On Sun, 3 Sep 2000, Ahbaid Gaffoor wrote:
> Any advatnges or disadvantages of setting the SCSI id's on the devices as they
> progress along the cable?
>
> That is, first device from controller - SCSI ID 0 (The cd-rom, first
> connector)
> Second device - SCSI ID 1 ( the first drive, second connector)
> Third Device - SCSIS ID 2 (the second drive, third connector)
> etc.
>
> Or can I just place them anywhere along the cable?
>
> I tend to think that it is easier to keep track of things by placing them in
> order...
>
> regards and thanks,
>
> your answer has cleared things up a lot.
>
> Ahbaid.
>
Well, the device number actualy affects who gets the SCSI bus if more then
one device want it at the same time. The higher the number wins. You
would think because of this that you would want the hard drives to have
the highest numbers, but this is not the case. Hard drives are usualy not
time critical when sending/recievign data - that is what they have buffers
for. Tape drives tend to have tighter timing requirments.
Also, it is normal practice to make the hard drive you are going to boot
from ID 0. (Some older hardware requires that you boot from ID 0.)
ALso, if you change SCSI IDs, you may change the mapping of your hard
drives under Linux. Linux maps the first SCSI hard drive it finds to
/dev/sda, the second to /dev/sdb, and so on. It does the same kind of
mapping for other SCSI devices. So if your SCSI CD is device 0 now, and
you deside to add another hard drive later, and put it as ID 0 because you
are plugging it in where the CD drive was, it now becomes /dev/sda. Now
your /etc/fstab is out of sync with your physical drives. Lots of fun.
NOw, if you were using a older Sparc Station, you would want the CD as ID
6, and if you have a SCSI tape, you want that as ID 5. This is because
the boot PROM on expects them there by default. You can do things like
boot CDROM or boot TAPE with them.
Mikkel
--
Do not meddle in the affairs of dragons,
for you are crunchy and taste good with ketchup.
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