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fdisk


SYNOPSIS

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

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

       fdisk -s partition ...

       fdisk -v


DESCRIPTION

       Hard  disks can be divided into one or more logical disks called parti-
       tions.  This division is described 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 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.

       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.    Disks   can  have  up  to  15  partitions.   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 parti-
       tion 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
       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  some-
       thing  that  can  be  described  in  simplistic Cylinders/Heads/Sectors
       form), but is the disk geometry that MS-DOS uses for the partition  ta-
       ble.

       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) geome-
       try 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  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.  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 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 sec-
       tor 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.
       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.)

       -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 num-
              ber, of course, but the number used for partition tables.)  Rea-
              sonable 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/parti-
              tions (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 par-
       tition 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 par-
       tition 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  nonin-
       teractively.)

       The  IRIX/SGI  type disklabel is currently not supported by the kernel.

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