path: root/disk-utils/fdisk.8

.\" Copyright 1992, 1993 Rickard E. Faith (faith@cs.unc.edu)
.\" Copyright 1998 Andries E. Brouwer (aeb@cwi.nl)
.\" May be distributed under the GNU General Public License
.TH FDISK 8 "11 June 1998" "Linux 2.0" "Linux Programmer's Manual"
.SH NAME
fdisk \- Partition table manipulator for Linux
.SH SYNOPSIS
.BI "fdisk [\-b] [\-u] [" device ]
.sp
.BI "fdisk \-l [\-b] [\-u] [" "device ..." ]
.sp
.BI "fdisk \-s " "partition ..."
.sp
.BI "fdisk \-v
.SH DESCRIPTION
Hard disks can be divided into one or more logical disks called
.IR partitions .
This division is described in the
.I "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
.IR /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.

.B 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
.I device
is usually one of the following:
.br
.nf
.RS
/dev/hda
/dev/hdb
/dev/sda
/dev/sdb
.RE
.fi
(/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
.I partition
is a
.I device
name followed by a partition number.  For example,
.B /dev/hda1
is the first partition on the first IDE hard disk in the system.
IDE disks can have up to 63 partitions, SCSI disks up to 15.
See also
.IR /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.

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 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,
.B 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 something
that can be described in simplistic Cylinders/Heads/Sectors form),
but 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 performed
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.
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, rebooting too quickly
might cause loss of not-yet-written data. Note that both the kernel
and the disk hardware may buffer data.

.SH "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
.B 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.

.B BE EXTREMELY CAREFUL
if you use the
.B 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.

.SH OPTIONS
.TP
.B \-v
Print version number of
.B fdisk
program and exit.
.TP
.B \-l
List the partition tables for
.BR /dev/hd[a-d] ,
.BR /dev/sd[a-h] ,
.BR /dev/ed[a-d] ,
and then exit.
.TP
.B \-b
When listing partition tables, also output a `Begin' column,
as earlier versions of fdisk did by default.
(Note: the values in this column, when given in cylinder units,
cannot be larger than 1023. There is nothing wrong if Begin and Start
differ, at least not as far as Linux is concerned.)
.TP
.B \-u
When listing partition tables, give sizes in sectors instead
of cylinders.
.TP
.BI "\-s " partition
The
.I size
of the partition (in blocks) is printed on the standard output.
This value is normally used as an argument to the
.BR mkfs (8)
program to specify the size of the partition which will be formatted.
(Older versions of fdisk would do this only if the partition id is
greater than 10, in an attempt to refuse DOS partitions;
this test has been deleted.)
Note that
.B "sfdisk -s"
gives different (namely, correct) answers.
Reasons for the difference are that the kernel and
.B fdisk
need not have the same idea about partition numbering
(e.g., in case you have BSD slices), and have different
ideas about the size of an extended partition.
.SH BUGS
There are several *fdisk programs around.
Each has its problems and strengths.
Try them in the order
.BR cfdisk ,
.BR fdisk ,
.BR sfdisk .
.\" .SH AUTHORS
.\" A. V. Le Blanc (LeBlanc@mcc.ac.uk)
.\" Bernhard Fastenrath (fasten@informatik.uni-bonn.de)
.\" Jakub Jelinek (jj@sunsite.mff.cuni.cz)
.\" and many others.