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From: "Jonathan M. Bresler" <jmb>
Message-Id: <199608290016.RAA15559@freefall.freebsd.org>
Subject: Re: [Q]: formula for calculating BPI needed
To: moos@degnet.baynet.de
Date: Wed, 28 Aug 1996 17:16:52 -0700 (PDT)
Cc: questions@FreeBSD.ORG, freebsd-hackers@FreeBSD.ORG
In-Reply-To: <3224D59B.74DF@degnet.baynet.de> from "Darius Moos" at Aug 28, 96 11:26:19 pm
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Darius Moos wrote:
> 
> 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 ?

	trying to use bpi for a DAT tape ;)
	bpi is from the days of 9-track tapes.  use B and b instead.

	B -- number of dump records
	b -- number of kilobytes per dump record.

	for a 2GB tape try 500000 40

	dump Bbf 500000 40 /dev/rst0 <filesystem>

> 
> 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,

	the trick there is helical scan.  the bpi may acutally be
	61000.  that 61000 bpi are recorded at an angle of ~6
	degrees from horizontal.  the width of hte recorded path is
	very narrow.  many recording paths lie stacked above one another.
	similar to repeated slash symbols at a shallower angle "////"
	as a result the effective length of a tape is increased many-fold.

> 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.

	no, you dont ;)  well maybe you do but others dont want you to :)
	the formulas in dump are there for the old 9-track tapes.
	just use the newer parameters: B and b.

jmb
--
Jonathan M. Bresler           FreeBSD Postmaster             jmb@FreeBSD.ORG
FreeBSD--4.4BSD Unix for PC clones, source included. http://www.freebsd.org/
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