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 partitions.  This division is recorded in the partition ta‐
       ble, found in sector 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 dedicated as swap partition.  On	Intel-com‐
       patible hardware, the BIOS that boots the system can often only access the first 1024 cylinders of the disk.  For this rea‐
       son 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 example, /dev/sda1 is the first partition on the first
       hard disk in the system.  See also Linux kernel documentation (the Documentation/devices.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 actually 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 partition 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 partitions.  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 num‐
       ber 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  num‐
       bers  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 something 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 performed 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.  Partitions beginning in cylinder 1 cannot begin on a cylinder boundary, but this is unlikely to cause difficulty
       unless you have OS/2 on your machine.

       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 any‐
       more  --  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 sector 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 informa‐
       tion 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 partition 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 parti‐
       tions 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 compatiblity mode, 'dos' or 'nondos'.  The default is non-DOS mode.  For backward compatibility,  it	is
	      possible	to  use the option without the  argument -- then the default is used.  Note that the optional 
	      argument cannot be separated from the -c option by a space, the correct form is for example '-c=dos'.

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

       -H heads
	      Specify the number of heads of the disk.	(Not the physical number, of course, but the  number  used  for  partition
	      tables.)	Reasonable values are 255 and 16.

       -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.

       -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/partitions (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  argument --  then  the  default	is
	      used.  Note that the optional  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.

BUGS
       There are several *fdisk programs around.  Each has its problems and strengths.	Try  them  in  the  order  cfdisk,  fdisk,
       sfdisk.	 (Indeed,  cfdisk is a beautiful program that has strict requirements on the partition tables it accepts, and pro‐
       duces high quality partition tables.  Use it if you can.  fdisk is a buggy program that does fuzzy things - usually it hap‐
       pens to produce reasonable results.  Its single advantage is that it has some support for BSD disk labels and other non-DOS
       partition tables.  Avoid it if you can.	sfdisk is for hackers only -- the user interface is terrible, but it is more  cor‐
       rect than fdisk and more powerful than both fdisk and cfdisk.  Moreover, it can be used noninteractively.)

       These days there also is parted.  The cfdisk interface is nicer, but parted does much more: it not only resizes partitions,
       but also the filesystems that live in them.

       The IRIX/SGI-type disklabel is currently not supported by the kernel.  Moreover, IRIX/SGI header directories are not  fully
       supported yet.

       The option `dump partition table to file' is missing.

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 2010							  FDISK(8)