Yesterday, Western Digital announced a breakthrough in microwave-assisted magnetic recording (MAMR) that completely took the storage industry by surprise. The takeaway was that Western Digital would be using MAMR instead of HAMR for driving up hard drive capacities over the next decade. Before going into the specifics, it is beneficial to have some background on the motivation behind MAMR.
Hard drives may be on the way out for client computing systems, but, they will continue to be the storage media of choice for datacenters. The Storage Networking Industry Association has the best resources for identifying trends in the hard drive industry. As recently as last year, heat-assisted magnetic recording (HAMR) was expected to be the technology update responsible for increasing hard drive capacities.
Slide Courtesy: Dr.Ed Grochowski’s SNIA 2016 Storage Developer Conference Presentation
‘The Magnetic Hard Disk Drive: Today’s Technical Status and Its Future’ (Video, PDF)
Mechanical Hard Drives are Here to Stay
One of the common misconceptions amongst readers focused on consumer technology relates to flash / SSDs rendering HDDs obsolete. While using SSDs over HDDs is definitely true in the client computing ecosystem, it is different for bulk storage. Bulk storage in the data center, as well as the consumer market, will continue to rely on mechanical hard drives for the foreseeable future.
The main reason lies in the ‘Cost per GB’ metric.
Home consumers are currently looking at drives to hold 10 TB+ of data, while datacenters are looking to optimize their ‘Total Cost of Ownership’ (TCO) by cramming as many petabytes as possible in a single rack. This is particularly prevalant for cold storage and archival purposes, but can also expand to content delivery networks. Western Digital had a couple of slides in their launch presentation yesterday that point towards hard drives continuing to enjoy this advantage, thanks to MAMR being cost-effective.
Despite new HDD technology, advancements in solid state memory technology are running at a faster pace. As a result SSD technology and NAND Flash have ensured that performance enterprise HDDs will make up only a very minor part of the total storage capacity each year in the enterprise segment.
The projections presented by any vendor’s internal research team always need to be taken with a grain of salt, but given that SanDisk is now a part of Western Digital the above market share numbers for different storage types seem entirely plausible.
In the next section, we take a look at advancements in hard drive technology over the last couple of decades. This will provide further technical context to the MAMR announcement from Western Digital.
Hard disk drives using magnetic recording have been around for 60+ years. Despite using the same underlying technology, the hard drives of today look nothing like the refrigerator-sized ones from the 1960s. The more interesting aspect in the story is the set of advancements that have happened since the turn of the century.
At a high level, hard disks are composed of circular magnetic plates or ‘platters’ on which data is recorded using magnetization and the patterns of magnetization represent the data stored. The patterns are laid out in terms of tracks. They are created, altered and recognized with the help of ‘heads’ mounted on an actuator that perform read and write operations. Modern hard disks have more than one platter in a stack, with each platter using its own individual ‘head’ to read and write.
There are additional hardware components – the motor, spindle, and electronics. The segment of interest from a capacity perspective are the platters and the heads. The slide below shows two ways to increase the capacity of a platter – increasing the number of tracks per inch (TPI) and/or increasing the number of bits per inch (BPI) in a single track. Together they yield a metric for areal density, which the industry gives as a value in bits per square inch, such as gigabits per square inch (Gb/in2) or terabits per square inch (Tb/in2).
Hard drives in the early 2000s primarily relied on longitudinal recording, with the data bits aligned horizontally in relation to the spinning platter – this is shown in the first half of the image below. One of the first major advancements after the turn of the century was the introduction of perpendicular magnetic recording (PMR) in 2005.
At the time PMR made its breakthrough, Hitachi commissioned an amusing video called ‘Get Perpendicular’, which was used to demonstrate this technology and reaching 230 gigabits per square inch. The video can be found here.
PMR was developed as a solution to the previous areal density limits of around 200 Gb/sq.in caused by the ‘superparamagnetic effect’ where the density of bits would cause the bits to flip magnetic orientation and corrupt data. PMR, by itself, can theoretically hit around 1.1 Tb/sq.in.
Alongside PMR, more technologies have come into play. The most recently launched hard drives (the Seagate 12TB ones) have an areal density of 923 Gb/sq.in. The industry came up with a number of solutions to keep increasing hard drive capacity while remaining within the theoretical areal density limits of PMR technology:
Helium-filled drives: One of the bottlenecks in modern drivers is the physical resistance on the heads by the air. Using Helium reduces that resistance, and requires sealed enclosures. The overall effect is improved head stability and a reduction in internal turbulence, allowing for a shorter distance between platters giving manufacturers the ability to stack up to seven platters in a single 3.5″ drive rather than six. Helium drives were first introduced to the market in 2012 by HGST. The latest helium drives come with as many as eight platters.
Shingled magnetic recording (SMR): In this technology, the track layouts are modified to give overlaps, similar to how roof shingles are laid (hence the sname). While this creates challenges in rewriting over old data to ensure that old data is not overwritten, there are sub-technologies and methods to mitigate some of these issues. The challenges can be either solved on the host side or the drive side. Seagate was the first to ship drive-managed SMR drives in 2013.
Improvements in actuator technology: In the last few years, Western Digital has been shipping ‘micro actuators’ that allow for finer positioning and control compared to traditional actuator arms. This directly translates to drives with a higher bit density.
Improvements in head manufacturing: Traditionally, PMR heads have been manufactured using the Dry Pole process involving material deposition and ion milling. Recently, Western Digital has moved to the Damascene process (PDF) that involves a etched pattern filled using electroplating. This offered a host of advantages including a higher bit density.
We had briefly mentioned PMR technology having theoretical limits earlier in this section. Traditional PMR can deliver up to 1.1 Tb/sq.in. with improved actuators and heads. Use of SMR and TDMR (Two Dimensional Magnetic Recording) can drive this up to 1.4 Tb/sq.in.
At those areal densities, the TPI and BPI need to be so high that the media grain pitch, the smallest size that the metallic elements that store individual bits can be, is around 7-8 nm. These small grains present a challenges, such as the head is not capable of creating a strong enough magnetic field for recording while maintaining stability at that small a grain size.
One solution to this would be to make it easier to write the data to the grain. By decreasing the resistance to magnetization, this is possible (this is technically called lowering the coercivity) and it allows the head to modify the magnetic state of the grain. This requires extra energy, such as thermal energy, to be directly applied to the grain for the short amount of time that is needed to write a bit. This is the point where the ‘energy-assist’ aspect comes into the picture.
Over the last several years, a lot of focus has been on heat-assisted magnetic recording (HAMR), where the lowered resitance (or coercivity) is achieved by locally heating the grains using a laser. This brings in a number of concerns that have prevented mass production of drives based on HAMR technology.
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MAMR, on the other hand, uses microwaves to enable recording. A primary reason for MAMR not being considered as a viable technology by industry analysts so far was the complexity associated with designing a write head to include a microwave generator. In the next section, we take a look at how Western Digital was able to address this.
The WD Breakthrough
Western Digital’s Microwave Assisted Magnetic Recording (MAMR) drives use platters very similar to those in the current-generation PMR drives*. This means that the innovation to enable MAMR is mainly to do with the heads that perform read and write operations.
As part of the MAMR design, WD pointed out to its shift to the damascene process for building the bit grains as the key enabler for the MAMR breakthrough. The process allows them to fabricate a spin torque oscillator (STO) capable of creating precise energy fields without any additional overheads. The embedded oscillator in the head is tuned to generate microwaves with a frequency of 20-40 GHz, and this provides the ‘energy-assist’ to make it easier to write to the bits (technically it lowers the coercivity of the underlying recording media).
* Current drives use an aluminium substrate with a cobalt-platinum layer.
WD pointed out that MAMR requires absolutely no external heating of the media that could lead to reliability issues. The temperature profiles of MAMR HDDs (both platters and drive temperature itself) are expected to be similar to those of the current generation HDDs. It was indicated that the MAMR drives would meet all current data center reliability requirements.
Based on the description of the operation of MAMR, it is a no-brainer that HAMR has no future in its current form. Almost all hard drive industry players have a lot more patents on HAMR compared to MAMR. It remains to be seen if the intellectual property created on the HAMR side is put to use elsewhere.
Western Digital has talked about timeframes for the introduction of MAMR drives. They had working prototypes on display at the press and analyst event yesterday. WD’s datacenter customers have their own four to six month qualification cycle, and MAMR drives for that purpose are expected to be out towards the middle of next year. Production-level HDDs based on MAMR technology are expected to start shipping in 2019.
Western Digital sees plenty of value in MAMR, and it is not hard to see why. MAMR technology allows for the bit densities of individual platters to scale to more than 4 Tb/sq.in. WD believes that it is well-positioned to bring 40TB drives by 2025 using MAMR alone.
Technologies such as SMR and TDMR are complementary to MAMR. Currently, WD does not use TDMR in any shipping enterprise drive, and SMR is restricted to a few host-managed models. It is possible that some MAMR drives will use those technologies to achieve higher capacity points compared to conventional drives. WD’s working prototype on display was a helium drive (HelioSeal), but, WD again stressed that helium is not a compulsory requirement for MAMR drives. It was also confirmed that drives of 16TB and more would have to be MAMR-based.
In 2005, when the shift from longitudinal recording to PMR happened, most vendors managed to release drives based on the new technology within a few years of each other. The shift to helium in 2012, though expected by everyone in the industry, proved to be a big win for HGST – they had a the markets that focus on high-capacity, or low-power, or low TCO to themselves for almost three years before Seagate eventually caught up. Toshiba is yet to release a helium drive publicly. It is going to be interesting to see how Seagate and Toshiba respond to this unexpected MAMR announcement from Western Digital.
The players in the hard drive industry have a robust cross-licensing program, and it is highly likely that other manufacturers will not face significant patent bottlenecks in bringing out MAMR drives on their own. WD stressed that the development is a multi-year effort, particularly if the heads are still being manufactured in the old dry pole process.
High-volume mature hard drives are often manufactured with the help of third-party suppliers – such as Showa Denko for the recording media and TDK for the heads. In the case of the MAMR drives, WD mentioned that all the components are being designed and manufactured in-house. It is possible for the competition to catch up faster if some of the third-party manufacturers are further along in their own R&D. In particular, TDK has been investing in MAMR R&D recently too. Toshiba has also shown interest in the same, but it is not clear how far along they are in the commercial development cycle. Currently, we believe WD has a clear lead in MAMR technology. It just remains to be seen how long it takes for the competition to catch up.
Autore: Ganesh T S AnandTech