Manual Pages  — FDISK

The FreeBSD utility, fdisk, serves a similar purpose to the DOS utility. The first form is used to display slice information or to interactively edit the slice table. The second is used to write a slice table using a configfile, and is designed to be used by other scripts/programs.

Options are:

-a
	Change the active slice only. Ignored if -f is given.
-b bootcode
	Get the boot code from the file bootcode. Default is /boot/mbr.
-B
	Reinitialize the boot code contained in sector 0 of the disk. Ignored if -f is given.
-f configfile
	Set slice values using the file configfile. The configfile only modifies explicitly specified slices, unless -i is also given, in which case all existing slices are deleted (marked as "unused") before the configfile is read. The configfile can be '-', in which case standard input is read. See CONFIGURATION FILE, below, for file syntax. WARNING: when -f is used, you are not asked if you really want to write the slices table (as you are in the interactive mode). Use with caution!
-i
	Initialize sector 0 of the disk. Existing slice entries will be cleared (marked as unused) before editing. (Compare with -u -.)
-I
	Initialize sector 0 slice table for one FreeBSD slice covering the entire disk.
-p
	Print a slice table in fdisk configuration file format and exit; see CONFIGURATION FILE, below.
-q
	Be quiet. Benign warnings (such as "GEOM not found") are suppressed.
-s
	Print summary information and exit.
-t
	Test mode; do not write slice values. Generally used with the -f option to see what would be written to the slice table. Implies -v.
-u
	Update (edit) the disk's sector 0 slice table. Ignored if -f is given.
-v
	Be verbose. When -f is used, fdisk prints out the slice table that is written to the disk.
-1234
	Operate on a single slice table entry only. Ignored if -f is given.

The final disk name can be provided as a "bare" disk name only, e.g.amp; da0, or as a full pathname. If omitted, fdisk tries to figure out the default disk device name from the mounted root device.

When called with no arguments, it prints the sector 0 slice table. An example follows:

        ******* Working on device /dev/ada0 *******
        parameters extracted from in-core disklabel are:
        cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

        parameters to be used for BIOS calculations are:
        cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

        Warning: BIOS sector numbering starts with sector 1
        Information from DOS bootblock is:
        The data for partition 1 is:
        sysid 165,(FreeBSD/NetBSD/386BSD)
           start 495, size 380160 (185 Meg), flag 0
                beg: cyl 1/ sector 1/ head 0;
                end: cyl 768/ sector 33/ head 14
        The data for partition 2 is:
        sysid 164,(unknown)
           start 378180, size 2475 (1 Meg), flag 0
                beg: cyl 764/ sector 1/ head 0;
                end: cyl 768/ sector 33/ head 14
        The data for partition 3 is:
        <UNUSED>
        The data for partition 4 is:
        sysid 99,(ISC UNIX, other System V/386, GNU HURD or Mach)
           start 380656, size 224234 (109 Meg), flag 80
                beg: cyl 769/ sector 2/ head 0;
                end: cyl 197/ sector 33/ head 14

The disk is divided into three slices that happen to fill the disk. The second slice overlaps the end of the first. (Used for debugging purposes.)

sysid	is used to label the slice. FreeBSD reserves the magic number 165 decimal (A5 in hex).
start and size fields provide the start address and size of a slice in sectors.
flag 80	specifies that this is the active slice.
cyl, sector and head fields are used to specify the beginning and end addresses of the slice.

Note: these numbers are calculated using BIOS's understanding of the disk geometry and saved in the bootblock.

The -i and -u flags are used to indicate that the slice data is to be updated. Unless the -f option is also given, fdisk will enter a conversational mode. In this mode, no changes will be written to disk unless you explicitly tell fdisk to.

The fdisk utility will display each slice and ask whether you want to edit it. If you say yes, fdisk will step through each field, show you the old value, and ask you for a new one. When you are done with the slice, fdisk will display it and ask you whether it is correct. It will then proceed to the next entry.

Getting the cyl, sector, and head fields correct is tricky, so by default, they will be calculated for you; you can specify them if you choose to though.

After all the slices are processed, you are given the option to change the "active" slice. Finally, when all the new data for sector 0 has been accumulated, you are asked to confirm whether you really want to rewrite it.

The difference between the -u and -i flags is that the -u flag edits (updates) the existing slice parameters while the -i flag is used to "initialize" them (old values will be ignored); if you edit the first slice, -i will also set it up to use the whole disk for FreeBSD and make it active.

You cannot use this command to completely dedicate a disk to FreeBSD . The bsdlabel(8) command must be used for this.