[stgt] Why LUN0?

[Braun, David]

I took a quick look at the code (not the RFC) and it I don't see
anything special about LUN0 being the only LUN that can respond to
REPORT_LUNS. Any LUN would work (assuming scsi_cmd_perform(...) is where
all the action is). Wouldn't this mean that one would have to ensure to
create at least a LUN0 (assuming the REPORT_LUNS isn't a LUN-less
command (I'm SCSI-ignorant)). My suspicion is that the REPORT_LUNS
command is a LUN-less command and would be responded to by the bus
controller in the non-internet implementation of SCSI.

Off to peruse the RFC.

Dave

-----Original Message-----
From: ronnie sahlberg [mailto:ronniesahlberg at gmail.com] 
Sent: Wednesday, November 28, 2012 11:34 PM
To: Braun, David
Cc: stgt at vger.kernel.org
Subject: Re: Why LUN0?

REPORTLUNS needs this. You must present a LUN 0 that can respond to
REPORTLUNS so that an initiator can discover which LUNs are available.

I think this is part of SAM for SCSI-3.


Without a LUN:0   your initiator might either not be able to access
any LUNs at all on your target,
or if you are lucky/unlucky (I dont really know which,  both options are
highly undesireable)  it might fall back to old style discovery and spin
for many many minutes in a "try to talk to each LUN from LUN 0 to LUN
<huge number> one at a time to see which ones exist"
everytime you reboot and the host needs to re-scan the bus.



regards
ronnie sahlberg

On Wed, Nov 28, 2012 at 5:53 PM, Braun, David <David.Braun at drs.com>
wrote:
> I'm trying to understand the need for LUN0. Is this required by the 
> iSCSI standard (RFC-3720) or is it an artifact of the implementation? 
> As a test I modified a copy of tgtd and tgtadm to allow the "-force"
> argument to the logicalunit delete function and it seems to work. BUT 
> I must confess I'm ignorant of what the ramifications could be. Could 
> someone explain the need for LUN0 or why I shouldn't be too surprised 
> when this hack blows up in my face?
>
> Thanks
>
> Dave
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