Prospects for the Evolution of Hard Drives: Advanced Technologies and Implementation Challenges

    For Western Digital, 2017 was marked by a number of high-profile premieres. On April 6, we announced an updated flagship in the line of specialized video surveillance drives WD Purple with a capacity of 10 TB, and on May 19, 10-terabyte models of the WD Red and WD Red Pro series were introduced, aimed at personal use and operation as part of corporate NAS. Autumn also did not go without fresh releases: in September, WD Gold began selling 12 TB, and in October the world saw the first 14 TB hard drives launched on the market under the brand name HGST Ultrastar Hs14.

    All this was a response to the growing needs of IT enterprises: the business urgently needs to increase the density of information storage, and every year the appetites of companies only grow. The question is, where is the limit, having reached which we can confidently say: “The time has come to look for a fundamentally different solution to the problem, the potential of hard drives is completely exhausted.” We will talk about this in today's article.

    The industry does not keep up with customers


    One of the key milestones in the history of our company is the development of the helium platform HelioSeal. Its essence is quite simple: the case of the hard drive is completely sealed and filled with helium - gas, which has seven times lower density in comparison with air. This design provides a number of important advantages:

    • significantly lower resistance of the gaseous medium inside the disk allowed the use of thinner magnetic plates, increasing their number from five to seven;
    • the decrease in turbulence helped to improve positioning accuracy, reduce the gap between the magnetic plates and the writing head, due to which it was possible to reduce its physical dimensions, achieving a higher recording density using the PMR method (perpendicular magnetic recording), and increase the speed of HDD by 21%;
    • a decrease in the friction force made the disks colder by 4 ° C and almost 49% more economical in terms of capacity (the latter is due to lower energy consumption for spindle unwinding).


    Benefits of the HelioSeal Platform


    Thanks to HelioSeal, it became possible at first to see 6- and 8-terabyte models, and then “heavyweights” of 10, 12 and 14 TB. But even this did not help to fully meet the demand. The rapid development of the sphere of information services (especially cloud technologies) has led to an explosive increase in the amount of processed information. While data center volumes increase by approximately 40% each year (according to estimates by the consulting company International Data Corporation), HDD capacity for the same period increases by only 20%. In this regard, it should not be surprising that if in the first year and a half of its existence HelioSeal Western Digital successfully sold about 1 million units of hard drives for 6 and 8 TB, then for the third quarter of 2015 alone, a total of 1.1 million were sold HDD

    The above statistics leave no reason for doubt: although the new drives have broken world records in their weight, even 14 TB per device is too little, taking into account the appetites of modern business, which already needs more productive equipment. What methods of solving this problem exist today?

    SMR - Promising File Storage Solution


    Although the HelioSeal platform has helped to significantly increase the volume of hard drives, a further increase in the number of magnetic plates is not possible: as practice has shown, adding at least one will make the devices extremely unstable, significantly reducing their reliability. However, the number of “pancakes” is only one of the many parameters that determines the capacity of the HDD, and since such characteristics as the size of the cell for storing a unit of information and the width of the tracks have remained unchanged for many years, there are still prospects for development.

    The first thing that comes to mind is SMR (Shingled Magnetic Recording) technology, which has found application in the new HGST Ultrastar Hs14. The “tiled” recording method differs from the classical perpendicular one in that each subsequent track overlaps the previous one: in this way it is possible to overcome the threshold of 1 Tbit / inch2 (the maximum that can be squeezed out of the PMR). The difference between the two approaches is clearly demonstrated in the diagrams below.


    Recording using the PMR method: the width of the tracks is significantly higher than the reading zone

    When we talk about increasing the recording density of PMR due to the high accuracy of head positioning in products based on HelioSeal, we are talking about reducing the gap between tracks (Guard Space in the figure). SMR, in theory, is able to provide volume growth of up to 20% or more.


    Recording using the SMR method: each new track is layered on the previous one

    But, we repeat, only in theory. In practice, there are significant problems with dubbing: since the recording head is wider than the reading area, when updating data, not only the required fragment is erased, but also subsequent tracks. As a result, each operation has to adjust both the target track and the neighboring ones. To minimize performance losses, tracks are placed in separate groups (“tapes”), which allows you to record the maximum number of tracks that need to be dubbed and ensure stable device performance. However, the need for additional space between the tapes makes SMR not so effective in terms of increasing disk capacity.

    An alternative to this approach is a combination of PMR and SMR. In fact, the HDD is divided into “working” and “archive” zones, in each of which different recording methods are used. The work area plays the role of a kind of cache, where files are used regularly. Subsequently, they are automatically transferred to the SMR zones without user intervention, which is somewhat reminiscent of the “garbage collection” procedure of the SSD.

    Another option is to create hybrid models, in which, along with the DRAM buffer, either SLC NAND memory or cheaper pseudo-SLC chips based on TLC will be present. At the output, we get devices that implement three-level caching, which greatly complicates the architecture of the HDD and requires fundamentally different read / write algorithms that could fully unleash the potential of such a bundle.

    But even with all of the above, SMR will remain a highly targeted solution for specific industries - for example, for archiving information and video surveillance (a combination of DRAM + SLC + PMR zones can provide a sufficiently high sequential recording speed in multi-threaded mode, when one drive serves from 32 to 64 cameras). In the retail segment, this technology is suitable only for the production of compact USB-drives: since such devices are primarily used as portable storage, the difference in performance between SMR and PMR is not so critical. At the same time, tile recording allows not only to increase the capacity of external HDDs, but also significantly reduce their overall dimensions. Alas, such devices, alas, will not work for operation in high-performance servers and desktops.

    TDMR, HAMR or MAMR?


    A more interesting technology is the two-dimensional magnetic recording TDMR, which can provide a stable increase in capacity up to 10% without loss of HDD performance.


    Workflow and Benefits of TDMR

    Its essence lies in physically reducing the size of the writing heads, which makes it possible to arrange magnetic tracks more compactly. Here another problem arises - the emergence of ITI (Inter-Track Interference). Simply put, in the process of reading information, the reader will perceive electromagnetic interference from neighboring tracks, which is fraught with recognition errors. Interference can be defeated through additional consecutive calls to one track. As a result, the controller will receive the necessary amount of data to suppress interference, but such an approach will significantly increase delays and require a significant expansion of the cache.

    Another solution is to create an array of reader modules that can read data from one track at the same time. This method allows you to reduce the signal-to-noise ratio without the need for additional passes, however, real-time filtering will require more efficient chips and an error control system (however, the latter can be implemented based on the Gallager code). In addition, the use of several readers on one head, if there was an effective ITI suppression system, would help increase the speed of hard drives due to the simultaneous reading of several tracks.

    HAMR (Heat-Assisted Magnetic Recording) technology opens up even more interesting prospects, which can increase the recording density up to 2 Tbit / inch2 and even higher. The essence of thermo-assisted magnetic recording is as follows: the writing head is equipped with a laser with a wavelength of 810 nm and a power of about 20 mW, which locally heats the magnetic plate to 450 ° C. High temperature helps to reduce coercivity (the value of the magnetic field strength required for complete demagnetization), which, in turn, reduces the area required for storing one bit of information, while eliminating the likelihood of a superparamagnetic effect (arbitrary transition of ferromagnetic particles to a single domain state) , which will lead to the loss of recorded data).


    HAMR principle of operation

    By tradition, here were not without pitfalls. The data carrier is heated by means of an optical near-field converter (NFT), which transmits thermal radiation to the magnetic plate, making it possible to record. Due to its excellent optical properties, gold was chosen as the main material for the manufacture of NFT. However, this metal has insufficient mechanical strength and quickly deforms under prolonged high-temperature exposure, which is why the optical converter fails too quickly. Therefore, the commercial implementation of this technology rests on the development of a thermally stable alloy.

    Another problem is that HAMR indirectly conflicts with HelioSeal. Since the thermal conductivity of helium is greater than that of air, in order to carry out local heating of the magnetic plate, it will be necessary to use a more powerful laser, which will increase the requirements for NFT characteristics. Rapid heating of the gaseous medium will lead to an increase in pressure inside the HDD, which means that the force of resistance to rotation of “pancakes” will increase, which will inevitably affect energy consumption. In turn, the temperature of the disk enclosure will also increase, and under no circumstances should it exceed 60 ° C, otherwise the costs of upgrading the air conditioning systems will increase significantly. All this is a serious obstacle to mass production of HDDs based on this technology.

    An alternative to these technologies is the innovative development of Western Digital - MAMR (Microwave Assisted Magnetic Recording - Microwave Magnetic Recording).


    MAMR - microwave magnetic recording

    The principle of its action is in many respects similar to HAMR with the only difference being that a spin moment generator (STO - Spin Torque Oscillator) is used in the recording layer to excite the magnetic domain. The module is a multilayer thin-film structure, in the gap of which, under the influence of direct current, a high-frequency (20–40 Hz) circular field arises that acts on the magnetic plate, due to which it is possible to significantly reduce coercivity and, therefore, significantly facilitate the change in the magnetization vector. MAMR is compatible with the classic PMR, while reducing the size of the “grain” (the area required to record one bit of information) from 8-12 nm to a record 4 nm or even less.


    Benefits of Using MAMR

    Thus, we managed to achieve a record recording density of up to 4 Tbit / inch2, which in the future will allow us to create drives with a capacity of 40 TB and higher in a classic case with a 3.5 ”form factor. The first serial devices are planned to be launched on the market in 2019. Their use will help to increase the data center's gross capacity by 40%, while the operating costs of disk storage will remain the same, because, unlike HAMR, new HDDs have low heat dissipation, and the STO generator is practically not subject to wear. If we talk about the migration from SSD-drives to hard drives based on MAMR, then their implementation will help reduce the cost of storing each gigabyte of information by 10 times.

    Specialized Products


    Talking about the prospects for the further evolution of hard drives, a simple truth should be clarified: the rules of the game have changed, the industry is slowly but surely heading for a clear segmentation and creation of highly specialized products that are “tailored” for specific tasks. When it comes to servers and nearline-applications, sacrificing speed for the sake of greater capacity becomes impractical and here the helium platform, capable of providing both high performance and impressive capacity, remains the best choice.

    In the case of network file storages and DAS, SMR capabilities are quite enough, and for DVRs, a tiled record in tandem with an advanced caching and information processing system based on SLC NAND will be a reasonable choice. TDMR will conquer the consumer market: the competent implementation of two-dimensional recording will allow you to create capacious, fast and inexpensive (compared to models based on HelioSeal) solutions that will be quite in demand among professionals and enthusiasts.

    Initially oriented to the corporate segment, MAMR drives in the future will be able to become the "gold standard" of high-performance data centers and cloud platforms. As for HAMR, there is no need to talk about the mass introduction of this technology - too many problems of a technical and economic nature have yet to be solved. Considering the appearance of Microwave Assisted Magnetic Recording, we can say that thermally-assisted magnetic recording looks like a dead end branch, and HAMR may never find wide application.

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