Hard disks can be divided into one or more logical disks called
R 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
R /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.
IDE disks can have up to 63 partitions, SCSI disks up to 15.
See also
R /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.
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