Another unquantified very small number.
You have a UPS, but the first time you discover it's not wired up
properly is when it fails.
This happened recently in a customer's office... The UPS power wiring
melted, leaving a big stinking hole, and all the servers went down -
quickly.
Another is misconfiguring the UPS daemon so it continues writing to
disk for 30 minutes until the battery runs down...
Like the chance of two disks going wrong in a RAID at the same
time... that would *never* happen would it?
*tip-toes out of the room*
-- Jamie
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