# Re: Two SCSI bus question...

Rogier Wolff (R.E.Wolff@BitWizard.nl)
Thu, 11 Dec 1997 00:55:08 +0100 (MET)

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Matto Marjanovic wrote:
>
>
> > > 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.
> > >
>
> Alright, out of curiousity then:
>
> Is there any particular reason why SCSI devices are assigned this way, rather
> than having the X in /dev/sgX be a direct function of device id and bus id?
> (e.g. /dev/sgA4 = device with ID=4 on the first bus)
>
> The way it is now, suppose one of 5 scsi disks blows out when you power up,
> then the rest all get their device assignments shifted and your server is
> *totally* screwed...

There are just 8 bits to a minor. You need about 4 bits to code for
the partition number, so you're left with just 4 bits to cram bus id
and device ID into. With wide scsi, you're left with 0 bits for the
bus ID.

Roger.

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