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RedHat 9 (Linux i386) - man page for fdisk (redhat section 8)

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

NAME
       fdisk - Partition table manipulator for Linux

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 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 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  manipu-
       lation  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 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  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 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 physi-
       cal geometry,  certainly  not  something  that  can  be	described  in  simplistic  Cylin-
       ders/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) 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 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 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 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,  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/hda1, 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/hda1 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, or 2048.   (Recent
	      kernels  know the sector size. Use this only on old kernels or to override the ker-
	      nel'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 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.

       -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.

       -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  parti-
       tion 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 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.)

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

Linux 2.0				   11 June 1998 				 FDISK(8)


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