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Date: Wed, 28 Aug 1996 23:26:19 +0000
From: Darius Moos <moos@degnet.baynet.de>
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Subject: [Q]: formula for calculating BPI needed
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Hi,

My first basic question is, how one would calculate the BPI
for a given DAT-tape-length and capacity. My problem is:
  - let a DAT-tape be 295 feet long <=> lf = 295
    => length of this DAT-tape is li = lf * 12 = 3540 inches
  - let the capacity of this tape be 2 GB
  => BPI = 2 GB / li = 2 GB / 3540 inches = 606633.8 BPI
  The maual i've read says the BPI is 61000 for such a tape.
So what am i doing wrong ?

i was trying to dump filesystems to a SONY-SDT-7000 with dump.
My problem is, that dumps estimated tapes for the dump are wrong;
for example dumping /dev/sd0a (about 50 MB) to a 295 feet long
DAT-tape makes dump to estimate 0.4 tapes (i set the commandlineoptions
to "0ufds /dev/sd0a 61000 295"). This can not be correct,
since the capacity of a 295 feet-DAT-tape is about 2 GB.
After scanning the sources of dump for the estmated-tapes-formula,
i found some strange constants being used there that i do not
understand and that are not documented nor explained anywhere.
I want to rewrite the formula for this calculation in
the dump-sources but i need some informations on this topic.
Maybe there is someone out there willing to give me some assistence
in understand what is going on in the dump-formula.

Many thanks in advance for all your help.

Bye, Darius.

email: moos@degnet.baynet.de