by D. A. Thompson The future J. S. Best of magnetic data storage technology In this paper, we review the evolutionary path tape and other magnetic technologies. Holographic storage of magnetic data storage and examine the [1] and microprobe storage [2] are treated in companion physical phenomena that will prevent us from papers in this issue. Optical storage is an interesting continuing the use of those scaling processes special case. If one ignores removability of the optical which have served us in the past. It is medium from the drive, optical disk storage is inferior in concluded that the first problem will arise from every respect to magnetic hard disk storage. However, the storage medium, whose grain size cannot when one considers it for applications involving program be scaled much below a diameter of ten distribution, or for removable data storage, or in certain nanometers without thermal self-erasure. library or “jukebox” applications where tape libraries are Other problems will involve head-to-disk considered too slow, it can be very cost-effective. It spacings that approach atomic dimensions, dominates the market for distributing prerecorded audio, and switching-speed limitations in the head and will soon dominate the similar market for video and medium. It is likely that the rate of distribution. However, it remains more expensive than progress in areal density will decrease magnetic tape for bulk data storage, and its low substantially as we develop drives with ten performance and high cost per read/write element to a hundred times current areal densities. make it unsuitable for the nonremovable on-line data Beyond that, the future of magnetic storage storage niche occupied by magnetic hard disks. The technology is unclear. However, there are no technology limits for optical storage [3] are not discussed alternative technologies which show promise in this paper. for replacing hard disk storage in the next ten The most important customer attributes of disk storage years. are the cost per megabyte, data rate, and access time. In order to obtain the relatively low cost of hard disk storage compared to solid state memory, the customer must Introduction accept the less desirable features of this technology, Hard disk storage is by far the most important member of which include a relatively slow response, high power the storage hierarchy in modern computers, as evidenced consumption, noise, and the poorer reliability attributes by the fraction of system cost devoted to that function. associated with any mechanical system. On the other hand, The prognosis for this technology is of great economic and disk storage has always been nonvolatile; i.e., no power is technical interest. This paper deals only with hard disk required to preserve the data, an attribute which in drives, but similar conclusions would apply to magnetic semiconductor devices often requires compromises in ௠Copyright 2000 by International Business Machines Corporation. Copying in printed form for private use is permitted without payment of royalty provided that (1) each reproduction is done without alteration and (2) the Journal reference and IBM copyright notice are included on the first page. The title and abstract, but no other portions, of this paper may be copied or distributed royalty free without further permission by computer-based and other information-service systems. Permission to republish any other portion of this paper must be obtained from the Editor. 311 0018-8646/00/$5.00 © 2000 IBM IBM J. RES. DEVELOP. VOL. 44 NO. 3 MAY 2000 D. A. THOMPSON AND J. S. BEST 105 Superparamagnetic effect Travelstar 18GT Travelstar 16GT 104 Travelstar 3GN,8GS Travelstar 4GT,5GS Deskstar 37GP Travelstar VP Ultrastar 36ZX Travelstar 2XP Deskstar 16GP 3 Travelstar XP Deskstar 16GP 10 Ultrastar 9ZX,18XP Shima Travelstar LP Spitfire Ultrastar 2XP Allicat Wakasa Ultrastar XP Corsair I&II 3390/9 ) 2 Sawmill 2 10 3390/3 3390/2 3380K 3380E 10 X 3380 3370 X 3350 Areal density (Mb/in. 1 3340 3330 2314 10 1 2311 1311 10 2 RAMAC 10 3 1960 1970 1980 1990 2000 2010 Year Figure 1 Magnetic disk storage areal density vs. year of IBM product introduction. The Xs mark the ultimate density predictions of References [4] and [5]. storage cost. Figure 1 shows the areal density versus time 1000 since the original IBM RAMAC* brought disk storage to computing. Figure 2 shows the price per megabyte, which 100 in recent years has had a reciprocal slope. Figures 3 and 4 show trends in data rate and access time. Since the long 10 history of continued progress shown here might lead to complacency about the future, the purpose of this paper 1 is to examine impediments to continuation of these 0.1 long-term trends. Price per megabyte (dollars) The sharp change in slope in Figure 1 (to a 60% 0.01 compound growth rate beginning in 1991) is the result 1980 1985 1990 1995 2000 of a number of simultaneous factors. These include the Year introduction of the magnetoresistive (MR) recording head by IBM, an increase of competition in the marketplace Figure 2 (sparked by the emergence of a vigorous independent Price history of hard disk products vs. year of product introduction. component industry supplying heads, disks, and specialized electronics), and a transfer of technological leadership to small-diameter drives with their shorter design cycles. The latter became possible when VLSI made it possible to processing complexity, power-supply requirements, writing achieve high-performance data channels, servo channels, data rate, or cost. and attachment electronics in a small package. It could be Improvements in areal density have been the chief argued that some of the increased rate of areal density 312 driving force behind the historic improvement in hard disk growth is simply a result of the IBM strategy of choosing D. A. THOMPSON AND J. S. BEST IBM J. RES. DEVELOP. VOL. 44 NO. 3 MAY 2000 1 102 Ultrastar 36ZX Ultrastar 36XP Deskstar 37GP Ultrastar 18XP Ultrastar 9ZX Deskstar 22GX,25GP Deskstar 16GP 10 1 10 Sawmill Allicat Ultrastar 2XP Ultrastar XP 3390/2 Spitfire 3380E 3380 3390/9 3370 3380K Corsair I&II 3390/3 10 2 1 3330 3350 3340 2314 2311 10 3 10 1 Internal data rate (Gb/s) Internal data rate (MB/s) 1311 10 4 10 2 RAMAC 10 3 1960 1970 1980 1990 2000 2010 Year Figure 3 Performance history of IBM disk products with respect to data rate. to compete primarily through technology. In IBM’s absence, the industry might well have proceeded at a 100 slower pace while competing primarily in low-cost design Ultrastar 2XP Ultrastar 9ZX Ultrastar XP Ultrastar 18XP and manufacturing. To the extent that this is true, the Accessing Ultrastar 36XP current areal density improvement rate of a factor of 10 Ultrastar 18ZX every five years may be higher than should be expected in Ultrastar 36ZX 10 a normal competitive market. It is certainly higher than the average rate of improvement over the past forty years. Time (ms) Seeking A somewhat slower growth rate in the future would not threaten the dominance of the hard disk drive over Rotational its technological rivals, such as optical storage and 1 1990 1995 2000 2005 nonvolatile semiconductor memory, for the storage Year of availability market that it serves. Our technology roadmap for the next few years shows Figure 4 no decrease in the pace of technology improvement. If anything, we expect the rate of progress to increase. Of Performance history of IBM disk products with respect to access course, this assumes that no fundamental limits lurk just time. beyond our technology demonstrations. Past attempts to predict the ultimate limits for magnetic recording have been dismal failures. References [4 – 6] contain examples of these, which predicted maximum densities of 2 Mb/in.2, operate at nearly a hundred times the latter limit. In each 7 Mb/in.2, and 130 Mb/in.2 Today’s best disk drives case, the upper limit to storage density had been predicted 313 IBM J. RES. DEVELOP. VOL. 44 NO. 3 MAY 2000 D. A. THOMPSON AND J. S. BEST L s L . Shrink everything, including tolerances, by scale factor s (Requires improved processes) . Recording system operates at higher areal density (1/s)2 (Requires improved disk, head, and electronics materials and designs) . Signal-to-noise ratio decreases Figure 5 Basic scaling for magnetic recording. on the basis of perceived engineering limits. In the first how to scale the velocity or data rate to keep the dynamic two examples, the prediction was that we would encounter effects mathematically unchanged. Unfortunately, there serious difficulties within five years and reach an is no simple choice for scaling time that leaves both asymptote within ten years. In this paper, we also predict induced currents and electromagnetic wave propagation trouble within five years and fundamental problems within unchanged. Instead, surface velocity between the head ten years, but we do not believe that progress will cease at and disk is usually kept unchanged. This is closer to that time. Instead, we expect a return to a less rapid rate engineering reality than other choices. It means that of areal density growth, while product design must adapt induced eddy currents and inductive signal voltages to altered strategies of evolution. However, the present become smaller as the scaling proceeds downward in size. problems seem more fundamental than those envisioned Therefore, if we wish to increase the linear density by our predecessors. They include the thermodynamics of (that is, bits per inch of track) by 2, the track density the energy stored in a magnetic bit, difficulties with head- by 2, and the areal density by 4, we simply scale all to-disk spacings that are only an order of magnitude larger of the dimensions by half, leave the velocity the same, than an atomic diameter, and the intrinsic switching and double the data rate.