosstalk” between separately recorded values.
There are two basic approaches to overcoming the Magnetic Recording Trilemma as presented: You can design a new recording pattern that works around those limitations, or you can employ some new technology that significantly changes the relationships between the three competing factors mentioned above. .
Staggered Magnetic Recording – A New Magnetic Writing Pattern
Digital West
Staggered magnetic recording is a technique that takes advantage of the fact that we can reliably read data on narrower tracks than we can write it. SMR disks divide your media into separate zones, where writes are placed on overlapping “overlapping” tracks.
While SMR techniques increase area density (currently approximately 10-15 percent) they come with significant performance and flexibility limitations. Since the write track is wider than the read track, an SMR disk cannot modify an individual sector within a zone; doing so would remove other sectors adjacent to the one being modified. To overcome this limitation, SMR disks are zoned, typically 256MiB in size.
There is enough linear distance between each zone and its neighbor to allow rewriting of the zone as a whole on demand, but the zone must be rewritten in its entirety. This in turn means that a single 4KiB sector rewrite may require a first read and then a new 256MiB zone, a worst-case amplification penalty of 13,107,200 percent.
We discussed Western Digital’s use of SMR with Phil Bullinger, Western Digital’s Vice President and General Manager, Data Center Business. Currently, there are no “hybrid” SMR drives – they are either managed by the host or by the drive, not both.
Consumer-facing SMRs, such as the highly maligned Western Digital Red and small desktop drives from Seagate, Toshiba, and Western Digital, are drive-managed SMRs. The firmware and processor in the unit itself handle alignment of writes to zones, garbage collection as zones are rewritten, and everything else needed to at least try to get the SMR unit working at expected levels. to accommodate a typical consumer workload.
Business-facing SMR disks, by contrast, are managed by the host. On a host-managed SMR disk, caching, zone management, and everything else necessary to optimize write patterns to the drive occur on the host computer. This allows not only larger caches, but also data optimizations further down the stack at the application level. If an application can deliberately produce data in 256MiB chunks instead of smaller, randomized operations, its performance will skyrocket in units with matching 256MiB zones.
Western Digital’s new 16TB and 18TB Gold units do not use SMR technology. The 20TB Ultrastar big brother does it, and should do it, host-managed SMR is what enables it to get an additional 11 percent data density of almost entirely similar hardware.
State-of-the-art magnetic recording technologies: HAMR, MAMR and EAMR
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Seagate’s HAMR technology uses a small laser to precisely heat magnetic beads before writing to them.
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Western Digital’s MAMR tests use microwave energy to excite the precise area of the magnetic medium in which it will be written, making it more susceptible to change.
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Western Digital EAMR is much less exotic than HAMR or MAMR: it simply applies additional current to the main pole of the write head as well as to the voice coil.
The other way to potentially increase storage density is to alter the parameters of the Magnetic Recording Trilemma by introducing substantially different recording technology.
Seagate’s HAMR (Magnetic Heat Assisted Recording) is an example of this new recording technology. HAMR uses a small laser to precisely heat the recording medium, temporarily reducing coercivity in an extremely small and well-defined area. Temporarily reduced coercivity allows smaller (and necessarily weaker) fields to alter that area without influencing the surrounding areas, recorded separately.
In previous research, Western Digital favored a similar technology: MAMR, or microwave-assisted magnetic recording. MAMR uses a microwave emitter to excite a small area of the disk to similarly reduce coercivity to the target area for long enough to perform the write operation.
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Western Digital EAMR is much less exotic than HAMR or MAMR: it simply applies additional current to the main pole of the write head as well as to the voice coil.
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The bias current applied to the main pole of the writing head reduces jitter, allowing faster and more accurate application of strong writing fields.
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In addition to EAMR recording technology, WD’s newest and largest discs feature triple-stage write actuators, linked together in three separate locations, similar to a human finger.
Although Western Digital continues its research on MAMR technology, the technology used in this month’s new records, EAMR, or energy-assisted magnetic recording, is considerably less exotic. Instead of altering the magnetic properties of the medium with microwave or laser emissions, EAMR simply stabilizes the writing field more quickly and precisely, using a bias current at the main pole of the writing head, as well as the current in the voice coils.
As you can see from the Western Digital graph in the gallery above, which Bullinger assured us was a real graph using real data, not a theoretical model, skewing the main pole results in much more consistent and predictable field curves from the head as a whole. This allows writes in significantly smaller areas than would be possible without bias from the main pole.
The new 18TB and 20TB drives also use three-stage actuators, which Bullinger tells us is the first in the industry. Each stage is a separately hinged pivot point that controls the location of the unit heads. The first stage makes rapid and radical changes in head positioning, while the other two, milliactuator and microactuator, make smaller and finer changes. The triple stage design contributes to the ability of an extremely dense unit to quickly and accurately position their heads as needed.
Finally, like all very large modern units, Western Digital’s EAMR drives are completely sealed, with an internal helium atmosphere. The lower density of helium at a given pressure creates a much less turbulent environment for heads and plates, allowing faster and more accurate tracking.
Product lines and availability
At this time, you will not find SMR information on any of the Gold data sheets. We called a Western Digital representative for clarification, since no unit on the Gold line uses SMR, the company thought that SMR notifications were not necessary. (We strongly recommend including this information in the data sheets of the units in the future).
Western Digital representatives tell us that the company’s Gold units can best be viewed as a subset of its Ultrastar DC line: All Gold discs have technically identical Ultrastar equivalents, but not all Ultrastar discs have Gold equivalents. The Gold product line are business units, but the entire line is all SATA, all CMR, all the time. Drives in the wider Ultrastar range can offer SATA or SAS interfaces, and conventional or staggered magnetic recording.
New 16TB and 18TB non-SMR Western Digital Gold units are generally available at the Western Digital Company Store and on today’s standard retail channels. Western Digital tells us that the host-managed Ultrastar DC HC650 SMR 20TB is a production sample for select customers now and will be widely available next quarter.
