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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  parti-
       tions,  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 adminis-
       tration 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 with-
       out  libata (a library used inside the Linux kernel to support ATA host controllers and devices) make a differ-
       ence 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'  par-
       tition, while the ninth should be labeled 'volume header'.  The volume header will also cover the partition ta-
       ble, i.e., it starts at block zero and extends by default over five cylinders.  The remaining space in the vol-
       ume 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  hold-
       ing  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  par-
       tition  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 FOR-
       MAT 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 use-
       less.

       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 num-
              ber 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  men-
              tioned 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  sup-
       port  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.ker-
       nel.org/pub/linux/utils/util-linux-ng/.



Linux 2.0                        11 June 1998                         FDISK(8)