FDISK(8)            Linux Programmer's Manual            FDISK(8)

NAME
       fdisk - Partition table manipulator for Linux

SYNOPSIS
       fdisk [-u] [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 par-
       tition 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 parti-
       tions, but the latter are more efficient. So, usually  one
       will  want a second Linux partition dedicated as swap par-
       tition.  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, typi-
       cally 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  parti-
       tions.

       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 num-
       ber 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  abso-
       lute  number of sectors (given in 32 bits) and as a Cylin-
       ders/Heads/Sectors triple (given in 10+8+6 bits). The for-
       mer  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.  Sec-
       ondly,  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 automati-
       cally.  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 prob-
       lems 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  operat-
       ing  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  parti-
       tion).

       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 informa-
       tion in the partition table.  DOS FORMAT expects DOS FDISK
       to clear the first 512 bytes of the data area of a  parti-
       tion  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 par-
       tition 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/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
       -v     Print version number of fdisk program and exit.

       -l     List   the   partition   tables  for  /dev/hd[a-d],
              /dev/sd[a-h], /dev/ed[a-d], and then exit.

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

       -s partition
              The size of the partition (in blocks) is printed on
              the standard output.  This value is  normally  used
              as  an  argument  to the mkfs(8) program to specify
              the size of the partition which will be  formatted.
              (Older  versions of fdisk would do this only if the
              partition id is greater than 10, in an  attempt  to
              refuse DOS partitions; this test has been deleted.)
              Note that sfdisk -s gives different  (namely,  cor-
              rect) answers.  Reasons for the difference are that
              the kernel and fdisk need not have  the  same  idea
              about  partition  numbering (e.g., in case you have
              BSD slices), and have  different  ideas  about  the
              size of an extended partition.

BUGS
       There  are  several  *fdisk programs around.  Each has its
       problems and strengths.  Try them  in  the  order  cfdisk,
       fdisk, sfdisk.

       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.

Linux 2.0                  11 June 1998                         1