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



NAME
       fdisk - Partition table manipulator for Linux

SYNOPSIS
       fdisk [-u] [-b sectorsize]device

       fdisk -l [-u] [-b sectorsize] [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
       -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.)

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

       -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.

       -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  inter
       face  is  terrible,  but it is more correct than fdisk and
       more powerful than both fdisk and  cfdisk.   Moreover,  it
       can be used noninteractively.)

       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), parted(8), sfdisk(8)



Linux 2.0		   11 June 1998 		 FDISK(8)