I'm not sure I got your question right, but...

>  I found out with some experimentation that
>TSM would take twice as long to backup a 60 GB compressed file as
>a 60 GB uncompressed file.

- A LTO2 tape drive can write data at 40Mbyte/s. 

- A LTO2 tape drive can do hardware compression. Assume it does 2to1.

If you send a 400Mbyte uncompressible file (already compressed) it will write 
the data in 10 seconds. 
40MB/s on the fibre/scsi , 40MB/s to the tape and 400Mbyte goes to tape.
        
If you send a 400Mbyte 2to1-compressible file it will write it in 5 seconds. 
80MB/s on the fibre/scsi , 40MB/s to the tape and 200Mbyte goes to tape.

It is normal that a compressed file of the SAME SIZE as an uncompressed file 
takes twice as long to backup.

/Anders Räntilä

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RÄNKAB - Räntilä Konsult AB      
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Email: Anders AT Rantila DOT com
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