Just to underline Darrens statement, drive manufacturers usually qoute 
compression as 2:1 so that the total capacity of a tape is doubled, the 
compression you get is dependant on the data and the 2:1 is not a hard 
limit.
On LTO1 (100GB native) I saw a tape with between 600-700 GB on it. A SAP 
database with little in it!
Presumably there is an upper limit to compression possibly based on the 
size of buffer on the tape drive and the compression algorithm used.

On Fri, 28 Mar 2008 16:28:52 +0000, A Darren Dunham <ddunham AT TAOS DOT COM> 
wrote:

>On Fri, Mar 28, 2008 at 11:53:39AM -0400, MIchael Leone wrote:
>> EMC NetWorker discussion <NETWORKER AT LISTSERV.TEMPLE DOT EDU> wrote on
>> 03/27/2008 03:34:41 PM:
>>
>> > Depends on your data.   File systems usually have a large volume of
>> > already compressed files, or files that don't compress much (jpgs, 
mpg,
>> > mp3, etc).  In our windows environments, we get about 300-400 on LTO-2
>> > and 700-900 GB on LTO-3 tapes.
>>
>> You get 900GB on LTO-3? Isn't LTO-3 400G native, 800G compressed?
>
>It is if you assume that you get exactly 2:1 compression.  But as
>mentioned above, the actual compression you get depends on the actual
>data.
>

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