Summary and comparison of alternative multiple port management proposals

[Bob Snively]

To:		X3T10

From:		Bob Snively

Date:		November 22, 1994

Subject:	Alternative multiple port implementations

To assist in selecting the set of functions to be provided for 
multiple port operation, the following summary of the two
proposals is provided.  Whichever combination of mechanisms
is selected, it is important that the implementation be robust,
simple, and sufficient.  Elegance is best demonstrated by
simplicity, not complexity.

Please review this summary and the referenced documents and
provide feedback about the best solution to the management
of multiple port devices.



1)	DUAL PORT IMPLEMENTATION

The present revision of SPC defines two functions required for dual port
operation.  Identification of ports is simple, since the only possible
ports are "this one" and "not this one".

  A)	Dual Port Option

	Section 5.3 defines the dual port option.  It states that
	the INQUIRY command shall have  DualP bit to indicate that
	dual ports are accepted.  The dual port has separate isolated
	paths and the service delivery function may be separated as
	viewed from the two ports.

	A SAM function to reset other ports is not yet defined.

  B)	PORT STATUS command

	Section 7.13 defines a 6-byte command that performs the following 
	functions.  The architecture assumes that there are only
	two ports on the target.

	a)	Disables opposite port

	b)	Enables opposite port

	c)	Form logical unit reservation to port issuing command,
		releasing any reservations on the opposite port.

	d)	Determine status of any reservations that may
		exist on the target device, including the SCSI device
		identifier of the initiator holding the reservation
		and identification of the port to which the 
		reserving initiator is attached.


2)	MULTIPLE PORT IMPLEMENTATION

The present revision of document X3T10/94-233 defines two functions
required for multiple port operation.  The number of ports is not
limited. The mechanism for defining the ports is outside the scope
of the standards, but the following methods may be used.

	Targets can be identified by their serial number or, if
	supported by the SCSI protocol, their world-wide name.
	Initiators communicate by methods outside the protocol to
	compare

	Initiators can be identifed by their world-wide name.
	For SIP devices which have only two ports, targets identify
	initiators as being  those installed on this port or those
	installed on the other port.

	All initiators bear the same relationship to a target, whether
	they are one of several initiators on the same port or 
	on another port.

  A)	ABORT TASK SET, OTHER INITIATOR

	This is a task management function that resets all tasks
	from the specified initiator.  This allows a host that
	is taking over from a failing host to clear all those target
	resources that had been under control of the failing host.
	This function is proposed to be added to SAM, section 7.
	The method for identifying the initiator is protocol dependent
	and outside the scope of SAM.

  B)	Priority Reserve

	This bit is added to the RESERVE(6) and RESERVE(10) commands
	to force the release of any conflicting reservations and the
	formation of the specified type of reservation to the issuing
	initiator.  




3)	COMPARISON


	Table of features for two approaches to multiple port management



Capability		DUAL PORT		MULTIPLE PORT


Number of ports		     2			  any number

Resource recovery	    tbd			  ATSOI

Priority Reserve	 logical unit		 any type

Initiator ID		  SIP compatible	 depends on protocol