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:
* "Lohmeyer, John" <JLOHMEYE at COSSYMWEST.CO.SYMBIOS.COM>
*
Brian Berg wrote:
----------
>Also, the process of determing how many bytes were written to a tape
record,
>say, would not be possible.
Brian,
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
before
the I/O process ends, it should not be used to test for actual
transfer
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
--
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
*
* For SCSI Reflector information, send a message with
* 'info scsi' (no quotes) in the message body to majordomo at symbios.com
More information about the T10
mailing list