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In Linux, partitions are represented by device files. A device file is a file with type c (for "character" devices, devices that do not use the buffer cache) or b (for "block" devices, which go through the buffer cache). In Linux, all disks are represented as block devices only. Unlike other Unices, Linux does not offer "raw" character versions of disks and their partitions.
The only important thing with a device file are its major and minor device number, shown instead of the files size:
$ ls -l /dev/hda
brw-rw---- 1 root disk 3, 0 Jul 18 1994 /dev/hda
^ ^
| minor device number
major device number
When accessing a device file, the major number selects which
device driver is being called to perform the input/output
operation. This call is being done with the minor number as a
parameter and it is entirely up to the driver how the minor
number is being interpreted. The driver documentation usually
describes how the driver uses minor numbers. For IDE disks,
this documentation is in Primary partitions on a disk are 1, 2, 3 and 4. So Each partition entry has a starting and an ending block address
assigned to it and a type. The type is a numerical code (a
byte) which designates a particular partition to a certain type
of operating system. For the benefit of computing consultants
partition type codes are not really unique, so there is always
the probability of two operating systems using the same type
code.
Linux reserves the type code 0x82 for swap partitions and 0x83
for "native" file systems (that's ext2 for almost all of you).
The once popular, now outdated Linux/Minix file system used the
type code 0x81 for partitions. OS/2 marks it's partitions with
a 0x07 type and so does Windows NT's NTFS. MS-DOS allocates
several type codes for its various flavors of FAT file systems:
0x01, 0x04 and 0x06 are known. DR-DOS used 0x81 to indicate
protected FAT partitions, creating a type clash with Linux/Minix
at that time, but neither Linux/Minix nor DR-DOS are widely
used any more. The extended partition which is used as a
container for logical partitions has a type of 0x05, by the way.
Partitions are created and deleted with the
/usr/src/linux/Documentation/ide.txt
.
For SCSI disks, one would expect such documentation in
/usr/src/linux/Documentation/scsi.txt
, but it isn't there. One
has to look at the driver source to be sure
(/usr/src/linux/driver/scsi/sd.c:184-196
). Fortunately, there
is Peter Anvin's list of device numbers and names in
/usr/src/linux/Documentation/devices.txt
; see the entries for
block devices, major 3, 22, 33, 34 for IDE and major 8 for SCSI
disks. The major and minor numbers are a byte each and that is
why the number of partitions per disk is limited.
device files have certain names and many system
programs have knowledge about these names compiled in. They
expect your IDE disks to be named /dev/hd*
and your SCSI disks
to be named /dev/sd*
. Disks are numbered a, b, c and so on, so
/dev/hda
is your first IDE disk and /dev/sda
is your first SCSI
disk. Both devices represent entire disks, starting at block
one. Writing to these devices with the wrong tools will
destroy the master boot loader and partition table on these
disks, rendering all data on this disk unusable or making your
system unbootable. Know what you are doing and, again, back up
before you do it.
/dev/hda1
is
the first primary partition on the first IDE disk and so on.
Logical partitions have numbers 5 and up, so /dev/sdb5
is the
first logical partition on the second SCSI disk.
fdisk
program.
Every self respecting operating system program comes with an
fdisk
and traditionally it is even called fdisk
(or FDISK.EXE
)
in almost all OSes. Some fdisk
s, noteable the DOS one, are
somehow limited when they have to deal with other operating
systems partitions. Such limitations include the complete
inability to deal with anything with a foreign type code, the
inability to deal with cylinder numbers above 1024 and the
inability to create or even understand partitions that do not
end on a cylinder boundary. For example, the MS-DOS fdisk can't
delete NTFS partitions, the OS/2 fdisk has been known to
silently "correct" partitions created by the Linux fdisk that
do not end on a cylinder boundary and both, the DOS and the
OS/2 fdisk, have had problems with disks with more than 1024
cylinders (see the "large-disk" Mini-Howto for details on such
disks).