[5153] in linux-scsi channel archive
Re: Linux as a SCSI _Target_ device?
daemon@ATHENA.MIT.EDU (Matthew Jacob)
Mon Nov 16 21:47:08 1998
Date: Mon, 16 Nov 1998 09:32:10 -0800 (PST)
From: Matthew Jacob <mjacob@feral.com>
Reply-To: mjacob@feral.com
To: "Dr. Michael Weller" <eowmob@exp-math.uni-essen.de>
cc: Jeff Noxon <jeff@planetfall.com>,
Gerard Roudier <groudier@club-internet.fr>,
linux-scsi@vger.rutgers.edu
In-Reply-To: <Pine.A32.3.95.981116124235.50020F-100000@werner.exp-math.uni-essen.de>
Yes- you're right-
There's another way to do all of this without target mode on both sides.
Leave one or more RECEIVE DIAGNOSTIC RESULT commands pending on the target
with an infinite timeout. When the 'target' wishes to send a message, it
just completes a RECEIVE DIAGNOSTIC RESULT command. The method for
divvying up a flat buffer returned into useful information is left as an
exercise.
It's not a kludge. It's called active receiver mode.
On Mon, 16 Nov 1998, Dr. Michael Weller wrote:
> 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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