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Home > Archive > Server + > January 2002 > SCSI Priority
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| mikehein 2002-01-24, 1:54 pm |
| Read a few articles, one or two books and I was sold on the idea that the SCSI Priority starts with the lowest number (highest 0 to lowest 7). I was quite comfortable with this idea until I read the following from Cramsession (page 7):
* Priority - Highest to Lowest - 7-0 /15-8
* 7 is the highest priority and decends to 0
* 15 is the highest priority and decends to 8
Please someone tell me that Cramsession is wrong and that when installing the first hard drive (boot drive) it gets the "highest" priority of 0 (or lowest available).
Who do I beleive? | |
| Paisleyskye 2002-01-24, 1:57 pm |
| The simplest way to remember your SCSI priorities is to remember that higher ID numbers have higher priorities. Therefore ID 7 is the highest priority in an 8-bit system. ID7 is usually used for the host controller card because it is such a vital part of the SCSI system. Many technicians set their primary hard drive to ID 0 in a SCSI system because it is the most active device. At this point you can set your other SCSI ID's giving slower devices higher priorities.
SCSI priorities in 16-bit and 32-bit systems get a little bit more complicated. 16-bit SCSI systems can support up to 16 devices, using ID's numbered 0-15, which is 16 ID's in all. The best way to understand SCSI priorities for this system is to break the ID's down in groups of 8. In ID group 0-7, the highest priority is 7, and the lowest is 0. In ID group 8-15, the highest priority is 15, and the lowest priority is 8. It is essential that you remember that ID's 0-7 have a higher priority than ID's 8-15. Therefore ID 13 has a lower priority than ID 7. To make things simpler the SCSI ID priorities in a 16-bit or 32-bit system from highest to lowest go like this:
7-6-5-4-3-2-1-0-15-14-13-12-11-10-19-8
SCSI priorities in 32-bit systems follow the same principle as 16-bit SCSI systems, where ID7 has the highest priority and ID 24 has the lowest priority. The SCSI priorities in a 32-bit SCSI system from highest to lowest go like this:
7-6-5-4-3-2-1-0-15-14-13-12-11-10-9-8-23-22-21-20-19-18-17-16-31-30-29-28-27-26-25-24
A major complication to understanding SCSI priorities stems back to IBM and their PS/2 computer. When IBM developed the PS/2 system they put the priorities all backwards!! For this reason, a lot of arguments have occurred regarding whether ID7 or ID0 has the highest priority. The answer to this question is that unless you are working on an IBM, ID 7 has the highest priority. | |
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| Paisleyskye 2002-01-24, 2:05 pm |
| Also, just to provide you with another source for the information:
In Upgrading and Repairing PCs 12th edition by Scott Muellar, on page 570 it clearly states that most SCSI host adapters are factory set to 7 which is the highest priority.
On page 573 it states that "Because the host adapter send this command to all the ID addresses in succession, from the highest priority address (ID 7) to the lowest priority address (ID 0)....
So, as I said in my first post, the highest priority is ID 7 and the lowest priority is ID 0. | |
| mikehein 2002-01-24, 2:24 pm |
| Hell... what a good answer! Thankyou.
So, if the only devices I have are a HD and CD and I want the best possible performance form my HD then I would give it an ID of 6 and give the ID of 0 to the CD.
-OR-
Would I give the CD an ID of 6 because it is a slow device where it would benefit from the attention and a lower ID to the HD? | |
| Paisleyskye 2002-01-24, 4:33 pm |
| Hi Mike,
Most technicians set the hard drive to the lowest priority which is 0 because it is the most active device. This however is not a set rule, and is just a convention. Technically you can put them where you want unless they have a preset ID from the manufacturer.
Newer SCSI devices that support Plug and Play are automatically given an available ID, however technicians need to be aware of SCSI priorities for troubleshooting purposes or when dealing with older devices.
If you are dealing with an older CD ROM and Hard Drive, where you set the hard drive is up to you. Like I say, most technicians set the hard drive to ID 0, but its only a convention and not a set rule. |
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Have a read of this