fdisk
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 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
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 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.
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
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.
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)
