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Andrew Joyce <joyce@××××××××××××.ca> wrote: |
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> If you consider the case where each drive alone has a a 10% chance |
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> of failing, two drives simultaneously failing should have a 1% |
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> chance of happening. |
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|
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This calculation assumes that the failure of one drive does not give |
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any information about the possible failure of the second drive. That |
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is not the case. If conditions were such that one drive failed, then |
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you should become less confident about your other drives. |
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|
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For example, I have seen an entire room of Sun workstations have their |
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hard drives go bad at once. Sun's explanation was that the bearings |
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in these drives were packed with the wrong lubricant. These |
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improperly lubricated bearings wore out and failed at a more or less |
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constant rate. |
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|
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On another occasion, a friend of mine bought two hard drives, same |
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make and model, with the plan of using the second drive for backup. |
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Both failed at the same time. Two of the same model drive, probably |
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made at the same place and time, and installed into the same |
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computer -- if you think about it, it wasn't a good idea. Whatever |
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would make one drive fail would likely cause the other to fail. |
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|
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(To put that in jargon: you have to multiply by the _conditional_ |
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probability that the second drive will fail, contingent upon the |
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failure of the first drive.) |
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|
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-- |
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Barry.Schwartz@×××.org |
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"I'm a war president!" |
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http://www.winternet.com/~trashman/Bush_or_Pooch.html |
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http://www.winternet.com/~trashman/Music.html |
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|
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-- |
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