readme.fdisk

Util-linux 软件包包含许多工具。其中比较重要的是加载、卸载、格式化、分区和管理硬盘驱动器

`fdisk': the Linux partition table editor
=========================================

`fdisk' is the Linux partition table editor.  In this section we
examine this utility and try to describe it thoroughly enough so that
anyone can use it.

* Contents:

* Disks and how they are described.
* Dividing up your disk.
* The `fdisk' command.
* Deleting and adding partitions.
* Active flags and system types.
* Extra commands for experts.
* Warnings for `fdisk' users.


Disks and how they are described
--------------------------------

A typical disk consists physically of one or more circular objects
called "platters", which rotate about a central axis.  Devices called
"heads" move to specified places on the disk surface to read or write
information.  There is usually one head on each side of every platter,
and all these heads are attached to a comb-like controller arm which
moves all of them at the same time, either closer to the centre of the
disk, or closer to the outer edge.

Suppose the arm is in one position, putting an area of the disk
surface within reach of one or another of the heads.  This total area,
everything that is accessible without moving the arm, is called a
"cylinder".  (A cylinder is a barrel-shaped cross section of a disk,
consisting of a circular strip from each side of each platter.)  The
part of a cylinder that one head can read or write without moving is
called a "track".

Each track is divided into several pie-shaped slices called
"sectors", which are the smallest parts of the disk which can be read
or written at a time.  The sectors on one disk are usually all the same
size.

In fact, there are not always two heads to every platter, there are
some disks which do not have the same amount of data in every cylinder,
and there may be disks which do not have the same amount of data in
every sector.  These features are usually hidden on PCs by the
controller card or the BIOS, which map the physical geometry of a disk
onto a logical geometry, which is what is actually used to access the
disk.

The numbers which describe the "geometry" of a disk are

  1. The number of cylinders it contains.

  2. The number of tracks per cylinder, which is the number of heads.

  3. The number of sectors per track.

  4. The number of bytes per sector.

These numbers vary from disk to disk, but a typical PC disk might
have about 1000 cylinders, half a dozen heads, and 15 or 20 sectors per
track, with each sector containing 512 bytes or characters; such a disk
contains 40 to 60 megabytes of data.  A "double density" floppy disk
contains 40 cylinders, with 2 heads (2 tracks per cylinder), and with 9
sectors per track; such a disk contains 360 kilobytes, or 360 * 1024
characters.  A "high density" 3.5 inch floppy contains 80 cylinders,
with 2 heads and 18 sectors per track, or 1.44 megabytes, or 1440 *
1024 characters.

The exact size of a track or cylinder in bytes varies from one disk
to another.  This `fdisk' for Linux deals mainly with cylinders, since
this is the best unit to use when allocating space for partitions.  It
reports partition sizes in "blocks" of 1024 bytes, or 2 sectors, since
`mkswap' and the various `mkfs' programs require this number.  A block
is the smallest amount of space which can be set aside for a file in
the current file systems.

An operating system, such as Linux or DOS or OS/2, may use a disk in
any way that it wishes, but if two operating systems share the same
disk, they must agree on who owns what, or else one will interfere with
the other (that is, by damaging the other's files).  A "partition" is a
section of a hard disk which is handled as a unit by all operating
systems which can access the disk.  The standard way to define
partitions (for the moment) is the "partition table", a list of
information which is stored in parts of the disk that don't belong to
any of the systems using the disk.  The beginning of the partition
table is stored in the disk's primary boot sector, and the rest is
stored in a chain of sectors scattered throughout the disk.

The first sector on the disk is called the "primary boot block" or
"primary boot sector" because (1) it comes first, before other, similar
sectors; (2) it tells where the other, similar sectors are found, so
that it is logically `prior' to them; and (3) it usually contains code
which is executed when the system boots up.  This sector contains a
table describing at most four partitions.  These areas are called
"primary partitions".

The partition table in the primary boot sector may also describe at
most one "extended partition".  This is a large area of the disk,
usually containing all the space which is not in any primary partition.
Within this space we can set aside other areas which are called
"logical partitions", because they look almost exactly like primary
partitions.  In fact, the main difference between them is that we can
boot from primary partitions, while we cannot boot from logical
partitions.  This happens because the address of a primary partition is
in a fixed place, whereas the address of a secondary partition is not,
so we require a more complicated process to discover it, one which is
too difficult for most primary boot programs.


Dividing up your disk
---------------------

It is a good idea to plan ahead before you start creating partitions
on your disk.  If you set aside a partition for some purpose, it is not
easy to change its size: you must backup all the data from the partition,
whether to floppies, to another partition, to another hard disk, or
somewhere else; then you must edit the table which describes this
partition, so changing its size; then you must reboot and initialise
the new partition, formatting it, for example, under DOS, or running
`mkfs' under Linux; finally you can copy all the data back.  It is
possible, if you have several partitions, to copy data back and forth
between them while you change their sizes, but this is a bit risky and
time consuming.  It is better to plan ahead what you will need, since
it is hard to change it afterwards.

Many people with large disks and recent versions of DOS have their
entire file system on one large partition.  They usually ask, `Isn't
there any way I can reformat my disk without copying everything off?'
There is no way to do it using standard DOS utilities, and there is no
truly safe way to do it using commercial software, because, if you make
a mistake, you will lose the entire contents of your disk.  If you are
going to back up your disk anyway, you might as well copy the data back
safely.  The Linux FAQ contains references to tools and procedures
which will allow you to do this, if you dare.

DOS and Linux both allow you to access several partitions on a
single disk; on DOS these are treated as if they were separate disks or
drives, and under Linux they are treated as different "devices".

You can have up to 64 partitions on a single IDE disk, or up to 16
partitions on a single SCSI disk, at least as far as Linux is
concerned; in practice you will rarely want so many.  The maximum size
of a Linux file system on a single partition depends on the type of
file system you use.  Minix file systems are limited to 64 megabytes.
You may have all of your Linux files in a single partition, or you may
have two, three, or more Linux file systems.  Similarly you may have
one or more DOS partitions.  If you have several small partitions, you
run much less risk of losing all your files if your disk gets
corrupted.  On the other hand, you may run out of space on a small
partition more easily.

Under DOS, you must refer to each partition by a separate drive
letter, but all partitions are automatically accessible.  Under Linux
only the root partition is automatically accessible, but once we mount
another partition, it is indistinguishable from the rest of the file
system.  Disks are usually mounted by a command in one of the system
startup files, `/etc/rc', so you need not worry about having to do it
yourself whenever you boot the system.  But even ordinary users may
be allowed to mount removable hard disks and floppy disks.

Linux requires at least one partition, which is the `root' of the
file system.  You may prefer to have a separate partition for `/usr',
which contains most of the executable files, or for `/home', which
contains most of your private files.  You may also wish to set aside a
partition to use for swap space, depending on the amount of memory your
PC has.  You will certainly need swap space if you have less than 4 MB
of RAM and wish to compile anything substantial.  You can reserve swap
space in a file, but you need a partition big enough to hold it, and
this will probably be less efficient than having a partition devoted to
swap.

The disk space you need for Linux is discussed in README.prepare.

Are you going to boot Linux from the hard disk, or will you boot
from a floppy?  Some boot programs place severe restrictions on where
the boot partition can be.  LILO is more relaxed about this, but does
require either the Master Boot Record on your first hard disk, or the
boot record on one of the first four partitions on your first hard disk.

If you have an extended partition with logical partitions in it, you
can have only three primary partitions containing data.


The `fdisk' command
-------------------

Every operating system, whether DOS, OS/2, or Linux, should provide
its own utility for editing hard disk partition tables.  At least four
of these utilities have been called `fdisk', for `Fixed DISK setup
program', where `fixed' means `not removable'.  I believe the first PC
program named `fdisk' came from Microsoft in about 1985; before that
time disks were too small to divide into separate sections.

Every operating system has its own peculiarities.  Normally you
should set up a partition for the use of one operating system by using
its own `fdisk' program.  Do not use the Linux `fdisk' to create
partitions for DOS or for any system other than Linux; otherwise you
may have problems.

An `fdisk' program performs two functions: it reports how the disk is
configured, and it changes that configuration by adding or deleting
partitions.  Most `fdisk' programs can also change other information in
partition tables.

This `fdisk' for Linux operates on one hard disk at a time.  If you
give the command

     fdisk

it reports on, and is able to change, `/dev/hda', the first hard
disk.  (If you have no `/dev/hda', `fdisk' uses `/dev/sda' as the
default device.) To look at or change the second hard disk, `/dev/hdb',
give the command

     fdisk /dev/hdb

To look at or change the first SCSI disk, give the command

     fdisk /dev/sda

There are some special forms of the `fdisk' command.  One of them,
suggested by Jim Winstead, simply lists all partitions on all available
disks:

     fdisk -l     (where `l' is a letter, not the digit `1')

The option `-v' is provided to list the current version of the
`fdisk' command.  Finally, there is an option `-s' which is not really
intended for interactive use.  It causes fdisk to print the size of a
partition in blocks of 1024 bytes as follows:

     fdisk -s /dev/hda7
     39934

Because this is intended to be used by `mkfs' and `mkswap' programs,
it does not return the size of extended partitions or of partitions
whose system type code is less than 10 (hexadecimal a).  If you start
`fdisk' without using one of these special options, it responds by
asking for a command:

     Command (m for help): _

Each `fdisk' command consists of a single letter, which must be
followed by <RETURN> before it is obeyed.  Upper and lower case are not
distinguished.  Anything you type after the first character is ignored.
Give the command `m', and you should see this menu:
      Command action
         a   toggle a bootable flag
         d   delete a partition
         l   list known partition types
         m   print this menu
         n   add a new partition
         p   print the partition table
         q   quit without saving changes
         t   change a partition's system id
         u   change display/entry units
         v   verify the partition table
         w   write table to disk and exit
         x   extra functionality (experts only)
     
      Command (m for help): _

The simplest commands are Print, Verify, and Quit.  On a small disk, the
Print command might produce a display like this one:

     Disk /dev/hda: 5 heads, 17 sectors, 977 cylinders
     Units = cylinders of 85 * 512 bytes
     
        Device Boot  Begin   Start     End  Blocks   Id  System
     /dev/hda1   *       1       1     236   10021+   1  DOS 12-bit FAT
     /dev/hda2         837     837     977    5992+   5  Extended
     /dev/hda3   *     237     237     836   25500   83  Linux native
     /dev/hda5         837     837     936    4249+  82  Linux swap
     /dev/hda6         942     942     977    1522    1  DOS 12-bit FAT

There are 5 partitions reported; `/dev/hda4' does not appear because
it is not allocated.  Partitions 1 and 3 are flagged as bootable.  The
size of each partition is reported in 1 kilobyte blocks; hence the
primary Linux partition, partition 3, is 25 1/2 megabytes in size.  The
`+' after three of the sizes warns that these partitions contain an odd
number of sectors: Linux normally allocates filespace in 1 kilobyte
blocks, so the extra sector in partition 5 is wasted.  Id numbers are
reported in hexadecimal and explained in English.

The display/entry units may be either cylinders or sectors.  The
default is cylinders, but changing the units makes the print command
display the following table for the system reported above:

     Disk /dev/hda: 5 heads, 17 sectors, 977 cylinders
     Units = sectors of 1 * 512 bytes