From:	SSDEVO::SHEN "14-Mar-1996 1904 -0700" 14-MAR-1996 21:16:35.85
To:	@[.DIS]BUSINESS_PARTNERS
CC:	MARKS,SHEN
Subj:	HSOF Implementation of Multi-SCSI bus failover

Attached is the summary of HSOF implementation on Multi-bus Failover. Next
msg will contain the actual Specification.

Diana
--------------------------

From:	SSDEVO::MARKS "Storage BU - Array Controllers DTN 522-2718  11-Mar-1996 1136 -0700" 11-MAR-1996 11:36:21.59
To:	SHEN
CC:	MARKS
Subj:	Summary of Multi-Bus Failover 

HSOF SCSI Multi-Bus Failover Summary					03/09/96
--------------------------------------------------------------------------------
* Contrasted to Transparent Failover

  Multi-Bus Failover is an alternative functionality to Transparent failover,
  which has been supported on HSZ20/40 controllers for some time.

* Basic Functionality

  Failover of LUNs to the surviving controller is automatic.  No host 
  intervention is required for LUNs to appear on the surviving controller.  
  A SCSI unit attention alerts the host to the failover event.  The host 
  must begin using the alternate path for failed over LUNs. LUNs do not 
  automatically failback to the returning controller. The host receives 
  a vendor-uniqe unit attention which indicates that a particular LUN 
  is eligible for failback, but the host must actually force the failback
  of individual LUNs.

* Configuration

  Each controller in the pair is connected by a separate SCSI bus to the host.
  Also, each controller must be configured to respond to the same set of
  target IDs.  Individual LUNs have a preference to a particular controller.
  The preference comes into play only when both controllers boot at the same
  time, in which case the controller preference determines which controller
  will service the LUNs.  Otherwise, the host controls which controller
  services a particular LUN while both controllers are up and running.