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FDISK(8)		     System Administration		      FDISK(8)

NAME
       fdisk - manipulate disk partition table

SYNOPSIS
       fdisk [-uc] [-b sectorsize] [-C cyls] [-H heads]	[-S sects] device

       fdisk -l	[-u] [device...]

       fdisk -s	partition...

       fdisk -v

       fdisk -h

DESCRIPTION
       fdisk  (in  the	first form of invocation) is a menu-driven program for
       creation	and manipulation of partition tables.  It understands DOS-type
       partition tables	and BSD- or SUN-type disklabels.

       fdisk  does  not	 understand GUID partition tables (GPTs) and it	is not
       designed	for large partitions.  In these	cases, use the	more  advanced
       GNU parted(8).

       fdisk  does  not	use DOS-compatible mode	and cylinders as display units
       by default.  The	old deprecated DOS behavior can	be  enabled  with  the
       '-c=dos -u=cylinders' command-line options.

       Hard  disks can be divided into one or more logical disks called	parti-
       tions.  This division is	recorded in the	partition table, found in sec-
       tor 0 of	the disk.  (In the BSD world one talks about `disk slices' and
       a `disklabel'.)

       Linux needs at least one	partition, namely for its  root	 file  system.
       It  can	use swap files and/or swap partitions, but the latter are more
       efficient.  So, usually one will	want a second  Linux  partition	 dedi-
       cated  as  swap partition.  On Intel-compatible hardware, the BIOS that
       boots the system	can often only access the first	1024 cylinders of  the
       disk.   For  this  reason  people with large disks often	create a third
       partition, just a few MB	large, typically mounted on  /boot,  to	 store
       the  kernel  image and a	few auxiliary files needed at boot time, so as
       to make sure that this stuff is accessible to the BIOS.	There  may  be
       reasons	of security, ease of administration and	backup,	or testing, to
       use more	than the minimum number	of partitions.

DEVICES
       The device is usually /dev/sda, /dev/sdb	or so.	A device  name	refers
       to  the entire disk.  Old systems without libata	(a library used	inside
       the Linux kernel	to support ATA host controllers	and  devices)  make  a
       difference  between  IDE	and SCSI disks.	 In such cases the device name
       will be /dev/hd*	(IDE) or /dev/sd* (SCSI).

       The partition is	a device name followed by a partition number.  For ex-
       ample,  /dev/sda1  is the first partition on the	first hard disk	in the
       system.	See also Linux	kernel	documentation  (the  Documentation/de-
       vices.txt file).

DISK LABELS
       A  BSD/SUN-type disklabel can describe 8	partitions, the	third of which
       should be a `whole disk'	partition.  Do not start a partition that  ac-
       tually  uses  its  first	 sector	(like a	swap partition)	at cylinder 0,
       since that will destroy the disklabel.

       An IRIX/SGI-type	disklabel can describe 16 partitions, the eleventh  of
       which should be an entire `volume' partition, while the ninth should be
       labeled `volume header'.	 The volume header will	also cover the	parti-
       tion  table,  i.e., it starts at	block zero and extends by default over
       five cylinders.	The remaining space in the volume header may  be  used
       by header directory entries.  No	partitions may overlap with the	volume
       header.	Also do	not change its type or make  some  filesystem  on  it,
       since  you  will	lose the partition table.  Use this type of label only
       when working with Linux on IRIX/SGI machines or	IRIX/SGI  disks	 under
       Linux.

       A  DOS-type  partition table can	describe an unlimited number of	parti-
       tions.  In sector 0 there is room for the description of	 4  partitions
       (called `primary').  One	of these may be	an extended partition; this is
       a box holding logical partitions, with descriptors found	 in  a	linked
       list  of	 sectors, each preceding the corresponding logical partitions.
       The four	primary	partitions, present or not, get	numbers	1-4.   Logical
       partitions start	numbering from 5.

       In  a DOS-type partition	table the starting offset and the size of each
       partition is stored in two ways:	 as  an	 absolute  number  of  sectors
       (given  in  32 bits), and as a Cylinders/Heads/Sectors triple (given in
       10+8+6 bits).  The former is OK -- with 512-byte	sectors	this will work
       up  to  2  TB.  The latter has two problems.  First, these C/H/S	fields
       can be filled only when the number of heads and the number  of  sectors
       per  track  are	known.	And second, even if we know what these numbers
       should be, the 24 bits that are available do  not  suffice.   DOS  uses
       C/H/S only, Windows uses	both, Linux never uses C/H/S.

       If  possible,  fdisk will obtain	the disk geometry automatically.  This
       is not necessarily the physical disk geometry (indeed, modern disks  do
       not  really have	anything like a	physical geometry, certainly not some-
       thing that  can	be  described  in  simplistic  Cylinders/Heads/Sectors
       form),  but  it is the disk geometry that MS-DOS	uses for the partition
       table.

       Usually all goes	well by	default, and there are no problems if Linux is
       the  only  system  on  the disk.	 However, if the disk has to be	shared
       with other operating systems, it	is often a good	idea to	let  an	 fdisk
       from  another operating system make at least one	partition.  When Linux
       boots it	looks at the partition table, and tries	to deduce what	(fake)
       geometry	is required for	good cooperation with other systems.

       Whenever	 a partition table is printed out, a consistency check is per-
       formed on the partition table entries.  This check  verifies  that  the
       physical	 and logical start and end points are identical, and that each
       partition starts	and ends on a cylinder boundary	(except	for the	 first
       partition).

       Some  versions  of MS-DOS create	a first	partition which	does not begin
       on a cylinder boundary, but on sector 2 of the first cylinder.	Parti-
       tions  beginning	in cylinder 1 cannot begin on a	cylinder boundary, but
       this is unlikely	to cause difficulty unless you have OS/2 on  your  ma-
       chine.

       A sync()	and an ioctl(BLKRRPART)	(reread	partition table	from disk) are
       performed before	exiting	when the partition  table  has	been  updated.
       Long  ago  it used to be	necessary to reboot after the use of fdisk.  I
       do not think this is the	case anymore --	indeed,	rebooting too  quickly
       might  cause  loss  of not-yet-written data.  Note that both the	kernel
       and the disk hardware may buffer	data.

DOS 6.x	WARNING
       The DOS 6.x FORMAT command looks	for some information in	the first sec-
       tor  of	the data area of the partition,	and treats this	information as
       more reliable than the information in the partition table.  DOS	FORMAT
       expects	DOS  FDISK  to clear the first 512 bytes of the	data area of a
       partition whenever a size change	occurs.	 DOS FORMAT will look at  this
       extra  information  even	 if the	/U flag	is given -- we consider	this a
       bug in DOS FORMAT and DOS FDISK.

       The bottom line is that if you use cfdisk or fdisk to change  the  size
       of  a  DOS partition table entry, then you must also use	dd to zero the
       first 512 bytes of that partition before	using DOS FORMAT to format the
       partition.   For	example, if you	were using cfdisk to make a DOS	parti-
       tion table entry	for /dev/sda1, then (after exiting fdisk or cfdisk and
       rebooting  Linux	 so that the partition table information is valid) you
       would use the command "dd if=/dev/zero of=/dev/sda1 bs=512 count=1"  to
       zero the	first 512 bytes	of the partition.

       BE  EXTREMELY CAREFUL if	you use	the dd command,	since a	small typo can
       make all	of the data on your disk useless.

       For best	results, you should always use an OS-specific partition	 table
       program.	  For  example,	 you  should  make DOS partitions with the DOS
       FDISK program and Linux partitions with the Linux fdisk or Linux	cfdisk
       program.

OPTIONS
       -b sectorsize
	      Specify  the  sector  size  of  the disk.	 Valid values are 512,
	      1024, 2048 or 4096.  (Recent kernels know	the sector size.   Use
	      this  only  on  old  kernels or to override the kernel's ideas.)
	      Since util-linux-2.17, fdisk differentiates between logical  and
	      physical	sector size.  This option changes both sector sizes to
	      sectorsize.

       -c[=mode]
	      Specify the compatibility	mode, 'dos' or 'nondos'.  The  default
	      is  non-DOS mode.	 For backward compatibility, it	is possible to
	      use the option without the <mode>	argument -- then  the  default
	      is used.	Note that the optional <mode> argument cannot be sepa-
	      rated from the -c	option by a space, the correct form is for ex-
	      ample '-c=dos'. This option is DEPRECATED.

       -C cyls
	      Specify the number of cylinders of the disk.  I have no idea why
	      anybody would want to do so. This	option is DEPRECATED.

       -H heads
	      Specify the number of heads of the disk.	(Not the physical num-
	      ber, of course, but the number used for partition	tables.)  Rea-
	      sonable values are 255 and 16. This option is DEPRECATED.

       -S sects
	      Specify the number of sectors per	track of the disk.   (Not  the
	      physical	number,	 of  course, but the number used for partition
	      tables.)	A reasonable value is 63. This option is DEPRECATED.

       -h     Print help and then exit.

       -l     List the partition tables	for the	 specified  devices  and  then
	      exit.   If no devices are	given, those mentioned in /proc/parti-
	      tions (if	that exists) are used.

       -s partition...
	      Print the	size (in blocks) of each given partition.

       -u[=unit]
	      When listing partition tables, show sizes	 in  'sectors'	or  in
	      'cylinders'.   The  default  is  to  show	sizes in sectors.  For
	      backward compatibility, it is possible to	use the	option without
	      the <units> argument -- then the default is used.	 Note that the
	      optional <unit> argument cannot be separated from	the -u	option
	      by a space, the correct form is for example '-u=cylinders'.

       -v     Print version number of fdisk program and	exit.

ENVIRONMENT
       FDISK_DEBUG=0xffff
	      enables debug output

SEE ALSO
       cfdisk(8), sfdisk(8), mkfs(8), parted(8), partprobe(8), kpartx(8)

AVAILABILITY
       The  fdisk  command  is part of the util-linux package and is available
       from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

util-linux			   June	2012			      FDISK(8)

NAME | SYNOPSIS | DESCRIPTION | DEVICES | DISK LABELS | DOS 6.x WARNING | OPTIONS | ENVIRONMENT | SEE ALSO | AVAILABILITY

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