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SCSI Priority

Daniel Spooner
Occasional Contributor

SCSI Priority

Can anyone tell me how the SCSI priority works?

If I have some very heavily used disks with a low priority, would it help to move them to a high priority - or would this cause the other disks to suffer?

The system is an N-Class with 6 CPU's ,2 SC10 disk arrays (mirrored). The disks are 18GB 15Krpm.

Thanks
Daniel
2 REPLIES
A. Clay Stephenson
Acclaimed Contributor

Re: SCSI Priority

Hi Daniel:

The priorities go in this order:
7,6,5,4,3,2,1,0,15,14,13,12,11,10,9,8

Seven is usually reserved from controllers. If you have multiple controllers on the same bus, they should get the next highest priorities and then the disks. You can certainly change the SCSI target ID's but as you suggest that simply moves the problem around. The real answer is to install another controller and split the disks across multiple scsi paths.

The following are more or less general 'rulkes of thumbs for the number od disks per FWD SCSI buss:

Heavy Sequential I/O: 5
Heavy Random I/O : 10
Very Light I/O : 15

If using SE-SCSI divide these by 2.

These apply ONLY to simple disks; if you are using arrays then you really need to reduce the number of arrays per scsi bus to no more than two and one is better.

Regards, Clay

If it ain't broke, I can fix that.
Stefan Saliba
Trusted Contributor

Re: SCSI Priority

Remember though that in the case of SC10 the ID's are slot dependent. The only control you have is wether to use high or low order ID's set on the dip switches of the bus controller.

Remember also that the Bus Controllers in the SC10 take an ID for themselves 14 and 15
if I am not mistaken.

Good luck

stefan