Global identifiers for initiators

Ralph Weber -- VMS -- ZKO3-4/U14 weber at
Thu Dec 29 14:09:53 PST 1994

When considering the question of global identifiers for initiators, one 
should think about the question, "What does an initiator global identifier 
represent?"  Stated in postal-service terms, "Does the global identifier 
represent the house or the people living there?"

For a SCSI target device (disk, tape, etc.), the house and the people living 
there are pretty much synonymous.  In the case of a non-removable medium, the 
two are synonymous.  The contents of the media cannot change except as the 
result of a transaction directed to the disk's global address (or a serogate 

Initiators, on the other hand, can have one family move out and a whole new 
family move in more or less on a whim.  It's done by shutting down operating 
system one and booting operating system two.  A shutdown and reboot of the 
same operating system has many of the same properties.  So, does the global 
identifier represent the running operating system and all its running applica- 
tions?  Or, does the global identifier represent the box that the operating 
system is running in?

My experience with VMS Clusters suggests that the latter type of representa- 
tion is not very useful.  But, the former case is very difficult to define 
and implement.  For example, the former case requires the operating system to 
define and present its new global identifier (that is different from the last 
boot) very early in the booting process.  Also, the operating system's global 
identifier must be the same on all access ports, which might get very ugly 
depending on which protocol layer defines the transmission methods for the 
initiator's global identifier.

I don't have a specific solution in mind here.  I just don't want the global 
identifier debate to attach too much value to global identifiers.  I've been 
there enough times to know that it doesn't work.


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