Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
FDISK(8)		  BSD System Manager's Manual		      FDISK(8)

     fdisk -- PC slice table maintenance utility

     fdisk [-BIaipqstu]	[-b bootcode] [-1234] [disk]
     fdisk -f configfile [-itv]	[disk]

     In	order for the BIOS to boot the kernel, certain conventions must	be ad-
     hered to.	Sector 0 of the	disk must contain boot code, a slice table,
     and a magic number.  BIOS slices can be used to break the disk up into
     several pieces.  The BIOS brings in sector	0 and verifies the magic num-
     ber.  The sector 0	boot code then searches	the slice table	to determine
     which slice is marked "active".  This boot	code then brings in the	boot-
     strap from	the active slice and, if marked	bootable, runs it.  Under DOS,
     you can have one or more slices with one active.  The DOS fdisk utility
     can be used to divide space on the	disk into slices and set one active.

     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, be-
	     low, 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 sup-

     -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

     -u	     Update (edit) the disk's sector 0 slice table.  Ignored if	-f is

     -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

     The final disk name can be	provided as a "bare" disk name only, e.g. 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/ad0	*******
	     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:
	     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 pur-

     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 up-
     dated.  Unless the	-f option is also given, fdisk will enter a conversa-
     tional 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 de-
     fault, 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

     The difference between the	-u and -i flags	is that	the -u flag edits (up-
     dates) 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.

     The automatic calculation of starting cylinder etc. uses a	set of figures
     that represent what the BIOS thinks the geometry of the drive is.	These
     figures are taken from the	in-core	disklabel by default, but fdisk	ini-
     tially gives you an opportunity to	change them.  This allows you to cre-
     ate a bootblock that can work with	drives that use	geometry translation
     under the BIOS.

     If	you hand craft your disk layout, please	make sure that the FreeBSD
     slice starts on a cylinder	boundary.

     Editing an	existing slice will most likely	result in the loss of all data
     in	that slice.

     You should	run fdisk interactively	once or	twice to see how it works.
     This is completely	safe as	long as	you answer the last question in	the
     negative.	There are subtleties that fdisk	detects	that are not fully ex-
     plained in	this manual page.

     When the -f option	is given, a disk's slice table can be written using
     values from a configfile.	The syntax of this file	is very	simple;	each
     line is either a comment or a specification, as follows:

     # comment ...
	     Lines beginning with a # are comments and are ignored.

     g spec1 spec2 spec3
	     Set the BIOS geometry used	in slice calculations.	There must be
	     three values specified, with a letter preceding each number:

	     cnum    Set the number of cylinders to num.

	     hnum    Set the number of heads to	num.

	     snum    Set the number of sectors/track to	num.

	     These specs can occur in any order, as the	leading	letter deter-
	     mines which value is which; however, all three must be specified.

	     This line must occur before any lines that	specify	slice informa-

	     It	is an error if the following is	not true:

		   1 <=	number of cylinders
		   1 <=	number of heads	<= 256
		   1 <=	number of sectors/track	< 64

	     The number	of cylinders should be less than or equal to 1024, but
	     this is not enforced, although a warning will be printed.	Note
	     that bootable FreeBSD slices (the "/" file	system)	must lie com-
	     pletely within the	first 1024 cylinders; if this is not true,
	     booting may fail.	Non-bootable slices do not have	this restric-

	     Example (all of these are equivalent), for	a disk with 1019
	     cylinders,	39 heads, and 63 sectors:

		   g	   c1019   h39	   s63
		   g	   h39	   c1019   s63
		   g	   s63	   h39	   c1019

     p slice type start	length
	     Set the slice given by slice (1-4)	to type	type, starting at sec-
	     tor start for length sectors.  If the start or length is suffixed
	     with a K, M or G, it is taken as a	Kilobyte, Megabyte or Gigabyte
	     measurement respectively.	If the start is	given as "*" it	is set
	     to	the value of the previous partition end.  If the length	is
	     given as "*" the partition	end is set to the end of the disk.

	     Only those	slices explicitly mentioned by these lines are modi-
	     fied; any slice not referenced by a p line	will not be modified.
	     However, if an invalid slice table	is present, or the -i option
	     is	specified, all existing	slice entries will be cleared (marked
	     as	unused), and these p lines will	have to	be used	to explicitly
	     set slice information.  If	multiple slices	need to	be set,	multi-
	     ple p lines must be specified; one	for each slice.

	     These slice lines must occur after	any geometry specification
	     lines, if one is present.

	     The type is 165 for FreeBSD slices.  Specifying a slice type of
	     zero is the same as clearing the slice and	marking	it as unused;
	     however, dummy values (such as "0") must still be specified for
	     start and length.

	     Note: the start offset will be rounded upwards to a head boundary
	     if	necessary, and the end offset will be rounded downwards	to a
	     cylinder boundary if necessary.

	     Example: to clear slice 4 and mark	it as unused:

		   p	   4	   0	   0	   0

	     Example: to set slice 1 to	a FreeBSD slice, starting at sector 1
	     for 2503871 sectors (note:	these numbers will be rounded upwards
	     and downwards to correspond to head and cylinder boundaries):

		   p	   1	   165	   1	   2503871

	     Example: to set slices 1, 2 and 4 to FreeBSD slices, the first
	     being 2 Gigabytes,	the second being 10 Gigabytes and the forth
	     being the remainder of the	disk (again, numbers will be rounded

		   p	   1	   165	   63	   2G
		   p	   2	   165	   *	   10G
		   p	   3	   0	   0	   0
		   p	   4	   165	   *	   *

     a slice
	     Make slice	the active slice.  Can occur anywhere in the config
	     file, but only one	must be	present.

	     Example: to make slice 1 the active slice:

		   a	   1

     /boot/mbr	The default boot code.

     boot0cfg(8), bsdlabel(8), newfs(8)

     The default boot code will	not necessarily	handle all slice types cor-
     rectly, in	particular those introduced since MS-DOS 6.x.

     The entire	utility	should be made more user-friendly.

     Most users	new to FreeBSD do not understand the difference	between
     "slice" and "partition", causing difficulty to adjust.

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

BSD				 May 24, 2009				   BSD


Want to link to this manual page? Use this URL:

home | help