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

       fdisk - Partition table manipulator for Linux

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

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

       fdisk-linux -s partition	...

       fdisk-linux -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 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 se-
       curity, 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/adN for IDE disks,	/dev/daN for SCSI disks, N=0,1,2...)  A	device
       name refers to the entire disk.

       The partition is	a device name followed by 's' and a partition  number.
       For  example,  /dev/ad0s1  is the first partition on the	first IDE hard
       disk in the system.

       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 ac-
       tually 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
       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

       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	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-

       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

       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 ma-

       A sync()	and a sys_bsd_ptsync() (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 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.

       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/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

       -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

       -s partition
	      The size of the partition	(in blocks) is printed on the standard

       -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_linux(8),	newfs(8), fdisk(8), sfdisk_linux(8)

Linux 2.0			 11 June 1998			      FDISK(8)


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