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Date: Wed, 27 Sep 1995 08:45:43 -0600
From: Nate Williams <nate@rocky.sri.MT.net>
Message-Id: <199509271445.IAA17099@rocky.sri.MT.net>
To: Bruce Evans <bde@zeta.org.au>
Cc: hackers@FreeBSD.org, nate@rocky.sri.MT.net, bde@FreeBSD.org
Subject: Re: Diskslice naming convention?
In-Reply-To: <199509270621.QAA08365@godzilla.zeta.org.au>
References: <199509270621.QAA08365@godzilla.zeta.org.au>
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Bruce Evans writes:
> >So, am I right in saying that other than the 'compatability' slice of
> >/dev/xd#a for the root partition, the naming scheme is as follows:
> 
> >X = generic device name.
> ># = disk number
> >Y = slice number (ie; one of the 4 'fdisk' partions')
> 
> >Xd#asY  (where 'a' could be any lower-case letter 'a-h')
> 
> No, Y may be any fdisk partition or logical drive, and 'a' follows the
> slice number.

Whoops, you're right. (of course)

Xd#sYa (where 'a' could be any lower-case letter 'a-h').

> >BTW - Are slices named 1-4 or 0-3?
> 
> No, slices are named 1-30.

Ok.  How do the 'fdisk' slices (of which there can only be 4) relate to
the 30 slices available in FreeBSD.  Excuse me for using DOS terminology
for a minute, but if I understand correctly, we can use 'extended'
partitions inside of 'primary' partitions.  However, 30/4 isn't a real
number, so where does the magic number 30 come from?

> >Anyway, we create a FreeBSD slice by setting it's ID to 0xA5.  Then, we
> >build a disklabel onto this 'slice' which gives us individual partitions
> >on that slice.  Are there any 'special' partitions in this disklabel?
> 
> c.
> 
> >How would I read the *entire* FreeBSD portion of the slice?  How about
> 
> There is no such thing as a `portion' of a slice.  (Would a portion be
> smaller or larger than a partition? :-).)  `dd if=/dev/rxd#sY' or `dd
> if=/dev/rxd#sYc' reads the entire slice sY.  These devices are
> identical.  /dev/MAKEDEV creates both for convenience.  The c partition
> is special whether there is a disklabel describing it or not.  If there
> is a disklabel, the c partition should match the whole slice.
> 
> >the entire disk?  How do I create a FreeBSD partition which accesses a
> >DOS slice?
> 
> `dd if=/dev/rxd#' reads the entire disk.  /dev/rxd# is a completely
> different device from /dev/rxd#c.  These devices are often confused
> because disklabel automatically translates from `xd#' to /dev/rxd#c'.

Ahh, so these are equivalent

# dd if=/dev/sd0c of=/dev/null
# dd if=/dev/sd0  of=/dev/null

(Read the entire slice)

Vs.

# dd if=/dev/rsd0 of=/dev/null

(Read the entire disk)

> You can't create a FreeBSD partition which accesses a DOS slice.  Just
> access the DOS slice directly.

Ok.  How do I know which slice is the DOS slice?  (This get's back to
the determination of the numbering scheme 1-30)



Nate