SAM-2 Revision 11

Ericson, George gericson at clariion.com
Fri Sep 10 08:05:50 PDT 1999


* From the T10 Reflector (t10 at t10.org), posted by:
* "Ericson, George" <gericson at clariion.com>
*
Ralph,

Here are my positions on the following topics:

On Device Server is associated with Zero or More Logical Units:
---------------------------------------------------------------

Both the Task Manager and the Device Server are Processing Elements.
Examples of Processing Elements are silicon based state machines, microcode,
an OS process, or an OS thread.  Consider a RAID Storage Subsystem that
presents some number of front-end Logical Units.  Suppose that the Subsystem
has a static pool of threads dedicated to executing Logical Unit Tasks.
Each of these threads would be a Device Server.  Some of these Device
Servers may not be assigned to a Logical Unit.  Alternatively, some Logical
Units may be served by the same Device Server.


On associations among Device Servers:
-------------------------------------

I agree with you, "the relationship of one Device Server to another is
outside the scope of the model."   In the model, Tasks are delivered to a
Logical Unit.  The associated Device Server executes those Tasks in an order
consistent with the Task ordering rules.  


On Logical Units Sharing Target Manager: 
----------------------------------------

As currently spec'd, a Target Manager implements a bridge function.  It
accepts a Task from a Service Delivery Port of a Target and, (through the
Target's Bus ports,) forwards it (or broadcasts it) to another Service
Delivery Port.  That Service Delivery Port may be attached to either a
Target or a Logical Unit Device.

The model allows other Target Devices to share the same Bus with Logical
Units.  The consequence of this is that multiple Tasks delivered to a
Logical Unit may be delivered via multiple Targets and therefore, multiple
Task Managers.

Proposal:  The current model can be better described with a fairly simple
adjustment. 
	1) Recast the current Target Device as a Bridge Device 
		with a superclass of Target Device.
	2) Recast the Target Manager as a Bridge Device Server
		associated with the Bridge Device
	2) The Bus Identifier address space is part of a SCSI Domain 
		associated with the Bridge Device.
	3) Make Logical Unit a subclass of the new Target Device.
	4) Logical Unit Numbers, Target Identifiers, and 
		Initiator Idenitifiers attached to a bus are 
		part of a single SCSI Domain associated with the Bus.

This adjustment preserves the current model, but describes it better.  (Note
that functions like Target Reset if delivered directly to a Logical Unit is
directly tranlatable as a Logical Unit Reset.)


On one Task Set for all Logical Units (TST=010b):
-------------------------------------------------

I'm not familiar with this one.  Can't find it in various revisions of SAM
and SPC.  Send me a reference.

Not sure how useful this feature would be.  However, in the current model,
the feature could be implemented relative to a Target by adding the new Task
Queue there.  In this case, the Task Manager would be responsible for
forwarding Tasks to other Devices in a manner that would preserve the
ordering rules.  The Task Manager would also be responsible for handling
Task Queue management (Clear/Abort Task) functions in a manner which
preserves consistency between the new Task Queue and the Task Queues in the
underlying Devices.

This feature would be very confusing in a subsystem having multiple Targets
with access to the same set of Logical Units. 


Regards, 
George Ericson                            <http://www.clariion.com/>
CLARiiON Advanced Storage Division     Data General Corp 
4 Coslin Drive, MS C44                         TL: (508) 480-7349 
Southboro, Ma. 01772                           FX: (508) 480-7913


-----Original Message-----
From: Ralph Weber [mailto:ralphoweber at CompuServe.COM]
Sent: Thursday, September 09, 1999 9:48 PM
To: Ericson, George; Milligan, Gene; T10, Reflector
Subject: Re: SAM-2 Revision 11


* From the T10 Reflector (t10 at t10.org), posted by:
* Ralph Weber <ralphoweber at compuserve.com>
*
George,

I feel like my comments here are splitting hairs, but since I'm
mailing this, I guess it's not going to stop me.

} This would imply Device Server is associated with zero or
} more Logical Units.  I prefer the latter.

I can't see how a Device Server can be associated with zero
Logical Units.  (Okay, it's a nit; but I warned you.)

Also, I tend to think of the association being among Device
Servers (of which there is one for each Logical Unit).
I.e., there is one unique Device Server for each Logical Unit
and how they interact with each other is the association that
is beyond the scope of the SCSI Architecture Model.

I think my hang-up on this comes from the fact that the object
shared by the Logical Units in the model is the Task Manager.
For modeling purposes, it's more elegant: Device Server
non-shared and Task Manager shared.

Incidentally, one reason the Task Manager is shared by all
Logical Units is that early definitions of Task Set boundaries
included rules that had, for example, one Task Set for all Logical
Units.  With the model setup as it is, that Task Set definition
could be reinstated (TST=010b) should someone want it.

Thanks.

Ralph...


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