Hi Geoff,
        The most likely reason is that some data sets are more compressible 
than others. If you have one server or backup job that is mostly Picture, music 
or movie files and another that has plain text or DB files, the tape with the 
Pictures will have less data on it because the JPG file format is 
"pre-compressed" at the file level. This means that the tape drive will not be 
able to compress the data further and will "reduce" the amount of data you can 
get on that tape. This is not (necessarily) true of a DB backup and the drive 
is normally able to compress this data significantly, which gives you more data 
on tape. 
        

Mark Pinder : 
Systems Engineer:
Spectra Logic : 

Message: 7
Date: Tue, 28 Feb 2006 09:36:19 -0800 (PST)
From: Geoff Hazel <geoff_hazel AT yahoo DOT com>
To: veritas-bu AT mailman.eng.auburn DOT edu
Subject: [Veritas-bu] Why don't tapes hold the same amount of data?

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We have all the same tapes in the robot library, but when I look at the "full" 
tapes I see a wide difference in the amount of data they have:
   
  A00085  HCART    TLD      0       16     -       3     151715893        
FULL
A00088  HCART    TLD      0       7      -       1     255478199        
FULL
A00089  HCART    TLD      0       10     -       1     343680501        
FULL
A00090  HCART    TLD      0       13     -       3     315433835        
FULL

  Why do some tapes hold twice as much data as other tapes?