RE: LUN field in R2T

Sounds like whatever the intent of the field was that the spec might be
somewhat ambiguous.   I'll err on the side of caution and make sure the
LUN field is copied from the R2T into the Data-Outs.   Personally in
neither our initiator or target implementations do I even look at that
LUN field in the R2T or Data-Out since the ITT and TTT are sufficient
for tracking the IO.  However I can conceed that some target
implementations may benefit from having the LUN field in the Data-Out.

 

Bill

 

________________________________

From: [email protected] [mailto:[email protected]] 
Sent: Friday, January 04, 2008 10:21 AM
To: Paul Koning
Cc: [email protected]; [email protected]; Sears, Bill
Subject: RE: [Ips] LUN field in R2T

 


Paul, 

Your comments make sense to me.   The initiator  *should* copy the lun
over from the R2T.    From a practical standpoint I don't think that  a
target should rely on the initiator copying that data, since the field
is, in fact, called "LUN" in the data out message. 

Further, I'm not sure that 10.7.4 is clear on this point:   

"If the Target Transfer Tag is
  provided, then the LUN field MUST hold a valid value and be
  consistent with whatever was specified with the command;" 

One could argue that 'command' is generally used in the context of a
SCSI command, not an R2T.  To reiterate - It's reasonable to say that an
initiator should copy the data, but I don't think that a target can
depend on that behavior. 

regards, 

david 

say that the

David J Cuddihy
Principal Engineer
ATTO Technology, Inc.


www.attotech.com <http://www.attotech.com/> 
Power Behind the Storage 



Paul Koning <[email protected]> wrote on 01/04/2008 09:53:59 AM:

> >>>>> "Julian" == Julian Satran <[email protected]> writes:
> 
>  Julian> Bill, This is left to the implementer. I would argue that a
>  Julian> "liberal" initiator should store the LUN-TTT fields as they
>  Julian> come and not fill them up.
> 
> That doesn't sound right to me.
> 
> Something can only be left to the implementer if any of the allowed
> choices interoperate.  And that is not the case here.
> 
> If the target treats the LUN field as an additional identifier along
> with the TTT, NOT necessarily the same as the LUN number used in the
> original I/O request -- which is what section 10.8.5 clearly permits
> -- then the only interoperable initiator behavior is to copy the TTT
> AND LUN from the R2T to the Data Out message.  If it uses the original
> request's LUN instead, it won't interoperate with a target that picks
> its own value for the R2T.
> 
> So there are two possible answers:
> 
> 1. A target can put anything it wants in the R2T LUN field, and the
>    initiator must use that value from the R2T in its Data Out, or
> 
> 2. A target must use the original request LUN in the R2T, and the
>    initiator can either copy the LUN field from the R2T, or use the
>    LUN it used in the request (since they are the same) when building
>    its Data Out.
> 
> It seems pretty clear to me that the current spec answer is #1.  And
> the underlying problem is that the field was called "LUN" rather than
> "64 bit cookie" or something like that.
> 
>  >>  In a nutshell all I want to know is this: Is it legal for an
>  >> initiator to set the LUN field in the Data-Out PDU based on what
>  >> it knows the LUN to be, or MUST the initiator copy the contents of
>  >> the LUN field from the received R2T into the Data Out.
>  >> 
>  >> Example: An initiator negotiates for MaxOutstandingR2T==4.  The
>  >> initiator sends a write command to a target.  The initiator
>  >> receives an R2T and starts sending multiple Data-Out PDUs.  The
>  >> initiator receives 3 more R2Ts but cannot start sending Data-Out
>  >> PDUs immediately due to outbound resource limitations.  The
>  >> initiator must now store off each R2T request so that it can honor
>  >> them when resources become available ? otherwise the initiator
>  >> will need to halt receive processing on the connection.  The
>  >> initiator will probably need to store the Target Transfer Tag,
>  >> Buffer Offset, and Desired Data Transfer Length for each R2T.
>  >> Does the initiator need to store the LUN from each R2T as well?
>  >> Or can it just fill in the LUN in the Data Out PDUs based on
>  >> knowing what LUN the command was sent to in the first
>  >> place?
> 
> 
> 
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