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Fixing Tape Errors

Long term retention

You usually get problems with old data which has been stored on tape for years. Before discussing the physical retention issues, how about starting with software issues? Say you have a requirement to keep a copy of application data for 7 years. You faithfully backup your application to tape, and six years later, you need to download it to check something. In the meantime, your application programs have changed, and so have your databases. You can get the data off tape no problem, but you can't read it in a meaningful way. The answer? Backup your data, and your database definitions, and your application programs, and anything else you might need, like database compression tables. Then, 5 years down the line you move to a different platform and the whole lot is useless. The point is that long term backups have to be thought through carefully. It is not enough just to get the data down to reliable tapes.

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Avoiding Tape Errors

OK, you've got a good application backup, but you're not sure about tape life. Your manufacturer quotes 10 years, but will EVERY tape last for 10 years? There are a number of reasons why tapes can go faulty. The obvious one is manufacturing errors, but also tapes will go progressively faulty due to chemical reactions between the tape surface, and water vapour and dust particles in the air. Fluctuating storage temperature speed these reactions up. Manufacturers recommend that tapes should be stored at a temperature of about 20 degrees Centigrade and relative humidity of 50%.

Keep your tape store clean. Its always a temptation to use free space in a tape store to hold other equipment. Avoid storing paper or frayed cardboard boxes with tapes, as these are a prime source of airborne dust.

Keep the drive heads clean. Its best to establish a cleaning routine of once per day, or even once per shift if the drive are busy. Automatic Tape silos will do this automatically

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Monitoring and detecting errors

Occasional read errors are usually corrected internally by the parity bits on the tape. These errors might be reported, either in an external error recording system like IBM's EREP, or internally on the tape. For example, the LTO Ultrium holds error information in the LTO-CM . The trick here is to intercept these errors, and use them to decide that a tape is deteriorating, and the data should be copied off it. You should also use EREP reports to identify faulty tape drives.

If you do not do this monitoring, then you might have a lot of old, faulty tapes around. Another avoidance technique is to check them out, and catch faulty tapes before they deteriorate too much. Software products that do this exist, for example FATS/FATAR from Innovation Software.

On final idea is to set up a schedule to copy tapes every 3 years or so. Backup products like DFHSM and Spectrum Protect (TSM) will do this for you, as they reclaim older tapes once the active data on them falls below a pre-defined threshold.

Fixing Tape Errors

If your tape has deteriorated so far that the oxide surface is peeling off it, then you are past the point of no return. However, if you can't read the data on a tape, and its critical, then it is worth trying the following

  • Store the tapes in a warm controlled environment for at least 24 hours to dry them.
  • Allow the tapes to cool back down to room temperature.
  • Clean and re tension the tape if possible.

An alternative is to try recovery software like Innovation's FATAR, which will try to clean the tape by moving the faulty spot backwards and forwards quickly under the drive head. If all else fails, FATAR will drop faulty blocks and strip off all the data it can read.

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