Ignore Wide Residue message

Lohmeyer, John JLOHMEYE at cossymwest.co.symbios.com
Fri Sep 20 12:26:00 PDT 1996

* From the SCSI Reflector (scsi at symbios.com), posted by:

Brian Berg wrote:
>Also, the process of determing how many bytes were written to a tape   
>say, would not be possible.


I agree with most of what you say and also encourage vendors to implement   
the IGNORE WIDE RESIDUE message.  However, this message is not necessary   
when reading variable length blocks from Sequential Access Devices.   
 There is a separate mechanism that uses the Incorrect Length Indicator   
(ILI) and a residue in the Information field to report such blocks.  Use   
of transport mechanisms for determining total bytes transferred is   
discouraged (because the target might leave the pointers not pointing at   
the end of the data transfer if, for example, MODIFY DATA POINTER is   
used).  From SCSI-2:

      NOTE 32 Since the data pointer value may be modified by the target   
      the I/O process ends, it should not be used to test for actual   
      length because it is not reliable.

The IWR message is most useful when the target needs to disconnect in the   
middle of a large transfer and an odd number of bytes have been   
transferred so far.  (Of course, the target could save the last byte to   
be transferred first on the next connection....)

John Lohmeyer                 E-Mail: john.lohmeyer at symbios.com
Symbios Logic Inc.             Voice: 719-533-7560
4420 ArrowsWest Dr.              Fax: 719-533-7036
Colo Spgs, CO 80907-3444    SCSI BBS: 719-533-7950 300--14400 baud

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