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Capacity per Tape Varies Widely - HP Ultrium 6

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I am running Retrospect 12.5 server with 20 clients on a Mac Mini running Yosemite server. The tape drive is a HP Ultrium 6 LTO drive. The amount of storage I get out of one tape is very inconsistent. I might only get 2.5TB on one tape and the next might get close to 5 or 6TBs. Same backup script, same media set, hardware compression is on. The data being backed up is a company fileserver and is not different enough to cause that dramatic a difference.


My current daily backup media set has two members in it. The first tape was full pretty close to 2.5TB, the second tape is still going and the media set has 8TB of data in it - 5.5TB on the second tape.


Any ideas on what may cause this? Is there a setting I am missing somewhere or something physically on the tape or drive that needs to be done?


Thank you.


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Here is some information:



Key facts from the FAQ:

Retrospect writes to a tape until its full, regardless of claimed capacity.

Some file compresses well, other not at all.

If data isn't provided to the tape drive fast enough, there will be blank blocks on the tape.

If there is a write error, it will show in log.


So, when the first tape (2.5 TB) was written, was the server (very) busy doing other things than running Retrospect?

Is it always the first tape that has (much) lower capacity?


Have you tried a cleaning tape?

One more thing: Most tape vendors provide a tape utility program for testing the tape drive. Have you tried that?

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I doubt the server was very busy when the backups are done since they are done after hours. Nothing in the logs to indicate there were problems while writing. It is not always the first tape - sometimes the first tape does well and the second one barely gets the 2.5TB. I have not tried a cleaning tape yet as the drive is only a couple of months old. I will look for a utility program.


Thanks for your reply.

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