IGNORE WIDE RESIDUE Message

Dave Cressman Dave.Cressman at quantum.com
Mon Nov 30 11:36:13 PST 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Dave Cressman <Dave.Cressman at quantum.com>
*
Hi,
IWR message is required for other cases (like parameter data returned on a
wide bus, although most implementations avoid this), but is particularly
needed for tape drives which support odd byte blocks.  This is because when
the host tries to read a block on tape, the blocksize is not known.  If the
host has SILI set in the CDB, and the block is smaller than requested, there
will be no indication to the host that the block was less than (vs. equal)
to the size requested, except by the # bytes transferred over the SCSI bus.
If the blocksize is odd (and SCSI bus is wide) the ONLY way for the drive to
indicate to the host the exact block size is by sending an IWR msg.  A
weirder case is when the host has written odd byte blocks and reads them
back in fixed block mode-multiple blocks per command.  Depending on hardware
support for odd byte blocks, the drive may have to "pause" after every block
and send an IWR msg. 

Since our customers test our drives for odd byte blocks I know that many, if
not most, systems which support wide bus tape drives, also support IWR
messages.

-Dave

David Cressman
Software Eng V
Quantum/ SSPG Division/ Advanced Tape Products group
Email: dave.cressman at quantum.com
Voice: 508-770-6877,  Fax: 508-770-5527



	-----Original Message-----
	From:	Norbert Lewalski [SMTP:norbert_lewalski at vortex.de]
	Sent:	Friday, November 27, 1998 2:50 AM
	To:	t10 at Symbios.COM
	Subject:	IGNORE WIDE RESIDUE Message

	* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted
by:
	* "Norbert Lewalski" <norbert_lewalski at vortex.de>
	*
	IGNORE WIDE RESIDUE message has been defined in the SCSI-2 
	specification and is also part of the SCSI-3 specification. As we 
	have never seen such a message being sent by a target we wonder if 
	anyone has ever made use of this message or will do so in the
future. 

	If wide transfer has been negotiated between initiator and target
and 
	transfer count is an odd number of bytes, then the message is not 
	required as both sides know that the high byte of the last word 
	transfered must be ingnored.

	If wide transfer has been negotiated and transfer count is an even 
	number of bytes, then why should the target disrupt the transfer at 
	an odd number of bytes?

	Regards,
	Norbert Lewalski



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	Norbert Lewalski                   Phone: +49-(0)7131-59720
	ICP vortex Computersysteme GmbH    Fax: +49-(0)7131-255063
	Falterstrasse 51-53                Email: n_lewalski at vortex.de
	74223 Flein                        WWW: http://www.icp-vortex.com  
	Germany
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