IP Storage: RE: LUN field in R2T

Thanks for getting back on this. I think I still need a little clarification.

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?

Thanks,

Bill

From: Julian Satran [mailto:Julian_Satran@xxxxxxxxxx]
Sent: Thursday, January 03, 2008 4:12 AM
To: Sears, Bill
Cc: ips@xxxxxxxx
Subject: Re: LUN field in R2T

Your interpretation is more liberal than the text says. The reason for requiring a LUN field in the first place was to remove the requirement for a target-wide unique TTT and enable distributed targets that do not require coordination. The reason LUN was chosen to tag the R2T is that it may simplify routing of the data to the appropriate destination.
But since it is called LUN some zealous initiator may check (although it is not mandated) the LUN validity versus a list obtained otherwise - so I would not go for a very liberal interpretation.

Julo

Sears_Bill@xxxxxxx

01/02/08 11:57 PM

To	<ips@xxxxxxxx>
cc
Subject	LUN field in R2T

Just a quick clarification:

10.8.5. Target Transfer Tag
[...]The Target Transfer Tag and LUN are copied in the outgoing data PDUs and are only used by the target.

Does this mean that the LUN field in the R2T does not need to contain the actual LUN number specified by the SCSI write? Since the field is “only used by the target” I assume this text means that the target may place anything it wants in the LUN field of the R2T and have it echoed back by the initiator.
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