SIP Question

Lohmeyer, John JLOHMEYE at
Wed Jan 4 14:44:00 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?

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 

As an example, imagine a RAID-3 application that has two outstanding 
requests, called A and B.  There are three disk drives in this example, each 
on a separate SCSI bus (1, 2, and 3).  Each request results in one I/O to 
each of the three disk drives (A1, A2, & A3) plus (B1, B2, & B3).  If A1, 
A2, and B3 were to reconnect at the same time, then the array controller 
could not service either outstanding request without buffering the I/Os and 
waiting for either A3 or B1&B2
to reconnect.

However, some array controllers wanted to support a large number of 
outstanding requests but did not want to manage a correspondingly large 
buffer memory.  So TTD and CIOP were invented to let the initiator manage 
the reconnection order on each bus.

John Lohmeyer             E-Mail:  John.Lohmeyer at FtCollinsCO.NCR.COM
NCR Microelectronics       Voice:  719-573-3362
1635 Aeroplaza Dr.           Fax:  719-574-5462
Colo Spgs, CO 80916     SCSI BBS:  719-574-0424 300--14400 baud

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