Command linking through switches

Witalka, Jerome J Jerome.Witalka at unisys.com
Mon Mar 2 07:27:45 PST 1998


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* "Witalka, Jerome J" <Jerome.Witalka at UNISYS.com>
*
The Unisys 2200 still uses command linking as described by Neil below.
While I agree that SAM and the original SCSI architecture did not
guarantee non interruptible ordering, I am not aware of any
implementation of command linking that did not preserve ordering.  I
recognize at this state in the game, it is much too late to change the
ANSI definition of how command linking works, I do believe there is a
place for a function that provides an implicit reservation of a device
through a small chain of commands.  
This function is provided by BMC, SBCON and IPI and I would be delighted
to find the equivalent implemented in a future edition of SCSI.  

 I believe that it will become increasingly useful to other companies as
networked storage and data sharing between multiple hosts become more
prevalent. As Neil pointed out, it is more efficient than
Reserve/Release since it requires two fewer commands and error recovery
is simpler since we don't have to deal with failures on the Reserve and
Release commands as well as the Linked commands.

Jerry Witalka     Unisys Corporation
jwitalka at unisys.com    612-635-7958


> -----Original Message-----
> From:	Neil T. Wanamaker [SMTP:ntw at crossroads.com]
> Sent:	Monday, March 02, 1998 9:00 AM
> To:	Bob Snively
> Cc:	t10 at Symbios.COM
> Subject:	Re: Command linking through switches
> 
> * From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
> * "Neil T. Wanamaker" <ntw at crossroads.com>
> *
> As the original guilty party: the motivation behind the Unisys driver
> was to
> provide indivisible read-before-write operations (the application on
> the
> 1100/2200 usually operates with a virtual disk with a different
> blocksize
> than is physically on the disk); note that this was implemented in the
> days
> of SCSI-I, when there was no standardized reserve-release mechanism.
> Even in
> today's environment, linking provides lower overhead and locks out
> other
> initiators for a shorter period than does reserve-release.
> 
> At 09:51 PM 3/1/98 -0800, you wrote:
> >* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted
> by:
> >* Bob Snively <Bob.Snively at Eng.Sun.COM>
> >*
> >
> >Ken,
> >
> >I am hoping that command linking through a switch will operate
> exactly
> >like command linking on a direct loop.  Each of the sequences in the
> >exchange is a separate Information Unit which is transmitted,
> received,
> >and managed in the proper sequence by the end-points (target and
> initiator)
> >only.
> >
> >If you need it, the architecture supports it.  However, the
> historical
> >usage that I am aware of (reducing selection overhead for firmware
> managed
> >selection) is not relevant any longer.  Most of the other potential
> >uses have also been over-taken by improved DMA and target
> architectures,
> >so the requirements for it are even weaker.
> >
> >Be aware that in multi-initiator environments, ordering relative to
> >commands outside the linked set is not mandatory, so it cannot be
> depended on
> >as a replacement for proper driver design.  This is stated implicitly
> >and explicitly by various SAM chapters.
> >
> >For all these reasons, I would be delighted to see it die from
> disuse.
> >
> >
> >
> >--------------------------------------------------------------------
> >Bob Snively			       Phone:	   (650) 786-6694
> >Sun Microsystems Computer Company      
> >Mail Stop UMPK 12-204
> >901 San Antonio Road		       E-mail:   bob.snively at sun.com
> >Palo Alto, CA 94303
> >--------------------------------------------------------------------
> >
> >
> >*
> >* For T10 Reflector information, send a message with
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> >
> >
> -------------------------------------------------------------------
> | Neil T. Wanamaker         Crossroads Systems, Inc.              |
> | ntw at crossroads.com        			  		  |
> |           Intelligent Connectivity for Storage Networks         |
> | (512)794-2727                                                   |
> | (512)349-0304 FAX    Web URL - http://www.crossroads.com        |
> -------------------------------------------------------------------
> 
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