EXPERT RESPONSE
This is a highly disputed question. Let's start with the fact that tapes advertise numbers like 30,000 passes before a tape should be discarded. One use per week might sound like it translates into 576 years (30,000 passes divided by 52 weeks). However, a pass is not the same as a use. A pass is the movement of a given section of media across a section of media. If you are sending less data to the drive than it wants (e.g., sending 20 MBps to an 80 MBps drive), then you're shoeshining. The more you shoeshine, the more you move the same part of media across the media multiple times. If you could figure out how many times you're shoeshining a given section of media, you could divide 30,000 by that number and have your answer.
Unfortunately, you'll never get that number, so it's really up to you. I know one customer who uses a fresh piece of media every time he does a backup. In my opinion, that's a ridiculous waste of a company's money. I also know people that keep reusing their media without any tracking. That's quite risky. I like something in the middle. Based on the numbers above, I don't think anyone will reach the "real" life of their media on a regular basis, so I don't throw away media based on that. I just throw away media the second it gives me a write error -- zero tolerance.
As long as we're on the subject -- please don't resell your media. Most modern tape drives can't reuse media that's been degaussed, so ask any vendor that's claiming to degauss tapes before reselling them how that's supposed to work.
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