Larry Osterman's WebLog

Confessions of an Old Fogey
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A bedtime story :)

A bedtime story :)

  • Comments 17

Once upon a time (all the best stories start with "Once upon a time"), there was a computer manufacturer, let's call it by a totally random collection of letters, let's say "JCN".

JCN had a brand new computer that it was producing (called the QD-BU).  One of the really cool things about this computer was that the hard disks on it were HUGE.  Twice the capacity of any PC available at the time.  This machine came with a 20 MEGABYTE hard disk!

Oh, and did I tell you it was FAST?  It was so fast, you could move the heads around on the hard disk in only 10 milliseconds!

JCN was quite proud of their new computer, and they made sure that it was rigorously tested.  They made sure that every component in the computer was of the highest quality, even the hard disks.  It was especially important that the hard disks work well, because with all that disk space, people would store more and more data on the hard disk.

After months and months of testing, JCN decided that their computer was ready to fledge its wings and take flight.  They launched it with MUCH fanfare, and it was very well received.

 

Unfortunately, they realized soon after the launch that there was a problem.  Users started reporting that hard drives in the QD-BU were starting to act "badly".  They would corrupt data seemingly at random.  JCN was NOT happy at hearing this, after all, they had spent a lot of time and money ensuring that the QD-BU would be better than any other computer available.

The problem was that the manufacturer of the hard drives couldn't produce them in quantity.  Their quality suffered when producing drives in quantity.

JCN worked to fix the problem with the manufacturer, and eventually everyone was happy.

 

Of course, JCN tried very hard to learn from the lessons of the QD-BU.  And their key takeaway?  Not that they needed to be careful about which vendor they chose to make their drives.

Instead the lesson they learned was "Fast hard drives break".  So for their next generation of computers (the QT/3), the hard drives were much slower.  They took 85 milliseconds to move the heads, but they were MUCH more reliable than the old QD-BU drives.

 

Of course this is only a bedtime story.  It has no relationship to the real world whatsoever.

  • Hi Norman. Your right. However, in the early PC environment, the distinguishment between Tracks and Cylinders (at least in the general software field) was not very disciplined. In the benchmark quoted (software), note that it says "Track to Track Seek Time" - emphasis on the Seek. Head to Head Switching time (track to track as your suggesting) was rarely ever quoted, much less tested. I mean I can’t think of an easy way to derive a stable, let alone moderately accurate result, given the capabilities of the hardware. The benchmark has to explicitly request a sector, in which case (as you’ve mentioned), sector interleave will play an ugly role. As for average seek, I recall that it was popular to perform this over one third of the disk surface. Ie 640Cyls / 3 = ~213 Cyls (if memory serves, the ST4038 has 733 Cyls, 5 Hds). As a metric, that is over simplifying things. Eg When the heads need to seek over larger distances, the timings/voltages are not a sum of a single (Cylinder) seek, it’s less (optimised). Indeed you could hear the differences, as the distances increased. Sorry to hijack your thread, Larry…
  • Tuesday, October 03, 2006 12:01 AM by RichardRudek > However, in the early PC environment, the distinguishment > between Tracks and Cylinders (at least in the general > software field) was not very disciplined. Um, aside from "general software field" I understand what you're saying. While the general software field was still dedicated to goals like making Y2K into a problem (due more to management than to programmers of course), the PC software field was dedicated to creating other future monsters. An inability to distinguish tracks from cylinders isn't a monster and I didn't even know it played a role on PC software development, but anyway, thank you for the reminder ^_^ By the way I remember head crashes on IBM disk drives too, which were half the size of a washing machine and didn't hold anywhere near 733 cylinders, but had more than 5 heads. In those days people tended to believe the manufacturers' specs on cylinder-to-cylinder seek and track-to-track seek etc. (well at least children tended to believe them ^_^) Anyway the important thing was that keeping things in the same cylinder tended to keep things faster.
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