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To: "David Langford" <langfod@dihelix.com>
Cc: arman@bico.co.id, freebsd-hackers@freebsd.org, freebsd-scsi@freebsd.org
Subject: Re: DLT exchanger 
Reply-To: Jason Thorpe <thorpej@nas.nasa.gov>
From: Jason Thorpe <thorpej@nas.nasa.gov>
Date: Thu, 27 Mar 1997 13:16:18 -0800
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On Thu, 27 Mar 1997 10:29:59 -1000 (HST) 
 "David Langford" <langfod@dihelix.com> wrote:

 > The driver works well on the cheapo 4mm Archive/Seagate drive.
 > The thing that took me awhile to realize was that you had to be very
 > explicit with chio. You cannot tell it to move a tape into the drive
 > and expect it to return the one it already has.

...right.  This was a concious design decision, which I'll rationalize
here:

	- the driver cannot reasonably keep the state of the
	  changer apparatus; many changers also have front
	  panel controls which may be used at any time
	  by a human operator.

	- in the case of multiple free slots, who is to make
	  the decision where the in-drive tape should be
	  returned to?  This decision is very hard to make,
	  esp. if a human operator has mucked with the
	  front panel controls.

	- the amount of state one has to keep around gets
	  larger as you add more drives and pickers into
	  the picture.  (I've used this with a multi-drive,
	  multi-picker apparatus under NetBSD.)

The chio(1) program is meant to be a simple interface for use
by scripts/command line.

Jason R. Thorpe                                       thorpej@nas.nasa.gov
NASA Ames Research Center                               Home: 408.866.1912
NAS: M/S 258-6                                          Work: 415.604.0935
Moffett Field, CA 94035                                Pager: 415.428.6939