👤
Home Man
Search
Today's Posts
Register

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:
Select Section of Man Page:
Select Man Page Repository:

Linux 2.6 - man page for fdisk (linux section 8)

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
       fdisk  (in the first form of invocation) is a menu-driven program for creation and manipu-
       lation 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  documen-
       tation (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  Cylin-
       ders/Heads/Sectors  triple (given in 10+8+6 bits).  The former is OK -- with 512-byte sec-
       tors 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 physi-
       cal  geometry,  certainly  not  something  that	can  be  described  in	simplistic Cylin-
       ders/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 parti-
       tion 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 bound-
       ary,  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, reboot-
       ing 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  com-
       mand "dd if=/dev/zero of=/dev/sda1 bs=512 count=1" to zero the first 512 bytes of the par-
       tition.

       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 exam-
       ple, 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  can-
	      not  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  parti-
       tion  tables.   Use it if you can.  fdisk is a buggy program that does fuzzy things - usu-
       ally 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 noninterac-
       tively.)

       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.ker-
       nel.org/pub/linux/utils/util-linux/.

Linux 2.0				   11 June 1998 				 FDISK(8)


All times are GMT -4. The time now is 01:30 AM.

Unix & Linux Forums Content Copyrightę1993-2018. All Rights Reserved.
×
UNIX.COM Login
Username:
Password:  
Show Password