©1999 IEEE VESIT Student Branch Hits :
Webauthor : Saumitra M Das
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TECHNOLOGY AT ITS SIMPLEST: HARD DISKS Hard disks have come a long way from 1957, when IBM introduced the first disks of 5 MB costing $35000 per annum on lease basis. The first major breakthrough in the hard disk market came with the introduction of the PC/AT by IBM with a 10 MB hard drive. Since then, there has been revolution after revolution in the secondary storage market, with the standard hard disk doubling in capacity every 18 months. The standard PC now comes with at least 1.2 GB, and even 2 GB raises no eyebrows. Even as disk space increases and physical disk sizes get smaller, seek time is falling drastically and data transfer rates are accelerating beyond the wildest dreams of developers.What is the techno behind this? A modern hard disk consists of several thin metal plates, called platters, coated with magnetic material, placed one on top of the other, around a spindle, with a drive arm for each platter to read and write data. Alongside, we have drive controller electronics that control the drive arm. The platters are usually made of aluminium, but highest performance drives are made of toughened glass. These platters revolve at speeds ranging from 3500 to 10000 rpm. However as disk rpm increases, the designer's problem multiplies, as both heat and friction increases. Head materials: Thin film heads: This is technology similar to that used in audio tape recorders. The head reads data off the disk by noting fluctuations in the magnetic field. But as the bits get more closely packed, the magnetic field becomes weaker and much more difficult to record, with a result that reading data becomes a problem. While this can be resolved by bringing the head nearer the disk , such a solution increases the risk of crashes. So, what's the solution? Magneto resistive heads(MR): Certain metals change their electrical resistance in the presence of a magnetic field. Fluctuations in the magnetic field around these metals produce fluctuations in their resistance. This property can be utilised to create the read element of the heads. The MR heads perform significantly better than thin film heads, especially, since the weakness of the magnetic field does not affect MR heads too adversely. But, this cannot be used to write to the disk and hence conventional thin film technology is used to create the write part of the head. An important factor in the quest for error-free performance of today's disks is head positioning. Consider your hard disk spinning at speeds of 7200-10000 rpm with the head positioned at a distance of a tenth of a hair's breadth away from it. Any inaccuracy in positioning the head at the right location at the right time would significantly degrade performance. Manufacturers now use a new system called embedded servo positioning to increase head positioning efficiency. In the older servo positioning method, all head positioning information was kept on a single platter used exclusively for that purpose. Obviously this brings about a lot of waste of drive space. So, now manufacturers "embed" the positioning information (called servo burst information) at particular positions on different tracks of the platters. When the head arrives at a particular track, it reads the signals from the servo bursts and, using this feedback, it readjusts itself until it is exactly over the centre of the track. Other cutting edge technology that are driving today's disks to higher levels of performance include PRML, command reordering, and data caching. PRML(Partial Response Maximum Likelihood): is one of the newest technologies to be incorporated into the hard disk. In conjunction with MR heads, PRML of drastically improves both capacity and performance of the hard disk. In normal disk drives, the analog signals reads by the head off the platter are converted into digital information by the drive electronics. To read a signal as 0 or 1, the head measures the strength of the signal and only the peak voltages are used. If the signal is less than particular voltage, it is rejected as background noise. But if bits get more closely packed together, the noise level increases and strength of the signal decreases. This makes it harder to increase data density beyond a certain point. So manufacturers have resorted to reading signals at a number of points and then determining the shape of the signal. This helps in reducing read-error rates and allowing increased data density. Command reordering: This is technique used to increase seek speeds. In a conventional command-queue system data requests are placed on a queue to the drive, and are executed one after the other. Consider a command requesting data from the innermost track.. The next command from the outermost one and the third command from the middle track. The head therefore will have to go all over the place. If the queue order could be changed so that commands that access data close to the head be given greater priority the performance and speed of the drive would increase. Data caching: Manufacturers have further improved performance by using cache memories. Tests conducted by Seagate and others have shown that 80-90% of the time if the computer asks for data from a particular location the next request for data will be from the succeeding location. All modern hard-disks have caches to "read-ahead " and thus improve efficiency. As I write this report, IBM has already come up with a hard disk that stores up to 11.6 GB per square inch. With these and other technologies the hard disk seems to be poised to take on the new millenium. -Rajaraman .P |
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©1999 IEEE VESIT Student Branch Hits :
Webauthor : Saumitra M Das |