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Lance Roberts
Asked: 2015-03-05 12:03:17 +0800 CST

Erase old dump tape in Tru64 UNIX

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We backup our OLD Tru64 UNIX system (Digital UNIX 4.0F) to DDS3 tapes using the dump command. We want to recycle those tapes to make future backups on when they get too old. I've found the mt erase command won't erase them, probably because the dump command puts blocks on the tape and it's no longer raw.

What's a simple way to erase the tape?

unix

3 Answers

  1. Best Answer

    Usually with tape devices you don't have to do anything special to overwrite them - simply start writing again from the beginning of the tape, and whatever was already on there is gone.

    As a FAQ for one classic backup program notes, the problem is usually the reverse: most tape devices are so keen to rewind to BOT and start overwriting their payload, that considerable design and implementation effort is expended on preventing them from doing exactly that:

    When appending to a tape, there is the possibility that, between the time that Amanda positions to the last image (that already is not really trivial!), and opening the device for writing, a tape rewind happens, and in that case Amanda would happily erase ALL of the tape, containing possibly many days worth of backup.

    (AMANDA solved that problem by simply never appending to a tape - you write everything you're going to write to that tape in a single pass during which time the device is kept locked. Draconian and wasteful, yes, but that gives you some idea how easy it is to accidentally rewind and overwrite a tape!)

    I would advise against degaussing for anything except secure destruction (and I wouldn't rely on it for that, either). As Wikipedia notes:

    For certain forms of computer data storage, however, such as modern hard drives and some tape backup drives, degaussing renders the magnetic media completely unusable and damages the storage system. This is due to the devices having an infinitely variable read/write head positioning mechanism which relies on special servo control data (e.g. Gray Code) that is meant to be permanently embedded into the magnetic media.

    [...]

    The servo patterns are normally never overwritten by the device for any reason and are used to precisely position the read/write heads over data tracks on the media, to compensate for sudden jarring device movements, thermal expansion, or changes in orientation. Degaussing indiscriminately removes not only the stored data but also the servo control data, and without the servo data the device is no longer able to determine where data is to be read or written on the magnetic medium.

    though I freely admit I don't know whether DDS-3 is one such medium.

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  2. dd if=/dev/zero of=/dev/TAPEDEVICE bs=1024

    or you can probably use a similar tar command to create an archive on the tape device from /dev/null

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  3. Try getting hold of a degausser (you just put the tape on it switch on and in a few seconds it's erased). Saves you the time of loading tapes up just to delete them.

    I don't mean to be rude but I have to ask: Are you adminstering this system through a hole in time to 1995?

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