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High-capacity data storage is needed now more than ever, as today's work-from-home world increases the use of cloud services, video content and image sharing. According to IDC, most of the data generated today gets stored on cost-effective HDDs.
With higher performance flash drives in the mainstream, capacity is the focus of HDD manufacturers. Vendors use areal density to describe HDD capacity, usually in terms of gigabits of data stored per square inch. Disk drive makers have worked to increase areal density to save space, facilitate higher capacity drives and drive down storage costs.
Emerging technologies are being developed to increase areal density as the industry moves beyond perpendicular magnetic recording (PMR) and the shingled magnetic recording (SMR) technologies. Heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording technology (MAMR) appear to be the next. The HAMR vs. MAMR contest may not be decided for some time, but it will influence the types of HDDs being used in the cloud and in data centers in the future.
Where we're at with PMR and SMR drives
The first PMR drives came out in 2005 and used a new approach to expand areal density. In these drives, bits are vertically aligned and positioned perpendicular to the disk platter instead of horizontally.
Manufacturers further refined PMR technology by reducing the head and media spacing and size, among other advances. But the technology is reaching its limit as to how much data can be stored in smaller spaces. At about 1.1 terabits per square inch, thermal stability becomes an issue in PMR drives. Bit values become less reliable as areal density and capacity increase into the terabyte range.
Two technologies have emerged to succeed PMR: SMR and two-dimensional magnetic recording (TDMR). SMR replaced PMR's parallel data track layout with partially overlapping write tracks. This approach increases the number of tracks per inch and narrows the space between tracks. However, SMR architecture requires that data be written sequentially and can have performance issues when it isn't. It requires significant system changes to optimize performance.
TDMR uses multiple read elements to read data from one or adjacent tracks. By combining signals from multiple read-back heads, this approach improves the signal-to-noise ratio from recorded tracks. By providing a stronger signal, TDMR enables higher-capacity drives and can be used on SMR and non-SMR hard drives.
HDDs get an energy-assist on the way to HAMR and MAMR
Thermal instability remains a problem with today's HDD technologies. Increasingly dense drives require higher energy barriers in storage media to maintain thermal stability. To record data reliably against higher energy barriers, HDD actuator heads must generate stronger magnetic fields. Drive manufacturers look to energy-assisted magnetic recording (EAMR) technology to do this. Using heat or microwaves, EAMR technology lowers the storage media's resistance to change in polar magnetization, increasing the magnetic field and thermal stability. And this is where HAMR and MAMR technologies come in.
Western Digital came up with indirect energy-assist PMR, also called energy-enhanced PMR (ePMR). The company is using ePMR in its recently released nine-platter Gold 16 TB and 18 TB HDDs. Western Digital claims these drives are the first commercial implementation of EAMR. With ePMR, greater bits per inch are achieved by applying an electrical current to the recording head to increase the magnetic field and improve writability, according to Western Digital.
Western Digital came up with ePMR while working on MAMR HDDs, which use microwaves to heat the storage medium. MAMR and HAMR, which uses lasers for the same purpose, are seen as the next step to increasing HDD capacity. In the HAMR vs. MAMR race, the next few years will determine which technology comes out on top.
What is MAMR technology?
MAMR technology was originally developed by Jimmy Zhu, a professor of electrical and computer engineering at Carnegie Mellon University. Western Digital demonstrated the first MAMR HDD in 2017.
MAMR technology uses a more stable recording medium than PMR that can accommodate a smaller bit area. The drive's actuator head incorporates a spin-torque oscillator and generates an electromagnetic field near the write pole. Data can be written to the media at a lower magnetic field, enabling denser, more reliable drives. MAMR technology doesn't use external heat, and write operating temperatures are similar to PMR drives. In addition, MAMR drive architecture is like PMR drives with changes only required to the actuator heads.
Western Digital uses damascene processing to manufacture its actuator heads. This approach lets it shape the heads and more precisely incorporate the spin-torque oscillator into the head assembly. The company also uses microactuators to fine-tune the positioning of the actuator heads, moving the articulation point of the actuator arms closer to the head. Actuators are the part of the drive that positions the heads to read and write data. Microactuators provide more control over the actuators for ultra-high-density drives.
MAMR technology can be used in conjunction with other technologies, such as helium-filled HDDs that to reduce resistance on the heads. MAMR drives are also expected to incorporate multi-actuator technology to increase throughput and speed reads, writes and drive rebuilds.
Western Digital plans to switch to MAMR-based drives after 2023. In the meantime, it's already using ePMR technology in its 18 TB and 20 TB Ultrastar DC drives.
What is HAMR technology?
HAMR technology uses recording material that's less prone to thermal instability and the resulting unreliable bit values, which allows smaller recording bits in HDDs. The media used in HAMR drives must be heated as data is written. Seagate Technology, for instance, is using a small laser diode in the recording head as the heat source. The laser heats Seagate's glass recording media to 450 degrees Celsius and then the media is rapidly cooled in a nanosecond to room temperature. This approach makes it easier to flip the magnetic polarity of each bit, allowing data to be written in a much smaller area for higher drive density.
The HAMR process has no effect on drive temperature or the temperature, stability and reliability of the media, according to Seagate. Some HAMR drives incorporate SMR technology and may suffer from some of the same performance issues as SMR when data isn't written in large sequences.
Seagate has trialed 16 TB HAMR drives with customers and plans to have a nine-platter 20 TB HDD out by the end of 2020. The company said it expects HAMR drive capacities will increase 30% a year, reaching at least 40 TB by 2023.
HAMR vs. MAMR
Seagate isn't alone working on HAMR HDDs. Western Digital and Toshiba are exploring both HAMR and MAMR drives. And HDD media manufacturer Showa Denko K.K. said in early 2020 it had developed thin film media made of an iron-platinum magnetic alloy for use in HAMR drives. With this new media, 3.5-inch HAMR-based HDDs with capacities of up to 80 TB could be developed.
In the HAMR vs. MAMR debate, it's unclear which will be the technology of choice, but one thing is clear: HDD technology will continue to be a viable force in the storage market. Which technology -- or technologies -- take the lead and, eventually, win out will be determined in the early EAMR products that come out over next few years. Keep an eye on these as the HDD future unfolds.