SC10 SCSI Bus Address Priority

[Mark Grossman]

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 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? If we had full cabinets would we see significant impact on the lower priority slots, or is it so little as to be neglibile?
thank you,
Mark Grossman

P.S. This thread has been moved from Disk to Disk array. - Hp Forum moderator

[paul courry · Accepted solution]

The answer, as always, is "it depends". The following is an extract from an HP manual and provides a basic explanaton:

"Each device on the bus is assigned a unique address in order to prevent conflicts during bus transactions. During bus operation, a device will only respond to its own unique address. Since there are 8 possible hardware connections on the bus, it is possible for all 8 devices to contend for the bus at any given point in time. In order to prevent conflict, one device is assigned the highest priority on the bus with subsequent priorities being assigned in descending numerical order. Since the SCSI bus interface is assigned an address of 7, it has the highest priority on the bus. Devices (targets) on the bus are assigned the next highest priority of 6 through 0. Note that 0 is the lowest. SCSI is "backwards" from most other bus implementations since it uses a descending priority scheme."


What this basically means is that when device id 6 requests the bus it gets priority over device 1. This should normally not be too much trouble because as a system administrator you have oh so carefully balanced the load on all your drives. Because you have done this, requests for reads or writes take place on all drives pretty equally, thus negating any effect from priorities assigned to drives.

[paul courry]

Additional info:

I am assuming you are using a performance tool such as Glance or SOS to gather the information about true I/O rates against each drive. Without this, you are basically taking a blind stab at it. You may even wish to get even more finely granulated information, down to the individual dataset with a database in order to optimize database performance.

[Mark Grossman]

Paul,
thanks for the info - the SC10 actually has 10 slots and 7,6,5,4 are not available for disks.
then they go 3,2,1,0,15,14,13,21,11,10,9,8. But 14 and 15 are for the Bus channel adapters.

We dont have much on there right now but I am now convinced we'd better plan carefully.

[Bob Inglis]

Please Note: Each Host bus adapter attached to the SC10 has to have a unique SCSI Bus address also. That is why addresses 7, 6, 5 & 4 are reserved. You can attach as many as 4 diferent host computers to the SC10 that completly uses up all 16 available addresses on the bus.
10 addresses for drives
2 sddresses for BCC's
4 addresses fot the hosts
SCSI addresses 7, 6, 5 & 4 are the highest priority addresses which is why they are reserved for the Host adapters.

[Mark Grossman]

Bob,
thanks for the input ,

Mark