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

NAME

       fdisk - Partition table manipulator for Linux

SYNOPSIS

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

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

       fdisk -s partition...

       fdisk -v

       fdisk -h

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.

       fdisk doesn't understand GUID Partition Table (GPT) and it is not designed for large partitions. In particular case use more  advanced  GNU
       parted(8).

       The  device  is	usually /dev/sda, /dev/sdb or so. A device name refers to the entire disk.  The 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  a  case  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).

       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.  Logi-
       cal 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 suf-
       fice.  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.   Par-
       titions	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/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-ng 2.17 fdisk differentiates between logical and physical sector
	      size. This option changes both sector sizes to sectorsize.

       -h     Print help and then exit.

       -c     Switch off DOS-compatible mode. (Recommended)

       -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  advan-
       tage  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-ng package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.

Linux 2.0							   11 June 1998 							  FDISK(8)