Date: Wed, 27 Sep 1995 09:31:51 -0600 From: Nate Williams <nate@rocky.sri.MT.net> To: Bruce Evans <bde@zeta.org.au> Cc: nate@rocky.sri.MT.net, bde@FreeBSD.org, hackers@FreeBSD.org Subject: Re: Diskslice naming convention? Message-ID: <199509271531.JAA17253@rocky.sri.MT.net> In-Reply-To: <199509271509.BAA28013@godzilla.zeta.org.au> References: <199509271509.BAA28013@godzilla.zeta.org.au>
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Bruce Evans writes:
> >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
>
> The `fdisk' slices (primary DOS partitions) are the first 4 of the 30.
>
> >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?
>
> Extended partitions aren't inside primary partitions. Each primary partition
> may be an extended partition.
Ahh, OK. We are using different terminology. I was using the
terminology 'extended' partition to represent a DOS drive letter inside
of an extended partition.
> Each extended partition begins with a
> secondary boot record with a partition table in it. The 4 entries in the
> partition table may be `logical drives' or extended partitions. Each
> extended partition begins with a secondary boot record...
So, we have:
Entire disk:
+------------------+------------------+------------------+------------------+
| | | | |
| Slice 1 | Slice 2 | Slice 3 | Slice 4 |
| | | | |
And possibily (can be divided as many times as necessary)
Single slice
+-------+-------+---+
| | | |
| S 5 | S 6 | 7 |
| | | |
And the numbering scheme for the slices > 4 is determined by how they
fall in the first 4 slices, correct?
> I decided that 5 bits in the minor number were enough to reserve for the
> slice number. 2^5 = 32 and 2 slice numbers are special.
>
> >> `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)
>
> No, the devices are completely different, as was just explained.
As I read the explanation, they are the same (other than the raw vs. block interaction).
disklabel automatically translated /dev/sd0 -> /dev/rsd0c
# dd if=/dev/sd0 of=/dev/null
But if I specifically hard-code in the device
# dd if=/dev/sd0c of=/dev/null
I should get the same results.
> >> 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)
>
> Look at sysinstall or libdisk/tst01 output for slices labeled as `fat".
> The numbering corresponds to a particular linearization of the tree of
> extended partitions so the relative order may vary with the OS.
Is there anyway to determine this outside of sysinstall?
Nate
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