Command linking through switches

Neil T. Wanamaker ntw at crossroads.com
Mon Mar 2 07:00:05 PST 1998


* 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
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>
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| Neil T. Wanamaker         Crossroads Systems, Inc.              |
| ntw at crossroads.com        			  		  |
|           Intelligent Connectivity for Storage Networks         |
| (512)794-2727                                                   |
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