Capacity of a compressed tape is dependant on the data being backed up and
the compression algorithm (which can be different on different drives),
with LTO-1 we typically get about 2:1 compression on fileservers and 4:1 on
database type data, with one database I backed up 600GB on one 100G tape
when the database had just been set up it later fell to the 400G region.
Conversly on a standalone DLT8000 I have seen it report compression of
0.9:1!
You are not doing anything wrong the drive manufacturers give the native
capacity and a compressed capacity but they can only assume that any
particular back up will achieve this.


On Mon, 24 Nov 2003 10:40:39 -0500, Matt Temple
<mht AT RESEARCH.DFCI.HARVARD DOT EDU> wrote

>
>We've been backing up with compression in various forms for a decade.
>I can tell you from our experience that 50 % - 60 % has been most
>typical of our compression ratio.   I don't think I actually said that
>clearly.   I think you should expect to get 150 to 160 gb on an AIT-3
>tape.  On rare occasions, 170 GB.. I've NEVER seen any real-world
>compression that looked like
>those AIT-3 claims.   I'd be most interested in knowing if anyone has.
>Then I'd know that we're doing something wrong.
>Our data is largely binary image data from microscopes, and scanning
>devices of various kinds.   Our network is gigabit backboned with
>a number of gigabit-capable computers.   Our Networker server generally
>claims to be running at about 11-12K KB per Sec  per drive on a good day
>which is fast enough to keep the drives moving, isnt it?
>
>

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