On Thu, Apr 21, 2005 at 10:49:00AM -0400, Vicki Stanfield wrote:
> 
> If the dump is 28GB, should it not fit on a 20GB tape if it is 
> compressed. Aren't the tapes basically 20/40GB?

Not in my mind.  They are 20GB tapes.
I.e. they can hold 20GB of ones and zero bits.

What those ones and zeros represent "could be"
40GB of data compressed ton 20GB of zeros and ones.
I.e. 50% compression.

But the level of compression varies greatly with
the type of data being compressed and the algorithm
being used.  In fact, the compressor used by your
tape drive can actually expand some types of data
rather than compressing it.

Using gzip as the software compressor I've tested data and
gotten 0, 3, 10, 25, 50, 75, even 90+ percent compression.
If the data in your dump compresses 50%, fine, it will fit
on a 20GB tape.  But if it only compresses 3%, I doubt it.
And if it was already compressed by gzip (or has lots of
gzipped files), it WILL EXPAND if the tapedrive compressor
works on it.

-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
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