Subject: Storage | October 11, 2017 - 11:16 PM | Allyn Malventano
Tagged: western digital, wdc, WD, STO, Spin Torque Oscillator, SMR, PMR, Microwave Assisted Magnetic Recording, microwave, MAMR, HAMR, FMR
Today Western Digital made a rather significant announcement in the field of HDD technology. We’ve previously talked about upcoming ways to increase the density of HDD storage, with the seeming vaporware Heat Assisted Magnetic Recording (HAMR) forever looming on the horizon, just out of reach.
WD, like others, have been researching HAMR as a possible way of increasing platter densities moving forward. They were even showing off prototypes of the technology back in 2013, but a prototype is a far cry from a production ready, fully reliable product. Seagate had been making stronger promises of HAMR, but since we are already 5 years into their 10-year prediction of 60TB HAMR HDDs (followed by further delays), it's not looking like we will see a production ready HAMR HDD model any time soon.
Ok, so HAMR is not viable for now, but what can we do? Seems WD has figured it out, and it's a technology they have been kicking around their labs for nearly a decade. Above we see the PMR limit of ~1.1 Terabits/square inch. SMR pushes that figure to 1.4, but we are running up against the so-called 'writeability limit', which is the point at which the write head / magnetic field is too small to overcome the paramagnetic threshold of the smaller magnetic domains of higher density media. We are used to hearing that the only way to raise that limit was to heat the media with a laser while writing (HAMR), but there is a different / better way - Microwave Assisted Magnetic Recording, or MAMR for short.
Don't let the 'microwave' part of the term fool you - we are not microwaving the media with sufficient energy to actually heat it. Instead, we are doing something *way* cooler. The slide above shows how smaller grain size (higher density) requires a stronger write field to reach sufficient energy levels to reliably store a bit of data. Now check out the next slide:
This is a lot to grasp but allow me to paraphrase greatly. Imagine a magnet with a north and south pole. If you came along with a stronger magnet and attempted to reverse its polarity by directly opposing the currently stored state, it's generally difficult to do so. Current HDD tech relies on the field being strong enough to overcome the stored polarity, but MAMR employs a Spin Torque Oscillator, which operates at a high enough frequency (20-40 GHz) to match the ferromagnetic resonance of the media. This causes a precession of the stored field (like a gyroscope) and tilts it about its vertical axis. This resonance adds the extra energy (in addition to the write field) needed to flip the field to the desired direction. What's amazing about this whole process is that thanks to the resonance effects, the STO can increase the effectiveness of the write field 3-4x while only consuming ~1/100th of the power compared to that needed to generate the write field. This reduction in the damping constant of the media is what will enable smaller magnetic domains, therefore higher platter densities in future MAMR-equipped HDDs.
One of the best things about this new tech is that it is just a simple addition to all of other technologies already in place today. Western Digital was already making their drive heads with an advanced 'damascene' process, silently introduced about three years ago. To oversimplify the description, damascene is a process that enables greater physical precision in the shape of the head, which helps increase density. What makes this process a bigger deal now is that it more easily enables integration of the Spin Torque Oscillator into the head assembly. Aside from this head-level change and another pair of leads to provide a very small drive current (~1-2mA), every other aspect of the drive is identical to what we have today. When it comes to a relatively radical change to how the writing can be accomplished at these upcoming higher densities, doing so without needing to change any of the other fundamental technologies of the drive is a good thing. By no change, I really mean no change - MAMR can be employed on current helium-filled drives. Even SMR.
Western Digital also slipped in another announcement, which is the shift from the older style 'nested actuator' (introduced with 2TB HDDs back in 2009), to a newer 'micro-actuator'. The newer actuator moves the articulation point much closer to the head compared to the previous technology, enabling even finer head tracking, ultimately resulting in increased track pitch. WD currently sits somewhere around 400 tracks per inch (TPI), but they hope to reach 1 million (!) thanks to this new tracking combined with MAMR and improved media chemistry.
Now this doesn't mean we will see a sudden influx of 40TB HDDs hitting the market next week. WD still has to scale up production of STO-enabled heads, and even after that is complete, the media technology still needs to catch up to the maximum capabilities of what MAMR can achieve (creating smaller magnetic domains on the disk surface, etc). Still, it's nice to know that there is a far simpler way to flip those stored bits around without having to resort to HAMR, which seems to be perpetually years away from production. Speaking of which, I'll leave you with WD's reliability comparison between their own HAMR and MAMR technologies. Which would you choose?
Oh yeah, and about that supposed SSD vs. HDD cost/GB crossover point. It may not be as soon as we previously thought:
Full press blast appears after the break.