SIP Question
Lohmeyer, John
JLOHMEYE at cosmpdaero.ftcollinsco.ncr.com
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
function.
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
--
John Lohmeyer E-Mail: John.Lohmeyer at FtCollinsCO.NCR.COM
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