Re: iSCSI: problem with LUN discovery

[John Muth <muth@veritas.com>]

Nelson,

I don't disagree, disk serial numbers are a good thing. I was
simply trying to point out (perhaps badly) that initiator specific
LUN mapping presents no problem to the initiator side since nobody
in their right mind uses the path to determine the identity of
a disk.

Now, if we could only convince all of the disk manufacturers
in the world (both real and virtual) to give us unique serial numbers
(we've seen drive models were every unit returned the same constant
serial number. and don't get me started about arrays....)

John Muth
VERITAS Software

Nelson Nahum wrote:
> 
> John,
> 
> The problem is that the signature written by the VxVM is not standard and
> only is recognized by the Veritas VxVM.
> Due to the fact that the important thing is to identify the media not the
> controller (as Veritas does), the best way is to have the serial number
> written in the media in a non accessible sector, and loaded by the
> controller to one of the Inquiry pages of the LU. In this manner no matter
> which channel, WWN, LUN  the specific LUN is presented can be recognized by
> every initiator.
> 
> Nelson Nahum
> CTO
> StoreAge Networking Technologies.
> 
> -----Original Message-----
> From: John Muth [mailto:muth@veritas.com]
> Sent: Wednesday, October 04, 2000 4:51 PM
> To: ips@ece.cmu.edu
> Subject: Re: iSCSI: problem with LUN discovery
> 
> Julo,
> 
> FYI. Assuming the initiators are running higher level disk
> management S/W like VxVM (VERITAS Volume Manager), this won't
> be a problem. VxVM already deals with device paths moving
> around by doing it's disk identification based on the label,
> not the device path.
> 
> John Muth
> VERITAS Software
> 
> julian_satran@il.ibm.com wrote:
> >
> > JP,
> >
> > Unless I misunderstood you this is entirely a T10 issue.
> > Presenting a different LU map to each initiator as identified by the port
> > or the ACCESS-ID is intentional (each initiator can view a different set
> of
> > LUs)and part
> > of the protection mechanisms of SCSI.
> >
> > With iSCSI at later stages (when we start handling discovery and
> > management)
> > the pain of getting to a specific volume might be eased.
> >
> > Julo
> >
> > Raghavendra Rao <jpr@divyaroot.India.Sun.COM> on 27/09/2000 23:05:22
> >
> > Please respond to Raghavendra Rao <jpr@divyaroot.India.Sun.COM>
> >
> > To:   Julian Satran/Haifa/IBM@IBMIL
> > cc:
> > Subject:  iSCSI: problem with LUN discovery
> >
> > Julian,
> >
> > Do you agree that following is a problem ? If so, can we fix it ?
> >
> >   SAM doesn't prohibit a target device from presenting initiator specific
> >   values for LUs. So it is believed that similar values for a LU may not
> >   necessarily address the same LU inside a target when accessed from
> > different
> >   initiators.
> >
> >   So the vendor ID page 83 of INQUIRY command is considered to be unique
> >   for a LU - There is one caveat to this though, that the page 83 values
> >   could optionally be target port specific. Anyway, if the hosts were to
> >   use a more persistent values such as page-83 identifier, say after
> reboot
> >   or after reconfiguration, it first needs to translate this page-83
> > identifier
> >   into a LUN. This can be done by first issuing a REPORT_LUN command to
> >   LUN 0 and issuing an INQUIRY page-83 command to each LU in the list
> >   obtained from REPORT_LUN response until a successful match is found.
> >   An initiator could cache the mapping for future speedier lookups, but
> >   it still has to linearly probe out all LUs to get the page-83 unique
> >   identifiers.
> >
> >   I hate to do this static probing; This problem can be fixed by either
> >
> >      a) mandating that LUN values are persistent and same for all
> >         initiators (more unlikely to be accepted)
> >
> >   OR
> >
> >      b) introducing a new page code for INQUIRY in which the initiator
> >         will pass down the page-83 identifier in the parameter list
> >         to LUN 0 and the target will respond with the LUN corresponding
> >         to this identifier; Of course, this has to go to T10 for approval.
> >
> > Thanks.
> > -JP