# Re: Two SCSI bus question...

Rogier Wolff (R.E.Wolff@BitWizard.nl)
Wed, 10 Dec 1997 09:29:24 +0100 (MET)

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Christoph Doerbeck wrote:
>
> > > The topic of having multiple SCSI buses has surfaced before, but
> > > my curiousity is how the /dev/(scanner) is assigned to a device.
> > > SCSI disks are simple, they're assigned in the order that they
> > > are detected (unless configured otherwise in the kernel). But,
> > > my scanner device seems to waffle around depending on whether or
> > > not some other devices are turned on/off at boot time. Any ideas???
> >
> > All detected devices are assigned a raw-device in the order in which they
> > are detected. Scanning the busses goes dev-id first, then bus-id.
> >
> > Thus you first get all devices on the first bus, then all on the second,
> > each in order of the SCSI IDs.
> >
> > Thus your scanner wanders whenever a device that is below it in that ordering
> > scheme is there or not.
> >
>
> But, let's say I have disk 1 (id=0), disk 2 (id=1), tape drive (id=2) and disk
> 3 (id =3). Isn't disk 3 always /dev/sdc regardless if the tape drive powered
> up? Let's say disk 4 is on controller 2 as id=0. Now /dev/sdd is always
> /dev/sdd unless one of the previous disks is not present at power up,
> regardless
> of the state of the tape drive.
>
> Shouldn't the scanner device behave the same way???? :-(

Actually it does, however the "sdX" devices are just "all the scsi
disks". The "sgX" devices are "all scsi devices". Thus if you want
your scanner to remain in the same place, make sure that no devices
with a power switch are before it.

Roger.

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