SIP Question

Jeff Williams jlw at
Thu Jan 5 06:44:27 PST 1995

> Duncan wrote:
> >I wasn't around for the origins of the TARGET TRANSFER DISABLE and
> >CONTINUE I/O PROCESS messages.  Am I correct in inferring that the
> >primary purpose of these is to allow an initiator to temporarily suspend
> >transfers from a high priority device in order to prevent bus starvation
> >on low priority devices?

> John L. replied:
> Your application of TTD and CIOP is reasonable, but the actual application 
> that drove the addition of these messages was to permit the initiator to 
> control the order of reconnections for disk array applications.  Array 
> controllers that didn't have gobs of memory (a technical term), needed this 
> function.

John's description is correct.  I have one issue with stating that Duncan's
application is correct....

Initiators have no ability to temporarily suspend the data transfer unless
they issued the TTD with the original nexus.  The TTD cannot be sent later 
in the I/O process to cause this behavior.  Therefore you need to know
before the I/O process begins that you wish to do this throttling of the
data.  If you send a TTD in the original nexus, then an outbound disconnect
can be used in conjunction with a CIOP to hold off data transfer.

Jeffrey L. Williams                                HP TelNet: 396-5030
Disk Controllers Lab             	 Telephone: (1) (208) 396-5030
Disk Memory Division                     Facsimile: (1) (208) 396-6858
Hewlett-Packard Co.                    ARPANET: jlw at

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