Background
MacOS users are familiar with two ways to designate SCSI
disks and the volumes stored on them: the volume name as it
appears on the desktop, and the SCSI bus id (the bus number,
target ID and, infrequently, the logical unit number). On
Mac OS, the SCSI bus ID is used only for administrative
tasks, such as installing a new disk or updating drivers.
Rhapsody adds a third SCSI device designator to this
picture: the Unix device name. Unix uses a tree-structured
file system where every directory (folder) and file is
located in a path starting at the top-level "root" directory
of the boot device.
Also, on Unix, hardware devices are located within this
tree, where they are identified as files stored in the
"/dev " directory. (These are not actual files,
but rather provide a name that the operating system uses to
locate the device driver for that particular device.)
When the system starts, the Rhapsody bootstrap looks at
each SCSI bus in turn, examining each SCSI bus ID in turn.
When it discovers a SCSI disk (either a hard disk or
CD-ROM), it initializes an entry in the "/dev "
folder. The first disk it discovers will always be
sd0 , the next sd1 , etc. Since each
hardware disk can contain more than one Unix file system
partition, the device name is extended with a letter
designating the partition. So, the actual Unix device will
be named sd0a , etc. If, for some reason, there
is more than one partition on this disk, the next will be
named sd0b , and so on.
The relationship between SCSI bus and target ID and the
raw device name that represents that particular SCSI bus and
target ID is computed on each Rhapsody restart. This means
that, if you change your configuration or power on (or off)
an external device, all of the devices found later will be
associated with a different Unix raw device name.
Unfortunately, because of its Unix heritage, the
developer release bootstrap requires the boot device's SCSI
bus and target ID, as well as the Unix device name.
Furthermore, the boot device name is also stored in the
fstab file in the /etc directory.
You will soon see how this complicates matters.
Another potential source of confusion for Macintosh users
is that, on MacOS, the internal bus is number zero and the
external number one. Rhapsody reverses this: it looks at the
external bus first and the internal bus second. The Rhapsody
developer release does not support third-party SCSI busses.
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