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No Compression on Compaq DLT VS 40/80 GB Tapes / Backup Exec 9.0

Posted on 2007-03-21
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No Compression on Compaq DLT VS 40/80 GB tapes with Backup Exec 9.0 Rev 4454
 - I have a single tape drive in a file server for backups of about 45GB presently.
 - I am having to use two tapes to get the complete backup
 - I have the setting in BU Exec set to Hardware compression, else software compression
   if hardware compression is unavailable.
I have accomplished this previously with no issues - but this is a configuration that was handed over to me from a previous admin's install.  There is another 8 tape Ultrium Autoloader that I had to reconfigure to maximize its function - so I suspect there may be an initial install problem with this tape backup unit as well.  I'd like to get the complete backup onto a single tape.  Any thoughts or ideas are welcom - Thank you!

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Question by:noelburke
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Lee W, MVP earned 250 total points
ID: 18764412
Typically, backup software doesn't provide compression if hardware compression is available.  And while I would think you could get 45 GB on a 40 GB native tape, it really depends on what you are backing up - typically, compression rates don't give you all that much extra (marketers LOVE to say you'll get 80 GB, but I've never seen anywhere near that capacity on a DLT tape - I've never seen anywhere near the compressed capacity of any tape.  Frankly - I tell people don't expect compression and if you get some, consider yourself lucky.

Remember, already compressed files (MPEG, JPEG, ZIP, and many other file types) are already compressed and cannot be compressed anymore.  In addition, trying to compress these files can slow the backup down and even make the files LARGER than they originally were.
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by:dovidmichel
dovidmichel earned 250 total points
ID: 18764845
hardware compression for the drive is enabled/disabled by the software.

A good first step is to overwrite a tape with a job that will fill the tape and then instead of spanning onto a second have it fail at that point. Then check the log to see exactly how much data was written to the tape. If the job wrote anything over the 40gb native capacity it is compressing, it may be a poor rate of compression but it is compressing.

The type of data will influence the compression ratio. Compressed files such as zip, jpg, mp3, etc.. will not compress at all. Binary files such as exe, dll, will compress very little.

It is also possible the drive is experiencing soft write errors. These are write failures that it recovered from so the job finishes as normal but for each write attempt space on tape is lost.

Next streaming. Streaming is were there is not a constant flow of data to the drive, so instead of the drive stopping and starting as data is available it just keeps going and pads the tape with empty space. The stop/start called shoe shinning is very hard on drives.  
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