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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
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.
fdisk doesn't understand GUID Partition Table (GPT) and it is not
designed for large partitions. In particular case use more advanced
GNU parted(8).
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 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-
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 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
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 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 parti-
tion. For example, if you were using disk to make a DOS partition ta-
ble entry for /dev/hda1, then (after exiting fdisk 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 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.)
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
cylinders.
-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.
BUGS
There are several *fdisk programs around. Each has its problems and
strengths. Try them in the order parted, fdisk, sfdisk.
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
mkfs(8), parted(8), sfdisk(8)
Linux 2.0 11 June 1998 FDISK(8)
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