johnl@ima.ISC.COM (John R. Levine) (08/15/87)
In article <5557@prls.UUCP> weaver@prls.UUCP (Michael Gordon Weaver) writes: >... If higher transfer >rates were required by a large part of the market, the rotation >speed of the drives could be increased from the current typical >3600 rpm to say 10,000 rpm. This would be expensive, but much >cheaper than using a vacuum. Aw shucks. The IBM 650, which was announced in July 1953, had as its main memory a drum that was 4" in diameter and rotated at 12,500 RPM. Since the drum was the main memory, access time was critical, so the fast rotation rate allowed access to any 10-digit word in 2.4 ms on average, less if you arranged your program around the drum to minimize rotational delay. Certainly we can do that well now. In any event, the speed of sound issue becomes less important as the size of the platters shrink. The first commercial disk, the IBM 350, had platters two feet across that rotated at 1200 RPM. The disk had 50 platters, and stored in total 5 MB. Seek time was on average 600 MS. To get seek time down by an order of magnitude, disk designers shrank the disk size by an order of magnitude, from 24" to as little as 3" these days. As noted, the limit on transfer rate currently is more likely to be the bus speed of the host computer than the speed of the disks. Even mainframe channels rarely run above 4MB/sec. If your computer had the I/O speed, there are all sorts of straightforward tricks to speed up the disk data rate. For example, you could run all of the disk heads at once so that a disk cylinder rather than appearing as N tracks each M bytes long looks like one track N*M bytes long, but with the same time to read or write a track. By the way, my reference for the ancient history is "IBM's Early Computers," by Bashe et al., published by MIT Press. -- John R. Levine, Cambridge MA, +1 617 492 3869 { ihnp4 | decvax | cbosgd | harvard | yale }!ima!johnl, Levine@YALE.something The Iran-Contra affair: None of this would have happened if Ronald Reagan were still alive.