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Date:	Thu, 5 Oct 2006 13:12:22 +0200
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: Max Block Size in mt
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On Oct  3 19:33, Jeffry T Ross wrote:
> I'm trying to read data from a Sony AIT-e260 SCSI drive under Cygwin.
> Cygwin 1.5.21-2
> mt 2.3.1
> 
> I know that the tape is good, because I can read from it (using the same
> drive) under Redhat.
> 
> The first file on the tape is 96 bytes so I do the following:
> >> export TAPE=/dev/nst0
> >> mt setblk 0
> >> dd if=$TAPE of=header.txt
> 
> This works fine.
> 
> The second file on the tape is 1GB and is written with a block size on
> 200kB.
> When I do the following:
> >> mt setblk 204800
> >> dd if=$TAPE of=segment.0
> 
> I get the following error:
> mt: /dev/nst0: Invalid argument
> 
> I've found that the maximum block size in mt can only be 64kB.
> 
> Am I missing something or is there a limitation to mt under Cygwin?

It's a limitation in Windows.  The default settings only allow
blocksizes up to 64K.  There's some information on the net that this is
a result of the default size of the scatter/gather arrays in the SCSI
controller driver settings.  Search for the keyword "MaximumSGList",
which should give you lots of hits.

However, I have tried this on three different machines with three
different SCSI controllers, and setting the MaximumSGList value only
works on one machine with a Adaptec U160 controller, even though it's
documented to work for most, if not all controllers. 

At one point I started to write code using direct SCSI access using the
IOCTL_SCSI_PASSTHROUGH ioctl codes, but hit the same limitation.  I
could set the blocksize to a value > 64K, but when reading data I never
got more than the first 64K from the block, plus the error code
ERROR_MORE_DATA.  Since I couldn't find a workaround, I eventually gave
up on this :(

I guess the bottom line is if you need to create portable tapes, stick
to blocksizes up to 64K.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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