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FDISK
!!!FDISK
NAME
SYNOPSIS
DESCRIPTION
DOS 6.x WARNING
OPTIONS
BUGS
SEE ALSO
----
!!NAME


fdisk - Partition table manipulator for Linux
!!SYNOPSIS


__fdisk [[-u] [[-b sectorsize]__''device''


__fdisk -l [[-u] [[-b sectorsize] [[__''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
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. IDE
disks can have up to 63 partitions, SCSI disks up to 15. 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 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, __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.
!!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 __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
__


__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.)


__-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 __cfdisk__,
__fdisk__, __sfdisk__. (Indeed, __cfdisk__ is a
beautiful program that has strict requirements on the
partition 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
partition 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
noninteractively.)


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), parted(8),
sfdisk(8)
----
13 pages link to fdisk(8):
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Last edited on Tuesday, June 4, 2002 12:31:11 am by "perry"
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