SAM definition of SCSI Device

Neil T. Wanamaker ntw at crossroads.com
Mon May 24 14:30:50 PDT 1999


* From the T10 Reflector (t10 at symbios.com), posted by:
* "Neil T. Wanamaker" <ntw at crossroads.com>
*
Some of this difference in perspective results from the fact that some of
the T10 membership comes from a small-systems world, where SCSI devices
began as single devices attached to PC HBAs, while some of us come from the
mainframe world, where multiple redundant paths to a logical unit are the
expectation. 

Some protocols, like IPI and ESCON, were designed with multipathing in
mind, and so incorporated facilities like path groups, dynamic reconnect,
etc., along with managing things like sense data on a LUN, rather than
path, basis. 

This large-system model strives to duplicate all parts of the path upward
|from the HBA - separate drive interfaces, separate controllers, separate
channels, separate I/O processors on the host - so as to eliminate single
points of failure upwards from the HBA  or tape head. 

This model also presupposes that all the paths are checked; not only that
the data paths are protected, but that the engines translating LBAs to
physical addresses, and the engines doing error recovery processes are also
protected. 

If SCSI is to migrate upwards into a data center environment, we need to
add the constructs to facilitate these capabilities, not just the names and
terminology. I would suggest the constructs from IPI-3 or ESCON as a
starting point.


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| Neil T. Wanamaker         Crossroads Systems, Inc.              |
| ntw at crossroads.com        			  		                      	 |    
| (512)794-2727                                                   |
| (512)349-0304 FAX    Web URL - http://www.crossroads.com        |
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