oldlinux-files/Linux-0.98/Yggdrasil-0.98.3/INSTALL/docs/fdisk

SUBJECT
     This file documents fdisk 0.92 - the version on the root disk is
     0.96, plus some other small modifications.  The documentation
     should be quite close, however.

	fdisk 0.93 was written by A.V. LeBlanc (A.V.LeBlanc@mcc.ac.uk)
		modifications for -l made by Jim Winstead Jr.
					(jwinstea@jarthur.claremont.edu)

TABLE OF CONTENTS:

     WARNINGS
     Introduction
     Disks and how they are described
     Dividing up your disk
     Running fdisk
     Deleting and adding partitions
     Active flags and system types
     Extra commands for experts
     Messages, errors, and warnings


WARNINGS:

  In general, you should not use this 'fdisk' program to create
  partitions for other operating systems, only for Linux.

  Dr. DOS 5.0 and 6.0 has been reported to have problems cooperating
  with Linux, and with this version of fdisk in particular.  This fdisk
  sets the system type to hexadecimal 81.  Dr. DOS seems to confuse
  this with hexadecimal 1, a DOS code.  If you use Dr. DOS, use the
  fdisk command 't' to change the system code of any Linux partitions
  to some number less than hexadecimal 80; I suggest 41 and 42 for
  the moment.

  PARTITIONING a hard disk may DESTROY data which is on that disk
  if you are not careful.  Go slowly, write down a description of
  what you started out with, and always verify before you write.
  See the explanations below for more more information.


INTRODUCTION

In this document we look at the utility 'fdisk', and we try to cover
enough of its general background to help anyone use it.  'fdisk' is
Linux's partition table editor.


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 typically one head on each side
of every platter, and all of 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.  Each of these strips is the part of a cylinder that one
head can read or write without moving, and it is called a 'track'.

Each track is divided into a number of 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.

(If you want to be accurate, 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.  I hope you
are not dealing with such disks, for both our sakes.)

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

     (a) the number of cylinders it contains,
     (b) the number of tracks per cylinder, which is the number of heads,
     (c) the number of sectors per track, and
     (d) the size of each sector.

Some of these numbers will vary, but a typical PC disk might have
about 1000 cylinders, about 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, at least in its present form,
deals mainly with 'sectors', which are 512 bytes each.  fdisk also
reports partition sizes in 'blocks' of 1024 bytes, or 2 sectors, since
you need the size in blocks in order to make a file system using mkfs.
A block is the smallest amount of space which can be set aside for
a file in the current file system.

An operating system, such as Linux or MS-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 primary
partitions is in a fixed place, whereas the address of secondary
partitions is not, and 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've got to copy all the data off of
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 MS-DOS, or running 'mkfs' under Linux; finally you can copy
all of 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 well
what you will need, since it is hard to change it afterwards.

Many people with large disks and recent versions of MS-DOS have their
entire file system on one large partition.  They always 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.
So if you are going to back up your disk anyway, you might as well
copy the data back safely.

MS-DOS and Linux both allow you to access several partitions on a
single disk; on MS-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 60 partitions on a single 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 is presently 64 megabytes.  You may, in other words,
have all of your Linux files in a single partition, or you may
have two, three, or more Linux file systems, and similarly with
MS-DOS.  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 MS-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 that you need not worry about
having to do it yourself whenever you boot the system.

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 megabytes 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.

If you wish to play with Linux, without compiling C programs or using
any large packages (like Emacs, TeX, or the X window system), you need
4 to 5 megabytes of disk space, plus 2 megabytes of RAM.  If you install
the C and C++ compilers, you will need 8 to 10 megabytes of disk space,
plus at least 4 megabytes of RAM: you can compile with 2 megabytes of
RAM plus some swap space, but performance will be poor.  Also, remember
that /tmp, which many programs (compilers and editors) use for scratch
files: you'll need to have a couple of free megabytes on the partition
containing this directory.  No matter how much space you have, it is
never enough.

Are you going to boot Linux up from the hard disk, or will you boot
from a floppy?  If you boot Linux from a hard disk, then the root
of the file system must be one of the four primary partitions.  If you
have an extended partition with logical partitions in it, you can have
only three primary partitions containing data.


RUNNING fdisk

Every operating system, whether MS-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
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
MS-DOS or for any system besides 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 fdisks 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.
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

efdisk responds by printing a list of partitions n the selected drive,
and asks 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
        v   verify the partition table
        w   write table to disk and exit
        x   extra functionality

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
        Device Boot  Begin   Start     End  Blocks   Id  System
     /dev/hda1   *       1      17   20059   10021+   1  DOS 12-bit FAT
     /dev/hda2       71060   71060   83044    5992+   5  Extended
     /dev/hda3   *   20060   20060   71059   25500   81  Linux/MINIX
     /dev/hda5       71061   71061   79559    4249+  82  Linux swap
     /dev/hda6       80002   80018   83044    1734    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.  Note that
the start of data in both DOS partitions is 16 sectors after the
beginning of the partition: this is one reason why you should use DOS's
own fdisk to create DOS partitions.  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 Verify command is useful because

(1)  It warns you if anything is wrong.  ALWAYS give a Verify command
     before writing any changes to disk.  See the section on Messages,
     Errors, and Warnings.

(2)  It reports how many unallocated sectors there are on the disk.

The Quit command is also useful.  fdisk does not actually change any
data on your disk unless you give a Write command.  If you are unhappy
about any changes you may have made, give the Quit command, and your
disk will remain as it was before you ran fdisk.  You can also interrupt
fdisk with CTRL-C.


DELETING AND ADDING PARTITIONS

Deleting a partition is simple.  Give the Delete command.  fdisk asks:

     Partition number (1-6): _

Once you get this far, you must either delete a partition or interrupt
the program with CTRL-C (or whatever your current interrupt character is).
Note:

     (1)  You may delete a nonexistent partition.  You will get a warning
          message.
     (2)  You may delete an extended partition.  This has the side effect
          of deleting all partitions greater than or equal to 5.
     (3)  You may delete a logical partition.  In that case, all partitions
          above it are renumbered at once.  For example, if you delete
          partition 5, then partition 6 becomes known as partition 5, and
          partition 7 as partition 6.

Adding a partition is just a bit more complicated.  Give the New command.
fdisk allows you to

     (1)  create a primary partition, if there is a free slot in the
          primary partition table;
     (2)  create an extended partition if there is a free slot in the
          primary partition table, and if there is no extended partition;
     (3)  create a logical partition if an extended partition exists.

If more than one of these actions is possible, you will be asked to
select Primary, Extended, or Logical, depending on what is currently
permissible.  Before you create a primary or an extended partition,
you are asked what slot it is to have in the table (1-4).

You may not add a primary or an extended partition if the selected slot
in the primary partition table is already occupied: in that case you
simply return to the main menu.  You are not allowed to add a new
primary partition unless there are sectors available outside of the
extended partition.  You are not allowed to add a new logical partition
unless there are sectors available inside the extended partition.

If space is available, you are prompted for the first sector:

     First sector (20060-71059): _

The limits are the lowest and the highest free sectors available in
the appropriate part of the disk.  Not all numbers in this range are
necessarily available: they may fall inside an existing partition.
If you select a sector which is already in use, you are told off
and prompted again for the first sector.  After selecting the
first sector, you are prompted again:

     Last sector (20060-71059): _

The limits are the sector you have chosen as the first sector, and the
highest available sector which is legitimate as the other boundary of
the partition.  In other words, all numbers in the given range are
legitimate, unlike those in the first sector range.

Adding or deleting partitions has no effect unless you subsequently
give the Write command.  Please remember to give the Verify command
first, just before giving the Write command: this is a safety
precaution.  After giving the Write command, you will see this
message:

     The partition table has been altered.
     Please reboot before doing anything else.

Here are some important things to note:

(1)  Before you reboot, you MAY run fdisk again, either to manage another
     disk, or to make additional changes to the same disk, or just to
     check that the changes have been made as you expected.

(2)  NEVER run any of the programs mkfs, mkswap, mount, or swapon before
     you reboot.  It is dangerous to run any programs, but these in
     particular may cause serious damage to any data on your disk,
     including the partition tables themselves.


ACTIVE FLAGS AND SYSTEM TYPES

The active flag is a bit in the partition table entry which marks a
partition as bootable.   This is important to some primary boot
sector programs, which will not boot from an unflagged partition.
Other such programs do not allow more than one partition to be flagged.
Some, like shoelace, ignore the flags completely.  I prefer to flag
all bootable partitions as active so that they stand out on the
menu shoelace displays.  fdisk prints a star after the name of a
partition's device file if its active flag is set.

The Active command changes, or toggles, a partitions active flag.
Give the Active command, and select a partition by number.  If it
was marked inactive, it will be flagged as active; if it was
flagged as active, it will be marked inactive.  You may not set
the active flag on an extended partition, but you may set it on
a logical partition, though the meaning of such a flag is
by no means clear.

The Type command changes the ID number which describes what type
a partition is.  fdisk currently recognises 24 system ID's, in the
sense that it prints a string for each of them, but it allows you
to change any system ID to any other, with the following exceptions:
you may not change any partition to or from the type Extended, and
you may not change a partition whose type is Empty (0) to any other
type.  You may, however, change the type of any data partition to
0, which is equivalent to deleting it.

The new system ID or type code is a hexadecimal number.  There are
two ways of listing the numbers which fdisk recognises: use the
List command, which prints the list, or use the Type command, which,
when it prompts you for the code, says

     Hex code (type L to list codes): _

where the upper case 'L' is used for clarity.  The codes printed are

     1 DOS 12-bit FAT    7 OS/2 HPFS    51 Novell?   81 Linux/MINIX
     2 XENIX             8 AIX          52 CP/M?     82 Linux swap
     3 Old XENIX         9 AIX bootable 63 386/IX    93 Amoeba
     4 DOS 16-bit <32mb  a OPUS?        64 Novell    94 Amoeba BBT
     5 Extended         10 OPUS?        75 PCIX      db CP/M
     6 DOS 16-b >=32mb  40 Venix        80 Old MINIX ff BBT

Some of these numbers are a trifle uncertain.  By default (for the
moment) fdisk uses a type of 81, the type used by the present
MINIX fdisk.  When our new file system is introduced, I shall
change the default to a new value.  The number 81 in any case gives
problems to Dr. DOS 5.0 and 6.0; if you use this software, change the
81 to any other value (except 0, 5, 80, or 85).

The value of 82 for Linux swap partitions is my own invention, and
is intended to give some recognisable distinctiveness to the
partitions when the values are displayed in hexadecimal, as they
are by shoelace.

New active flags and new system type codes are not written to the
disk until you exit from fdisk with the Write command, as described
above, in the section on deleting and adding partitions.


EXTRA COMMANDS FOR EXPERTS

The eXtra command 'x' puts fdisk into 'expert' mode, in which a
slightly different set of commands is available.  The Active, Delete,
List, New, Type, Verify, and 'eXpert' commands are not available in
expert mode.  The commands Write and Quit are available as in
ordinary mode, the Print command is available, but produces output
in a slightly different format, and of course the Menu command prints
the expert menu.  There are several new commands.

(1)  The Return command brings you back to the main menu.

(2)  The Extended command prints the list of table entries which
     point to other tables.  Ordinary users do not need this
     information.  The data is shown as it is stored.  The same
     format is used for the expert Print command.

(3)  The dangerous Begin command allows you to move the start of
     data in a partition away from its beginning.  I believe
     shoelace has difficulties with partitions changed in this
     way.  But other systems create partitions with this format,
     and it is sometimes useful to be able to reproduce it.

(4)  The slightly dangerous Cylinders command allows you to change
     the available number of cylinders.  This will allow you to end
     a partition at the end of a cylinder, if you wish.

(5)  The extremely dangerous Heads and Sectors commands allow you
     to change the number of heads and sectors.  It should not be
     necessary to use these commands, but I understand that there
     are disks whose geometry generic Linux is unable to determine,
     so I have provided the ability to cope with such cases.
     WARNING: If you set either of these numbers to a false value,
     you may lose all data on your disk.

Always, after giving any of the commands Begin, Cylinder, Heads, or
Sectors, you should Return to the main menu and give the Verify
command.