DEC And Computer System Hardware Integrity
Entry posted by xxmikexx
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In the Backup thread j flanagan brought up the subject of computers manufactured by DEC a long time ago. I said I would open a new thread for that subject. This is that thread ...
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j flanagan,
It wasn't all sweetness and light regarding DEC computer system integrity. While our CPUs were very very reliable, our disk drives were not -- could not be -- comparably reliable. The affordable technology of the time simply did not permit it. The reason is that the hard drives of the time were prone to head crashes, something that is encountered today only very infrequently but was a constant concern during the 70s, which was the time frame of my employment at DEC.
The classical head crash happened as follows ...
At some point, for whatever reason a particle would become dislodged from the recording medium. If the particle came from a recording track then the parent sector would go bad. But more likely the particle came from the area between tracks so that the flanking sectors would remain good. However ...
Now we have a particle flying around inside the drive case because it was flung off the recording medium by the fast rotation of the drive. Usually such a particle would bounce around between the walls of the housing and the spinning recording medium, ending up settling at the bottom of the housing. Unfortunately, once in a while the dislodged particle would strike hard elsewhere on the medium, gouging it, and now we have two particles flying around. Most of the time the two particles will settle, but sometimes instead a cascade will build. When the cascade begins, death of the drive is only minutes away.
When a cascade gets going, sooner or later a particle is going to encounter one of the flying heads, trying to fit itself in between the head and the recording medium. If the size of the particle is roughly that of the head flying height the particle is quite likely to induce head flutter, and this flutter inevitably resulted in a literal head crash, with the head impacting the recording medium just like an airplane hitting the ground, with equally catastrophic results.
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So the wise customer had data on duplicate hard drives, and the hard drives themselves would be backed up to magtape. Trouble is, with magtape the heads wear and without constant finicky maintenance there can be no guarantee that an tape years or even months earlier will be readable on the drive that created it. Thus multiple tape drives were a good idea, and of course tapes had to go offsite in duplicate if one was to minimize risk of data loss. The head wear issue encouraged the three-generations of tapes backup philosophy of the day.
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Let the record show that webmaster Nels Anderson later worked at Prime Computer, who addressed the problem of system reliability by having a pair of computers for a single system, sharing memory, with one computer "shadowing" the other. The result of their very carefully designed system architecutre was that in general no data would be lost even if one of the machines went down.
But of course Nels cut his programming teeth on a high school computer donated by DEC, as so many other later industry professionals did. :)
Edited by xxmikexx
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