*BSD News Article 71737


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From: j@uriah.heep.sax.de (J Wunsch)
Newsgroups: comp.unix.bsd.freebsd.misc
Subject: Re: Cannot Read a SCSI Tape Written by an Unknown Machine
Date: 22 Jun 1996 12:53:50 GMT
Organization: Private BSD site, Dresden
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chapman@allmalt.cs.uwm.edu (Mark Chapman) wrote:

> I have a SCSI DAT tape that was written on an unknown machine type using
> tar.

The `unknown' is most likely an SGI.  (At least, that's the only
machine that is known by now to use such a big tape blocking.)

> tar: read error on /dev/rst0 : Input/output error
> st0: 128874-byte record too big
> ********************************

The problem has been discussed on the FreeBSD mailing lists, you might
be able to find the traces via the Web interface.  Basically, our
physio(9) [kernel-internal routine] limits all transfers to 64 KB,
i.e. it does split larger requests into chunks of at most 64 KB.  This
is primarily done to circumvent through problems with busmaster-DMA
SCSI controllers that can only handle 16 scatter/gather segments, so
no special handling is required to deal with the problems caused by a
VM architecture.  (16 segments == up to 16 non-adjacent physical pages
== 64 KB)

The sad news for you: it's impossible to read these tapes with FreeBSD
by now.

-- 
cheers, J"org

joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/ -- NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)