[5149] in linux-scsi channel archive
Re: Linux as a SCSI _Target_ device?
daemon@ATHENA.MIT.EDU (Dr. Michael Weller)
Mon Nov 16 12:15:02 1998
Date: Mon, 16 Nov 1998 12:52:27 +0100 (MEZ)
From: "Dr. Michael Weller" <eowmob@exp-math.uni-essen.de>
To: Jeff Noxon <jeff@planetfall.com>
Cc: Gerard Roudier <groudier@club-internet.fr>, linux-scsi@vger.rutgers.edu
In-Reply-To: <19981115230640.C4383@planetfall.com>
On Sun, 15 Nov 1998, Jeff Noxon wrote:
> On Sat, Nov 14, 1998 at 09:27:21PM +0100, Gerard Roudier wrote:
> > YES!
> > I read you have been told about some SCSI project for Linux that
> > implements SCSI target mode. It seems guys wanted to implement kind of
> > network or cluster link. For that, just implementing the PROCESSOR device
> > should have been enough. You should have a look at this code, I think.
>
> Are you talking about IP over SCSI? They did not use target mode.
> If I recall correctly they used a SEND MESSAGE command or similar. I
> need to look back at that project and find a copy of the SCSI spec.
Although SCSI specifies a SEND MESSAGE command (for an SCSI network card
or similar; well is there any one outside actually?) host A would need to
send a SEND MESSAGE command to host B, hence host B is target of the scsi
command and must be in 'target mode' then.
Target mode here merely only means that the host can accept data from the
SCSI bus which it did not explicitly ask for. Either A or B (probably
both) must be able to do that in order for communication between them.
The only scenario to connect two hosts over scsi w/o target mode I can
imagine is with a third SCSI target device to which 'A' sends his message
and 'B' polls on occasion: 'Do you have a message for me from 'A''. Well,
actually 'B' could issue a read to this target for the message, the target
disconnects then reconnects and transmits later if a message actually
arrived (thus no 'active poll' required.)
I'm not aware that the IP-SCSI project uses a device like this. A way out
with standard hardware would be a SCSI disk where both host communicate by
scribling over the same disk blocks. Hmm, I don't think this is anything
else than a bad bad kludge.
Michael.
--
Michael Weller: eowmob@exp-math.uni-essen.de, eowmob@ms.exp-math.uni-essen.de,
or even mat42b@spi.power.uni-essen.de. If you encounter an eowmob account on
any machine in the net, it's very likely it's me.
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