Advances in disc storage technology

by DEREK COOK

D ata storage systems present computer manufacturers, as well as systems houses and end

users for that matter, with the two ‘textbook’ problems of a technically accelerating industry. Users have dif- ficulty keeping themselves informed of the improvements taking place and, because science fiction is aiways more interesting than fact, are temp- ted to wait for the ‘dramatic advances’ which some callous suppliers insist are ‘almost with us’.

Plated media, vertical recording, thin film heads, intelligent disc drives - all are under development and hold out promise of leap-frogging each other in the improvement race. There are, however, some important new products already on the market and

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Abstract: The article surveys advances in storage technology and likeb future developments. Microfloppy discs, intelligent disc drives, thi~~lm and plated media and o~t~~a~ storage are all discussed in detail, with special attention paid to the cost/benefits promised by these new technologies.

i&yz.oords: storage media, discs, disc drives, Winchesters, optical storage.

Derek Cook is UK manager of the Shugart (:orporation.

imminent developments which, al- though they may not all be dramatic enough to catch the imagination - some are simply extensions to existing trends - do provide a practical pic- ture of the storage technologies we shall be seeing in the next few years and their effect on computer systems.

Floppy progress

The 3% in microfloppy disc drive is now accepted by ANSI and, more important, by the industry in a de facto manner, as the standard sub- ~n~oppy size.

Three and a half inch drives, how- ever, need some degree of 5 l/4 in compatibility and this is not neces- sarily available in all designs. Since most applications for the 3% in drive will be in redesigned and upgraded computers formerly using 5*/4 in, the need for backward compatibility is obvious. Even in portable computers, compatibility will ease the transfer of both data and software because prac- tical track formats and data layouts can be used. In fact, the physical configuration of the disc drive can be completely transparent to both the hardware interface and the operating system software.

Microfloppy capacities of 0.5 Mbyte and 1 Mbyte are now available.

For today’s marketplace and for standalone portable systems these are necessary capacities. Higher capacity 5% in drives have been announced or are in development, though they re- quire a large, well-established manu- facturer to create the standard. No great transformation in technology is needed, simply the familiar trend in higher track densities, but, of course, using a servo-type positioner for greater accuracy.

People who suggest that the 3Y2 in will take over from the 5% in disc in business computers are probabiy allowing enthusiasm for anything new to cloud their judgement and are unaware of developments in 5% in drives.

Using 192 track/in, the capacities of minifloppy drives are already on their way to the 3-4 Mbyte mark. The problem then becomes one of compa- tibility with existing 96 and 48 track/ in drives and media. That problem too is being solved, allowing high capacity drives to read and/or write from/to lower capacity discs. The solution uses ‘write wide, read narrow’ techniques. The high capa- city drives ought to be able to read and write 96 track/in diskettes and read from 48 tracks/in with complete reliability. This high degree of com- patibility is needed because of the

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huge popularity of the 5% in floppy It is not generally realized, inciden- as selfarbitration, command chaining, disc drive - almost a case of future tally, that the same design of drive can and multiple host/drive configura- development being hampered by its be used for vertical as for conven- tions, can also be integrated through success to date. tional media. Only the drive interface the use of the SCSI interface.

electronics and the read/write head The next step is to integrate the Intelligent drives require redesign. This suggests that drive and controller electronics into a

Intelligent disc drives will be used in the compatibility area to allow drives to recognize the type of media being inserted. This will enable automatic selection of the appropriate step dis- tance and allow compensation for the differing requirements of various media, for write current, read signal amplification, etc.

It is relatively easy for the enhance- ments to be added. Many upgrades are possible at ~~~ cost - only additional firmware for the drive’s processor is required. In fact, pro- gress like this shows that the initial development cost of the cheapest possible drive is not, today, going to be the lowest cost way of making a whole new range of disc drives. Up- grade paths must be preplanned.

Microfloppies stand up!

~crofloppy drives, I suggest, are the best candidates for the introduction of vertical recording techniques. The media for vertical recording is at present unreliable, difficult to manu- facture and, at best, somewhat vul- nerable to rni~~d~g. Considerable research into vertical recording media is taking place currently but Shugart feels that the medium capable of being expanded into full volume mass production has yet to appear.

Disc drive m~ufacturers, there- fore, are eager to find a good, reliable vertical recording disc. Some techni- ques like coating oxide particles with cobalt (one of the ingredients in plated media) are showing promise.

The microfloppy disc already has its own hard plastic cartridge protect- ing its thin 50 p in oxide coating from mishandling and accidental damage. Vertical recording diskettes would be safer with such a well protected pack- aging.

vertical recording will be a relatively easy technology to accommodate when the media is ready.

Winchesters

As with the floppy disc drive market, the trend toward half heights and more drive intelligence is a firm one and, though it may be less glamorous than the technical wizardry also show- ing through in this sector, it is, in the short term, the major influence. We are already seeing announcements of 3% in Winchesters, but the micro- Winchester demand is not yet fully clear. The portable and hobby market for which the 3% in floppy drive was developed is not a great consumer of Winchester-type storage capacities - not yet at any rate (one is always wary of dismissing future upgrades in this business).

The Winchester, in the foreseeable future, will come into computers only at the small business capability levels. For these applications, the smaller 3% in size offers few advantages - discs in offices are carried around in files, folders and wallets, not shirt pockets. Again 3% in drives will need compatibility with 5% in sizes them- selves, offering a huge upgrade path. They will also need cost advantages over the 5% in drives in the relevant capacities - a problem which will be solved within a few months.

Intelligent Winchesters

Today’s intelligent Winchester drives are in essence integrated drives and controllers in a single unit. Format- ting, error detection and correction, defect mapping, data buffering, auto- matic retries, formerly work for the CPU, are now done by the drive itself. Additional controller functions, such

single circuit board using LSI and VLSI technology. This also allows greater reliability by compacting more functions into less space.

The enhanced data rate currently being proposed is dependent upon new controllers, and therefore will probably manifest itself more easily in intelligent drives than in common designs.

Data rates of 10 MHz apply mainly to the high performance minicom- puter Winchesters rather than the low cost, low performance micro systems. More widespread application must await increased demand for develop- ment to be worthwhile and for the LSI chips to be designed. Shugart has an advantage in this market being the only manufacturer to also manu- facture controllers. Faster data rates and intelligent disc drives are both under development.

Further developments include the use of magneto-optics which com- bines magnetic and laser technologies. Magneto-optics produces capacities of around 17000 tracks/in and 25 000 bit/in, though these capacities are two or three years away.

Thin and plated media

Thin film and plated media are inter- esting future developments, though not, as yet, necessarily the most profitable way for systems designers to spend their time. We shall not see the maximum benefit from either technique until systems are designed which use both together - perhaps also with vertical recording formats.

For the next few years, oxide media will match plated media byte for byte. But as oxide media is limited by head and positioner technologies, plated media will eventually outstrip the performance of oxide.

Electroplat~g allows very thin

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systems Focus error prism -,_ Objective lens two clxis actuator

Tekxapic lens assembly Detector

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coating, higher densities and a harder surface better able to withstand damage from shock and vibration. A 5’/j in Winchester drive with two platters could, on current capacity levels for plated media, achieve 50 Mbyte of storage instead of the more typical 10 Mbyte.

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Servo-assisted

Somewhat less dramatic, but never- theless a practical improvement, is the use of a servo, or stepper compensa- tion positioner. A servo, instead of a simple stepper motor head actuator on the two platter 5% in Winchester will further double the capacity to 100 Mbyte.

i?wting mirror assembly

Thin film heads allow an increase in the linear bit density on oxide media though in the immediate future they will continue to apply only to 14 in drives.

How the optical disc drive works. ction lens in the optics unit of Shugurt’s U~~i~~ ~~~ ~p~~c~~ &c drive gathers light em~ging from t+ laser diode and fms a ~ollimuted ~purullel~ beam of light. The correction prisms then gulfs this elliptical beam into a circular beam. A routing mirror bends the beam 90” into a polarizing beam splitter. This transmits the beam through the quarter wave plate, rotut~ng the po~u~‘zuti~ of the beum by 9U”, and passes the beam to the spot- fming objective lens, which is part of a two-axis actuator system.

The higher costs of such heads are more easily absorbed in the higher performance drives, which are charac- terized by their higher price tag. The density increase is in the urder of 400%. In the 14 in drive area, for example, units in excess of 0.5 Gbyte using five platters are on their way.

Optical starage -the: b~eak~ro~gh

The actuator moves the lens vertically under focus servo control to keep the beam of light faked on the ~e~~d~~g luyer of the disc, u%d in the rud~u~ direction zmnder ~ucki~ senro control to keep the beam centered in the pre~o~ed truck. The reflected light from the disc is now collected and made parallel by the objective lens and goes through the quarter wave plate again, rotating another 90”. Since the beam has lutzo hem rotated l&F, it is reflected by the po~u~~ng beam and splits into u~iti~ul optics which reluy the beam onto a quad photodetectors The signa from the quud detector is ampli~ed and produces feedback info~ution for the focus and tracking servos us well us the raw read channel data signal. The read channel dutu signal is then processed in a manner similar to that in magnetic disc drives to provide NRZ data and clock at the drive interfuc~~CtIOniC5 COVer

Opticaf disc drives present a complete departure from existing storage sys- tems technology. Shugart’s Optimem Division announced the f&t available optical system at the end of f983. The current 12 in, 1 Gbyte disc is a first step into a whole new Ievel of capaci- ties. A smaller disc will follow, prob- ably based on the 5 in digital audio disc platter, which will be more suited to the small end of the business microcomputer market. Capacities of optical discs are likely to increase as the technolagy matures.

Much of an office’s input still corns in paper form and the capacity of the opt&i disc allows the construc- tion of a computer database. Docu- ments can be optically scanned and

Front panet -------

This exploded kx.v shows the basic elements of tke @timem IOOO optical disc drive.

stored. If we assume that half a disc’s input comes from scanning and half from keyboard input the capacity is some 56000 pages of information. High-end optical drives are also approaching the market.

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Metal polymer

Optical media is both removeable and nonerasable. Removeability takes away the problems of backup associ- ated with high capacity Winchester storage and there are several advan- tage of nonerasability. Having a per- manent record gives, say, account- ants, an unassailable audit trail through the computer. How this will affect the legal acceptance of com- puter data evidence remains to be seen but it will be an interesting evolution over the next few years. Certainly, most computer frauds rely on the happy habit of computers deleting old transactions from discs to save storage space. With optical capa- cities, who needs to save space this way?

Substrate

When the laser beam generated by the optical disc drive contacts the surface of the media, a bubble 0.1 micron in diameter, or smaller, is formed. The bubble is the equivalent of a bit in a magnetic-based system.

problems currently associated with dynamic databases, updates, reboots when the drive goes down, access times to main computers etc., may be eliminated or at least relegated to a comparatively small, hyperactive seg- ment of the database content.

Optical discs can be reproduced System integration of optical disc

quickly and cheaply for the distribu- drives is no real problem from either

tion of software or localized database software or hardware points of view.

contents. Methods of holding infor- Using a controller with an intelligent

mation on distributed databases may host-level interface like SCSI, allows

well change dramatically and the easy integration of optical drives into

Bubble IS one micron or less

existing systems. The designer’s main problem is the second-stage one of software development to use write- once media instead of rewriteable magnetic discs. The file structures and access methods, for example, must handle updating without reloca- tion of individual records, and must be able to rapidly reassemble a file which has records scattered around the disc.

SCSI bus

ODI

The Optima 1000 optical disc drive is designed to operate with a controller incorporating the industy-standard Small Computer System Interface (SCSI). Use of an intelligent, host-level interface like SCSI enables designers to quickly integrate new peripherals, including optical disc drives, into new or existing systems. This diagram shows the basic functions of the controller included with the Uptimem disc drive.

Conclusion

As I stated at the beginning of the article, tec~ological advances are be- ing incorporated into new disc and disc drive products and there are several more being developed. The overriding factor in marketing new systems will, however, always be cost. All microcomputer markets are very cost conscious, sometimes to the ex- tent of neglecting cost/performance ratios.

Mi~computer markets, in con- trast, take cost/~rformance ratios ex- tremely seriously. If advances cannot be made with cost-effective design, manufacture and marketing, demand wiI1 not grow. Thus, the future for storage technologies lies in the adap- tion of new technologies in a cost- sensitive manner. cl

Shugart Corporation, AMD House, Golds- worth Road, Woking, Surrey GUZI 1 JT, UK.

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