SC10 SCSI Bus priority

[Mark Grossman]

this is a repost of issue i opended in the mass storage forum whcih doesnt seem to get much action:
We just purchased 2 SC10's that we are running Full Bus with 1 BCC on each box and we are going to use mirror disk between the two.
On page 79 of the SC10 User and Service Guide manual(newest manual only) there is 'table 10' which shows bus priority for each slot.
My question is: Does anybody have a feel for how much impact this priority scheme really has on traffic? If we had full cabinets would we see significant impact on the lower priority slots, or is it so little as to be negligable?
thank you,
Mark Grossman

[Madhu Sudhan_1 · Accepted solution]

I think SCSI priority does matter.
Here is the information on how priorities work.

The wires of a SCSI bus are shared among all the SCSI devices. Because the wires can only carry one piece of data at a time, a device wishing to use the bus must get permission from all the other devices (arbitration phase). It performs this by activating the BSY control line with its unique SCSI ID. If more than one device attempts to gains control over the bus at the same time, the device with the highest SCSI ID number gains control (a useful tip when you want certain devices on the SCSI bus to have priority).

After winning the arbitration, the device must get the attention of the device with which it wishes to communicate (selection phase). The SEL control line is asserted and the SCSI for the 'target' device on the bus is added to the calling device releases the SEL control line and the data bus, then the information transfer begins.

Enjoy !
Madhu

[Mark Grossman]

Madhu,
thanks for the detailed explanation and response. We will definitely plan ahead as we fill these cabinets and pay attention to the scsi priority.

Mark

[Chris Moore]

Just a small clarification - the statement that the highest SCSI ID wins arbitration is only true on a narrow bus. The priorities from highest to lowest are targets 7 down to 0, then 15 down to 8, then 23 down to 16, then 31 down to 14. I'm not sure but I think the SC10 has a wide bus, so the priorities are 7 (normally the system), 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8.

[Mark Grossman]

Chris,
thanks for the input - actually we caught that here, but assumed Madhu was referring to the highest priority as listed on the chart (which you also described), not the actual highest numeric id. On the SC10 7,6,5,4 are all for system and unavailable for disks. And 15,14 are for the BCC units.