FDISK linux command manual

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

       fdisk - Partition table manipulator for Linux

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

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

       fdisk -s partition ...

       fdisk -v

       Hard disks can be divided into one or more logical disks called parti-
       tions.  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 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.

       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/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

       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  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 different problems. First of all, these
       C/H/S  fields can be filled only when the number of heads and the num-
       ber 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 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-

       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 the
       partition starts and ends on a cylinder boundary (except for the first

       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

       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.

       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/hda1,  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/hda1  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.

       -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.)
              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/parti-
              tions (if that exists) are used.

       -u     When listing partition tables, give sizes in sectors instead of

       -s partition
              The  size  of the partition (in blocks) is printed on the stan-
              dard output.

       -v     Print version number of fdisk program and exit.

       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-

       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.

       cfdisk(8), mkfs(8), parted(8), sfdisk(8)

Linux 2.0                        11 June 1998                        FDISK(8)