FDISK - phpMan

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 com-
patible 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 ker-
nel 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 par-
titions.

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 sec-
tor (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 correspond-
ing 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 neces-
sarily the physical disk geometry (indeed, modern disks do not really have anything
like a physical geometry, certainly not something that can be described in simplis-
tic 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 sys-
tems, 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 any-
more - 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 con-
sider 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 ta-
ble 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
disk to make a DOS partition table 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 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 num-
ber, 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 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.