FCP_CONF/CRN

Douglas Hagerman Douglas.Hagerman at digital.com
Mon Jul 27 13:04:20 PDT 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Douglas Hagerman <Douglas.Hagerman at digital.com>
*
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I agree with Dave that Command Reference Numbers should be on a target
basis. There are two ways to look at this--target or LUN--and I think
that the target approach is better. It seems to me that the way you look
at the purpose of the CRN determines which way you want to go.

A basic feature of Fibre Channel is that Exchanges are completely
independent. There is no inherent ordering of Exchanges, so you need a
way to keep track of which one is which (even in the case of a fabric
that is ordered in the FLA sense).

One way of doing this is to use a CRN at the SCSI level to keep the
order of the SCSI commands under control on a per-LUN (I-T-L nexus)
basis. There is some merit in this, particularly in multiple-path
situations. However, multiple-path situations are not defined in SAM or
anywhere else in the T10/T11 standards.

Another way to do this is to use the CRN at the FC-2 level to keep the
order of the Exchanges under control on a port-to-port basis. This
allows the port-to-port communication to mimic the communication you get
with a parallel SCSI bus, which makes all the upper layers work properly
without any further worrying.

Perhaps we need to break this down into two functions, one at the SCSI
level (the CRN) and one at the FC-2 level. In this case I would offer
that  the second of these functions be called the "Exchange Index". It
would be reset at N_Port login time, and would wrap with a minimum
period greater than R_A_TOV.

Doug.

> -----Original Message-----
> From: David A. Peterson [mailto:dap at nsco.network.com]
> Sent: Monday, July 27, 1998 2:47 PM
> To: fc at nsco.network.com; t10 at Symbios.COM
> Subject: Re: FCP_CONF/CRN
> 
> 
> * From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
> * "David A. Peterson" <dap at nsco.network.com>
> *
> 
> 
> Bob Snively wrote:
> 
> >
> >   7)  Commands with a CRN of zero shall not participate in 
> the ordering and
> >   verification procedures specified in 5.
> >
> >   Any command, including such initialization commands as 
> INQUIRY, TEST UNIT
> >   READY, and MODE SENSE/SELECT may always use a CRN of zero 
> if the state of the
> >   ECRN bit is not known or if execution ordering is not 
> required for that
> >   command.
> 
> I have no real objections to the above other than I am not 
> fond of a transportlevel
> concept finding its way into a SCSI mode page. This should be 
> a FC/FCP port
> characteristic only (i.e. target based).
> It is unfortunate that it is deemed neccessary to be lun 
> based today . Hopefully
> disk drive vendors will see
> the benefits of CRN (especially in a class 3 environment) in 
> the future.
> 
> 

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