SAM-3 proposal: ... Data-In and Sense Data sizes

Elliott, Robert (Server Storage) Elliott at
Thu Oct 31 06:57:27 PST 2002

* From the T10 Reflector (t10 at, posted by:
* "Elliott, Robert (Server Storage)" <Elliott at>
> -----Original Message-----
> From: Ralph Weber [mailto:ralphoweber at] 
> Sent: Wednesday, October 30, 2002 9:31 PM
> To: t10 at
> Subject: Re: SAM-3 proposal: ... Data-In and Sense Data sizes
> * From the T10 Reflector (t10 at, posted by:
> * Ralph Weber <ralphoweber at>
> *
> Rob,
> The exposition below appears to be confusing implementation 
> with architecture.
> Adding a SENSE LENGTH parameter recommends (or maybe 
> requires) that SCSI Transport Protocols include equivalent 
> information in their Response IUs. 

They all do so to date.  

>                                    This may, in due course, 
> affect how HBAs are designed but from a SAM perspective that 
> is nowhere near as interesting as the requirements placed on 
> targets as a result of the necessary requirements in the SCSI 
> Transport Protocols.
> As for SENSE LENGTH options, the most straight forward option 
> appears to have been overlooked. SPC-3 should be modified to 
> require that when allocation length is greater than 8, at 
> least 8 bytes of Sense Data shall be returned.

There's no allocation length communicated to the target
in autosense.  

For REQUEST SENSE, there is this rule:
"If the allocation length is eighteen or greater, and a device 
server returns less than eighteen bytes of data, the application 
client should assume that the bytes not transferred would have 
been zeros had the evice server returned those bytes."

> The problems with the DATA-IN LENGTH proposition are similar
> to those with SENSE LENGTH. In this case, however, I take the 
> absence of suitable information in the SAS Response IU as an 
> indication that the current thinking in the industry that 
> provision of a DATA-IN LENGTH parameter is inappropriate.

A residual is not provided in SAS response IUs for the same
reason as parallel SCSI - the initiator is lockstepped with
the target and its count of data received should match the 
target's count of data transmitted.  We're not worried about 
losing entire data frames in a vast store-and-forward fabric.

Rob Elliott, elliott at
Industry Standard Server Storage Advanced Technology

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