Changes to SDA Model for multiple controllers

Gerald Fredin gfredin at
Fri May 13 11:27:43 PDT 1994

        The following is a list of operations needed to manage multiple 
        controllers in an SDA, and how they could be accomplished within the 
        current model.  If it is not possible (or at least not apparent how) to 
        perform an operation, I have suggested an addition or modification to 
        the model.
        I have assumed that there is a C-LUI type of Controller.
        1.  The initiator must be able to determine that there is more than one 
        path to an SDA.  This can be accomplished by first scanning with Inquiry 
        commands and locating all SDA device types.  For all SDA device types, 
        the Unit Serial Number Vital Product Data Page should be requested.  If 
        identical serial numbers are returned, these identify paths to the same 
        2.  The initiator needs to know for each path, which controller C-LUI is 
        being accessed.  By using the Report C-LUI Service, the initiator can 
        determine what controllers the SDA contains, but there is no mechanism 
        to determine which controller is being accessed.  A "report current 
        connection" or similar service to identify the addressed C-LUI is 
        3.  If the SDA is able to take advantage of the redundant controllers 
        without any host intervention, then the capability is needed to specify 
        and/or report this sparing capability.  This is available through the 
        Create/Modify or Report P-LUI/C-LUI  Spare Service.
        4.  The model needs to account for implementations that:
             a.  Only allow access through one controller to all volumes.
                 The other controller is a hot standby.
             b.  Each volume has a dedicated access path through one of the
                 controllers.  If one controller fails, the other controllers
                 can be instructed to provide an access path to the volumes
                 which are no longer accessible.      
             c.  Allow access to any volume through any controller concurrently. 
        By providing the ability to attach volumes to the controller C-LUIs, any 
        of these implementations can be accomplished.  The model currently 
        allows attachment of C-LUIs, P-LUIs, and R-LUIs to C-LUIs, but does not 
        address the need to attach a V-LUI to a C-LUI.
        5.  A service is needed which would enable and disable the controllers.  
        This is similar to the enable/disable port in the dual port model.  If 
        the model allows for operations to other types of C-LUIs (turning LEDs 
        on/off, resetting power supply fault indicator, etc.) then the same 
        mechanism could be used for controller C-LUIs to provide this service.

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