attached MC Op Codes

Bob Snively Bob.Snively at Eng.Sun.COM
Wed Dec 14 22:19:59 PST 1994

John Lohmeyer writes:

>Actually, that isn't quite what I had in mind.  In fact, I did not expect to 
>see drivers written to deal with both attached and non-attached medium 
>changers, but I do not want to preclude such drivers if they are indeed 
>The intent of the committee decision to support attached medium changers was 
>to simplify life for low-end changer applications.  The SCSI-2 medium 
>changer model uses a separate LU (perhaps on a separate SCSI device) from 
>the one or more LUs that read and write the medium.  This permits an 
>elaborate functionality with multiple robotics and more than one read/write 
>device per changer.  But there is no defined way for an initiator to 
>discover (through the interface) the association of the separate LUs.  This 
>association is changer and/or system specific.
>The intent of the attached medium changer model was to permit simple changer 
>devices to share the same LU with the read/write device.  This solves the 
>association problem and enables some simple changer products (e.g., a device 
>with 6 pieces of media and a single CDROM reader).  These products can share 
>a single SCSI protocol chip and a single processor between the changer 
>function and the player function.  They will happen (have happened?) whether 
>or not we choose to document them.
>The problem we were attempting to solve is the association problem, not the 
>inability of some systems to cope with nonzero LUNs.  Of course, this 
>solution also helps with that problem.
>A second problem that was being addressed was the need for a separate medium 
>changer driver.  The attached medium changer model was simple enough to 
>imbed the changer control in the device driver.  This is why I did not 
>expect common driver software.

I am tempted to agree with Mr. Ridder's view.  It seems to me that
in a very sophisticated system with separate targets for 
the many attached changers and drives, the association problem
may exist.  That clearly requires a special driver for proper

For a single target using LUN addressing, the association
problem does not exist, since each element of the system has the
same target address.  This has a lot in common with the RAID model
and certainly does not require combining motion operations with
media access operations in the same LUN.  If tapes would like to use a simple
stacker as is presently defined,  I have no strong objection.  
I propose, however, that the generic SCSI approach
be to use separate LUNs and that that approach be used for CD-ROMs,
those tapes that need greater functionality, and all future

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