SCSI Copy (op code 18h)

Lohmeyer, John JLOHMEYE at cossymwest.co.symbios.com
Tue Oct 22 14:41:00 PDT 1996


* From the SCSI Reflector (scsi at symbios.com), posted by:
* "Lohmeyer, John" <JLOHMEYE at COSSYMWEST.CO.SYMBIOS.COM>
*

Howard Johnson wrote:
 ----------
>I am looking for a SCSI hard drive that supports the Copy command and   
have
>been unsuccessful.
>
>I have been told by one HD manuf. that no one supports this command.  If   
not
>does anybody know why?

At one time (about 10 years ago) most SCSI tape drives did support the   
COPY command and many customers asked for it (few actually used it,   
however).  On the surface, COPY makes a lot of sense.  You only need to   
move the data once over the bus to get it where it is going.

But when you dig deeper, COPY starts looking less desirable.  COPY only   
works on the same bus.  If you have more than one bus or devices on some   
other interface, then you have to do the copy function the old fashioned   
way (into CPU memory and back out to the other device).  As long as you   
have to write and test the code for the old fashioned way, you might as   
well use that code all the time --  no sense in writing and testing two   
sets of code.  (Besides, Ultra SCSI and Ultra2 SCSI reduce the transfer   
times on the bus.)

The disk drive vendors are having a hard enough time with the downward   
spiral of disk prices without supporting a feature that only a few   
customers might use.  Implementing COPY on a disk drive is more than just   
adding the additional microcode, it also involves supporting the   
initiator role.  Even if all of this involved no added costs, there would   
still be the testing and support costs.

The bottom line is the bottom line...

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



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