Path: utzoo!attcan!uunet!munnari.oz.au!cluster!metro!bunyip!moondance!uqcspe!qfagus!cxbne!phil
From: phil@cxbne.cx.oz (Phil Chadwick)
Newsgroups: comp.sys.mips
Subject: Re: QIC-120 tapes with dump (was Re: /proc and other questions)
Summary: dump does not switch tapes on physical EOT
Message-ID: <20520@cxbne.cx.oz>
Date: 5 Feb 90 04:23:49 GMT
References: <35264@mips.mips.COM> <1529@uakari.primate.wisc.edu>
Organization: Comperex Pty. Ltd., Brisbane, Australia
Lines: 35

In article <1529@uakari.primate.wisc.edu>, bin@primate.wisc.edu (Brain in Neutral) writes:
>  
> To John: I've found that that the dump program will sense the end of the
> tape and ask you to switch properly, but will not correctly estimate the
> amount of tape it's going to use, if the parameters are off.  Question: do
> you find on large filesystems that the number of tapes that dump estimates
> that it will use it accurate?
> 
> I've found that 570/25000 will produce estimates that are slightly too low.
> E.g., dump estimates that it will use .98 tapes, and actually uses slightly
> more than 1 tape (it *does* sense end of tape and request the next).

Dump works by figuring out how many blocks it can fit on a tape -
i.e. given tape length, density and block size, dump calculates how
many blocks it can write on the tape, then proceeds to count the
blocks as it writes them.  When the count is exhausted, the tape
is switched.

If there is insufficient tape to write the calculated number of
blocks, dump reports a "write error" and asks if you want to restart
the tape (and will do so indefinitely, as long as the tape supplied
is "too short").

If dump initially estimates that it requires 0.98 tapes and actually
uses one and a bit, then dump has simply underestimated the number of
blocks required to be written to tape.  This is quite common, and I
seem to recall that the BSD manual for dump(8) mentions the likelyhood
of an underestimate.


Phil Chadwick.  Comperex, Brisbane.

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