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Tapes not reporting actual capacity

 
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Advisor

Tapes not reporting actual capacity

Has anyone experiences that their tapes don't display the formatted capacity when the protection expires?
Have a full library of newly formatted 320GB tapes. Specified the size when preparing, but they're reporting full after five days with only 140-160GB available.

We're running an MSL5000 with 160/320 drives, hardware compression is enabled.
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Respected Contributor

Re: Tapes not reporting actual capacity

Are you by any chance using software compression as well, or encrypting the data before it's sent to the library?

Compression is a funny thing; a particular file will only compress how it will compress -- and despite the common marketing '2:1' assumption, most customers I talk to tell me that their average is more like 1.3:1; come customers see almost no compression, it just depends on the data.

Things that don't compress well (or actually take up more space compressed due to metadata) can include mpg, wmv, jpg, encrypted data, and truly random data. Things that compress well will have lots of patterns -- text, databases with zeroed-out areas, possibly raw photographs.

If you encrypt or compress your data on the server, it will become random data, and the tape drive will store no more than 160GB of data (less, perhaps, because of the overhead of the ineffective compression attempt's metadata).

A native-160GB tape, by the way, will always write 160GB of bits. If the data stream is highly compressible, those 160GB of bits might represent 320GB of data on the sending server.

There is an option in the newer versions of HP's Library and Tape Tools that lets you measure the compressibility of your data. It might be worthwhile to use that to see if there is something unusual about your data, or wrong with your tape drive. You can also run a complete set of diagnostics on an HP tape drive through L&TT. www.hp.com/go/tape, look for it under 'value add software'.
--
Liberty breeds responsibility; Government breeds dependence