Provided by: util-linux_2.20.1-1ubuntu3_i386 bug

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 table, 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-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
       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  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
       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 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
       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 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
       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 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 compatiblity 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
              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 <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.

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  produces  high  quality  partition
       tables.   Use  it if you can.  fdisk is a buggy program that does fuzzy
       things - usually it happens 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 correct 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/.