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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 >

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 >

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!