Global identifiers for initiators
Ralph Weber -- VMS -- ZKO3-4/U14
weber at star.enet.dec.com
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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