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fdisk


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 manipulation 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 parti-
       tions.  This division is recorded in the partition table, found in sec-
       tor 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  dedi-
       cated  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   documentation   (the   Documenta-
       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 parti-
       tions.  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 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 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 physical geometry, certainly not some-
       thing that  can  be  described  in  simplistic  Cylinders/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 per-
       formed on the partition 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 boundary, but on sector 2 of the first cylinder.   Parti-
       tions  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.

       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 parti-
       tion 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-2.17, fdisk differentiates between logical  and
              physical  sector size.  This option changes both sector sizes to
              sectorsize.

       -c[=mode]
              Specify the compatibility 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 cannot be sepa-
              rated from the -c option by a space, the  correct  form  is  for
              example '-c=dos'. This option is DEPRECATED.

       -C cyls
              Specify the number of cylinders of the disk.  I have no idea why
              anybody would want to do so. This option is DEPRECATED.

       -H heads
              Specify the number of heads of the disk.  (Not the physical num-
              ber, of course, but the number used for partition tables.)  Rea-
              sonable values are 255 and 16. This option is DEPRECATED.

       -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. This option is DEPRECATED.

       -h     Print help and then exit.


       -v     Print version number of fdisk program and exit.


ENVIRONMENT

       FDISK_DEBUG=0xffff
              enables debug output


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.kernel.org/pub/linux/utils/util-linux/.


util-linux June 2012 FDISK(8)



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