Provided by: util-linux_2.12r-4ubuntu6_i386 bug

NAME

       fdisk - Partition table manipulator for Linux

SYNOPSIS

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

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

       fdisk -s partition ...

       fdisk -v

DESCRIPTION

       Hard  disks  can  be  divided  into  one  or  more logical disks called
       partitions.  This division is described 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.

       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.

       The device is usually one of the following:
              /dev/hda
              /dev/hdb
              /dev/sda
              /dev/sdb
       (/dev/hd[a-h]  for IDE disks, /dev/sd[a-p] for SCSI disks, /dev/ed[a-d]
       for ESDI disks, /dev/xd[ab] for XT disks).  A device name refers to the
       entire disk.

       The  partition  is  a  device name followed by a partition number.  For
       example, /dev/hda1 is the first partition on the first IDE hard disk in
       the  system.   IDE disks can have up to 63 partitions, SCSI disks up to
       15.  See also /usr/src/linux/Documentation/devices.txt.

       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 and make some file system
       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 different problems. First of all,  these
       C/H/S fields can be filled only when the number of heads and the number
       of sectors per track are known. Secondly, 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 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  the
       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 a BLKRRPART ioctl() (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/hda1,  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/hda1 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,
              or 2048.  (Recent kernels know the sector size. Use this only on
              old kernels or to override the kernel’s ideas.)

       -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.

       -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.

       -u     When  listing partition tables, give sizes in sectors instead of
              cylinders.

       -s partition
              The size of the partition (in blocks) is printed on the standard
              output.

       -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), mkfs(8), parted(8), sfdisk(8)