--On Monday, February 24, 2003 10:03:53 -0800 John Oliver <joliver AT john-oliver 
DOT net> wrote:

On Fri, Feb 21, 2003 at 09:10:35PM -0500, Jon LaBadie wrote:

On Fri, Feb 21, 2003 at 03:49:32PM -0800, John Oliver wrote:
> On Fri, Feb 21, 2003 at 06:14:37PM -0500, Gene Heskett wrote:
>
> > Throw in that marketing is usually a bit optimistic in saying its a
> > 20 gigger without compression, and that always needs a fudge factor
> > when actually estimating, and it likely this will happen.
>
> But fudging by 100%?  I don't buy that... :-)

You don't have to.  Gene was only talking about a few percent.

No... if my tape is theoretically capable of 20GB uncompressed and 40GB
compressed, and after compression amanda can only fit 20GB on it, that
would hypothetically demonstrate a 10GB un-compressed capacity.  Or,
half of what it's actually doing.  I do not believe Quantum sells a
10/20 tape drive as a 20/40  I'm sure there *is* some fudging going on,
but not, like I said, 100%.  I'm apparently "loosing" about half of the
capacity of my tapes, and I'm puzzled why I'm the only one who sees a
problem with that... :-)

The only capacity number that matters on a tape is the raw 'uncompressed'
capacity.  The 'compressed' numbers are just a marketing ploy to make the
tape capacity seem higher.  Yes, some data will compress nicely, giving
you the appearance of writing 40 or even 60 GB to that tape, but you are
still only writing 20 GB of ones and zeroes to the tape.
  If you are backing up filesystems with mostly uncompressible data, then
the amount of disk space used will approximate the amount of tape used,
or possibly be even larger on tape if you try to compress already compressed
data. I have one 5.4GB directory that is 5.3GB compressed, but a 6.2GB
directory I have only uses 2.6GB compressed.  Depending on your data you
could get anywhere from 20 to 100 GB on that same tape, with the odds
much greater of being near the low end of that range rather than the high
end.

Frank



--
Frank Smith                                             fsmith AT hoovers DOT 
com
Systems Administrator                                  Voice: 512-374-4673
Hoover's Online                                          Fax: 512-374-4501