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2. The FreeBSD way of labelling hard drives
Linux and FreeBSD label hard drives and partitions after two differents schemes. This section explains the main differences between the two schemes. In fact the FreeBSD labelling scheme is an adaption of the traditional BSD labelling style ported to live within the PC's fdisk partitions. Thus it is very similar to other BSD-based Unix systems such as NetBSD, OpenBSD, Ultrix, Digital Unix, SunOS, and Solaris.
2.1 FreeBSD ``slices'' and ``partitions''
FreeBSD needs one of the four entries in the partition table on your PC's
hard drive. This primary partition is called a ``slice'' in FreeBSD
terminology. It then uses the disklabel program to make up to eight
partitions in this primary partition. These logical partitions are called
``partitions'' in FreeBSD terminology. This concept is similar to the way
Linux (and DOS) handles logical partitions in an extended partition. You
cannot install FreeBSD in an extended partition made by Linux (or DOS). Note
that the Linux fdisk program doesn't display the BSD partitions in a
FreeBSD slice from the main menu, but it can display BSD disklabel information
if you give the command `b'. The output is something like this
(/dev/hda4 is the FreeBSD slice):
bash# fdisk /dev/hda Command (m for help): p Disk /dev/hda: 64 heads, 63 sectors, 621 cylinders Units = cylinders of 4032 * 512 bytes Device Boot Begin Start End Blocks Id System /dev/hda1 * 1 1 27 54400+ 83 Linux native /dev/hda2 28 28 55 56448 83 Linux native /dev/hda3 56 56 403 701568 83 Linux native /dev/hda4 404 404 621 439488 a5 BSD/386 Command (m for help): b Reading disklabel of /dev/hda4 at sector 1624897. BSD disklabel command (m for help): p 8 partitions: # size offset fstype [fsize bsize cpg] a: 64512 1624896 4.2BSD 0 0 0 # (Cyl. 404 - 419) b: 104832 1689408 swap # (Cyl. 420 - 445) c: 878976 1624896 unused 0 0 # (Cyl. 404 - 621) e: 64512 1794240 4.2BSD 0 0 0 # (Cyl. 446 - 461) f: 645120 1858752 4.2BSD 0 0 0 # (Cyl. 462 - 621) BSD disklabel command (m for help): q bash#
The letters `a'...`f' in the first column are the same labels as shown below in the example for a FreeBSD slice. There are three special partitions in BSD parlace. The letter `a' designates the root partition, `b' designates the swap partition, while `c' designates the whole slice. See the FreeBSD documentation for more information on the ``standard'' way of assigning these letters to different partition types.
2.2 Drive and partition labelling in Linux and FreeBSD
The hard drives are labelled in the following way in Linux and FreeBSD:
Linux FreeBSD
First IDE drive /dev/hda /dev/wd0
Second IDE drive /dev/hdb /dev/wd1
First SCSI drive /dev/sda /dev/sd0
Second SCSI drive /dev/sdb /dev/sd1
The partitions (FreeBSD slices) on an IDE drive are labelled in the following
way (/dev/hda is used as an example):
Linux FreeBSD
First primary partition /dev/hda1 /dev/wd0s1
Second primary partition /dev/hda2 /dev/wd0s2
Third primary partition /dev/hda3 /dev/wd0s3
Fourth primary partition /dev/hda4 /dev/wd0s4
The partitions in my FreeBSD slice is labelled in the following way. It is the
labelling you get by default. It is possible to change the labelling if you do
a custom installation of FreeBSD (/dev/hda4 is the FreeBSD slice in
the example):
Linux label FreeBSD label FreeBSD mount point /dev/hda5 /dev/wd0s4a / /dev/hda6 /dev/wd0s4b swap /dev/hda7 /dev/wd0s4e /var /dev/hda8 /dev/wd0s4f /usr
If you run dmesg in Linux you will see this as (The linux kernel must be
build with UFS filesystem support for this to work. See section
Installing and preparing Linux):
Partition check: hda: hda1 hda2 hda3 hda4 < hda5 hda6 hda7 hda8 >
If you have installed FreeBSD in the /dev/sd1s3 slice
(/dev/sdb3 in Linux parlace), and /dev/sdb2 is a Linux
extended partition containing two logical partitions (/dev/sdb5 and
/dev/sdb6), the previous example would look like this:
Linux label FreeBSD label FreeBSD mount point /dev/sdb7 /dev/sd1s3a / /dev/sdb8 /dev/sd1s3b swap /dev/sdb9 /dev/sd1s3e /var /dev/sdb10 /dev/sd1s3f /usr
This will be shown as
Partition check: sdb: sdb1 sdb2 < sdb5 sdb6 > sdb3 < sdb7 sdb8 sdb9 sdb10 >in the output from
dmesg.
If you have a Linux extended partition after your FreeBSD slice you're in
for trouble, because most Linux kernels installation floppies are build
without UFS support, they will not recognise the FreeBSD partitions inside the
slice. What should have have been seen as (/dev/hda3 is the FreeBSD
slice and /dev/hda4 is the Linux extended partition)
Partition check: hda: hda1 hda2 hda3 < hda5 hda6 hda7 hda8 > hda4 < hda9 hda10 >is seen as:
Partition check: hda: hda1 hda2 hda3 hda4 < hda5 hda6 >
This can give you the wrong device assignment and cause the loss of data. My advice is to always put your FreeBSD slice after any Linux extended partitions, and do not change any logical partitions in your Linux extended partitions after installing FreeBSD!
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