I don’t remember and I don’t have access to the numbers any more. Our system was massive. Because it was massive, we designed it to minimize cost and minimize human effort required to run it. Errors were mostly invisible unless they required human intervention—

—which happened often enough. Tape stuck in a drive, operator has to go into the tape library to fish it out. You have to turn the robots off first. Individual tape errors were invisible because we used erasure codes. We would have to see, like, four correlated failures in order to lose data.

I did the math and we were comfortable with that level of redundancy, based on our data for how often tapes fail. When I say I did the math, I mean I personally did the math, but I don’t have the figures handy.

If the error is 1 in 1,000 it's likely never to be found. How often is data retrieved from tape?