SCSI Copy (op code 18h)

Skip Jones sk_jones at qlc.com
Tue Oct 22 15:28:46 PDT 1996


* From the SCSI Reflector (scsi at symbios.com), posted by:
* "Skip Jones" <sk_jones at qlc.com>
*
Howard,

There are two key reasons I sight for lack of COPY command support:

1) Its hard to do right.  Copy quickly becomes a rat's nest of issues, bugs,
problems, and basically a firmware nightmare, especially related to error
recovery should a problem occur during the copy.  For the most part, disk
drives are at their best when being a target and except for the most basic
functionality, asking them to become initiators is asking for trouble.

3) No market demand. Most system designers are less than crazy about the idea
of having peripherals write to each other with limited-to-no host involvement. 
Actually, in the mid-'80s, some drives did make some effort at implementing
COPY, primarily because Wang Corp. demanded it in their purchase
specificcation.  We all had  "Wang Code" as a special version of firmware used
to try closing Wang.  It was during those early days that most drive suppliers
realized the can of worms that COPY quickly surfaces.  Since system software
toying with the notion of COPY (such as Wang) intended to use it for data
back-up/archiving onto a tape drive, us disk folks soon learned how much we
didn't understand tape drives!  However, since it is arguably easier to be an
initiator for a disk drive than a tape drive, and since system houses such as
Wang specified COPY as a requirement on both tape and disk drives, it turned
out that COPY was embraced by the tape folks.  Having come to our rescue, most
of us disk folks gleefully left COPY commands up to our self-masochistic tape
friends!

I believe that you will still find COPY supported on many tape drives.


Best Regards,

Skip Jones
Director, Planning and Technology
QLogic Corp.
_______________________________________________________________________________
From: Howard Johnson on Tue, Oct 22, 1996 1:49 PM
Subject: SCSI Copy (op code 18h)
To: 'scsi at symbios.com'

* From the SCSI Reflector (scsi at symbios.com), posted by:
* Howard Johnson <hjohnson at tacsys.com>
*
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?

Howard Johnson
R&D
TAC Systems

*
* For SCSI Reflector information, send a message with
* 'info scsi' (no quotes) in the message body to majordomo at symbios.com

*
* 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