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<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN">
<HTML
><HEAD
><TITLE
>Partitions</TITLE
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><HR
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN876"
>Partitions</A
></H1
><P
>A hard disk can be divided into several
<I
CLASS="GLOSSTERM"
>partitions</I
>. Each partition functions as if
it were a separate hard disk. The idea is that if you have one
hard disk, and want to have, say, two operating systems on it,
you can divide the disk into two partitions. Each operating
system uses its partition as it wishes and doesn't touch the
other one's. This way the two operating systems can co-exist
peacefully on the same hard disk. Without partitions one would
have to buy a hard disk for each operating system.</P
><P
>Floppies are not partitioned. There is no technical reason
against this, but since they're so small, partitions would be
useful only very rarely. CD-ROM's are usually also not
partitioned, since it's easier to use them as one big
disk, and there is seldom a need to have several operating
systems on one.</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN881"
>The MBR, boot sectors and partition table</A
></H2
><P
>The information about how a hard disk has been partitioned
is stored in its first sector (that is, the first sector of the
first track on the first disk surface). The first sector is the
<I
CLASS="GLOSSTERM"
>master boot record</I
> (MBR) of the disk; this
is the sector that the BIOS reads in and starts when the machine
is first booted. The master boot record contains a small program
that reads the partition table, checks which partition is active
(that is, marked bootable), and reads the first sector of that
partition, the partition's <I
CLASS="GLOSSTERM"
>boot sector</I
>
(the MBR is also a boot sector, but it has a special status and
therefore a special name). This boot sector contains another
small program that reads the first part of the operating system
stored on that partition (assuming it is bootable), and then
starts it.</P
><P
>The partitioning scheme is not built into the hardware, or
even into the BIOS. It is only a convention that many
operating systems follow. Not all operating systems do follow
it, but they are the exceptions. Some operating
systems support partitions, but they occupy one partition on
the hard disk, and use their internal partitioning method
within that partition. The latter type exists peacefully
with other operating systems (including Linux), and does not
require any special measures, but an operating system
that doesn't support partitions cannot co-exist on the same
disk with any other operating system.</P
><P
>As a safety precaution, it is a good idea to write down the
partition table on a piece of paper, so that if it ever corrupts
you don't have to lose all your files. (A bad partition table
can be fixed with <B
CLASS="COMMAND"
>fdisk</B
>). The relevant information
is given by the <B
CLASS="COMMAND"
>fdisk -l</B
> command:
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>fdisk -l /dev/hda</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>Disk /dev/hda: 15 heads, 57 sectors, 790 cylinders</TT
>
<TT
CLASS="COMPUTEROUTPUT"
>Units = cylinders of 855 * 512 bytes</TT
>
<TT
CLASS="COMPUTEROUTPUT"
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
> Device Boot Begin Start End Blocks Id System</TT
>
<TT
CLASS="COMPUTEROUTPUT"
>/dev/hda1 1 1 24 10231+ 82 Linux swap</TT
>
<TT
CLASS="COMPUTEROUTPUT"
>/dev/hda2 25 25 48 10260 83 Linux native</TT
>
<TT
CLASS="COMPUTEROUTPUT"
>/dev/hda3 49 49 408 153900 83 Linux native</TT
>
<TT
CLASS="COMPUTEROUTPUT"
>/dev/hda4 409 409 790 163305 5 Extended</TT
>
<TT
CLASS="COMPUTEROUTPUT"
>/dev/hda5 409 409 744 143611+ 83 Linux native</TT
>
<TT
CLASS="COMPUTEROUTPUT"
>/dev/hda6 745 745 790 19636+ 83 Linux native</TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
> </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN905"
>Extended and logical partitions</A
></H2
><P
>The original partitioning scheme for PC hard disks allowed
only four partitions. This quickly turned out to be too little
in real life, partly because some people want more than four
operating systems (Linux, MS-DOS, OS/2, Minix, FreeBSD, NetBSD, or
Windows/NT, to name a few), but primarily because sometimes it
is a good idea to have several partitions for one
operating system. For example, swap space is usually best put
in its own partition for Linux instead of in the main
Linux partition for reasons of speed (see below).</P
><P
>To overcome this design problem, <I
CLASS="GLOSSTERM"
>extended partitions</I
> were
invented. This trick allows partitioning a <I
CLASS="GLOSSTERM"
>primary
partition</I
> into sub-partitions. The
primary partition thus subdivided is the <I
CLASS="GLOSSTERM"
>extended partition</I
>; the
subpartitions are <I
CLASS="GLOSSTERM"
>logical partitions</I
>. They behave
like primary
<A
NAME="AEN913"
HREF="#FTN.AEN913"
>[1]</A
>
partitions, but are created differently. There is no speed
difference between them.</P
><P
>The partition structure of a hard disk might look like that
in <A
HREF="x876.html#HARD-DISK-LAYOUT"
>Figure 4-2</A
>. The disk is divided into
three primary partitions, the second of which is divided into
two logical partitions. Part of the disk is not partitioned
at all. The disk as a whole and each primary partition has a
boot sector.</P
><DIV
CLASS="FIGURE"
><P
><B
><A
NAME="HARD-DISK-LAYOUT"
>Figure 4-2. A sample hard disk partitioning.</A
></B
></P
><P
><IMG
SRC="hd-layout.gif"></P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN920"
>Partition types</A
></H2
><P
>The partition tables (the one in the MBR, and the ones for
extended partitions) contain one byte per partition that
identifies the type of that partition. This attempts to
identify the operating system that uses the partition, or what
it uses it for. The purpose is to make it possible to avoid
having two operating systems accidentally using the same
partition. However, in reality, operating systems do not
really care about the partition type byte; e.g., Linux
doesn't care at all what it is. Worse, some of them use it
incorrectly; e.g., at least some versions of DR-DOS ignore the
most significant bit of the byte, while others don't.</P
><P
>There is no standardization agency to specify what each byte
value means, but some commonly accepted ones are included in
in <A
HREF="x876.html#PARTITION-IDS"
>Table 4-1</A
>. The same list is
available in the Linux <B
CLASS="COMMAND"
>fdisk</B
> program.</P
><DIV
CLASS="TABLE"
><P
><B
><A
NAME="PARTITION-IDS"
>Table 4-1. Partition types (from the Linux <B
CLASS="COMMAND"
>fdisk</B
> program).</A
></B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>0</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Empty</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>40</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Venix 80286</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>94</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Amoeba BBT</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>1</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>DOS 12-bit FAT</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>51</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Novell?</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>a5</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>BSD/386</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>2</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>XENIX root</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>52</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Microport</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>b7</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>BSDI fs</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>3</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>XENIX usr</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>63</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>GNU HURD</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>b8</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>BSDI swap</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>4</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>DOS 16-bitf <32M</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>64</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Novell</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>c7</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Syrinx</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>5</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Extended</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>75</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>PC/IX</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>db</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>CP/M</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>6</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>DOS 16-bit >=32M</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>80</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Old MINIX</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>e1</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>DOS access</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>7</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>OS/2 HPFS</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>81</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Linux/MINIX</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>e3</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>DOS R/O</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>8</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>AIX</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>82</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Linux swap</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>f2</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>DOS secondary</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>9</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>AIX bootable</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>83</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Linux native</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>ff</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>BBT</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>a</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>OS/2 Boot Manag</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>93</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Amoeba</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
> </TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
> </TD
></TR
></TABLE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1008"
>Partitioning a hard disk</A
></H2
><P
>There are many programs for creating and removing
partitions. Most operating systems have their own, and it
can be a good idea to use each operating system's own, just
in case it does something unusual that the others can't.
Many of the programs are called <B
CLASS="COMMAND"
>fdisk</B
>,
including the Linux one, or variations thereof. Details on
using the Linux <B
CLASS="COMMAND"
>fdisk</B
> are given on its
man page. The <B
CLASS="COMMAND"
>cfdisk</B
> command is similar
to <B
CLASS="COMMAND"
>fdisk</B
>, but has a nicer (full screen)
user interface.</P
><P
>When using IDE disks, the boot partition (the partition
with the bootable kernel image files) must be completely
within the first 1024 cylinders. This is because the disk is
used via the BIOS during boot (before the system goes into
protected mode), and BIOS can't handle more than 1024 cylinders.
It is sometimes possible to use a boot partition that is only
partly within the first 1024 cylinders. This works as long
as all the files that are read with the BIOS are within the
first 1024 cylinders. Since this is difficult to arrange,
it is <I
CLASS="EMPHASIS"
>a very bad idea</I
> to do it; you never know when
a kernel update or disk defragmentation will result in an
unbootable system. Therefore, make sure your boot partition
is completely within the first 1024 cylinders.</P
><P
>Some newer versions of the BIOS and IDE disks can, in fact,
handle disks with more than 1024 cylinders. If you have such
a system, you can forget about the problem; if you aren't quite
sure of it, put it within the first 1024 cylinders.</P
><P
>Each partition should have an even number of sectors,
since the Linux filesystems use a 1 kilobyte block size, i.e.,
two sectors. An odd number of sectors will result in the
last sector being unused. This won't result in any problems,
but it is ugly, and some versions of <B
CLASS="COMMAND"
>fdisk</B
>
will warn about it.</P
><P
>Changing a partition's size usually requires first backing up
everything you want to save from that partition (preferably the
whole disk, just in case), deleting the partition, creating
new partition, then restoring everything to the new partition.
If the partition is growing, you may need to adjust the sizes
(and backup and restore) of the adjoining partitions as well.</P
><P
>Since changing partition sizes is painful, it is preferable to
get the partitions right
the first time, or have an effective and easy to use backup
system. If you're installing from a media that does not require
much human intervention (say, from CD-ROM, as opposed to floppies),
it is often easy to play with different configuration at first.
Since you don't already have data to back up, it is not so
painful to modify partition sizes several times.</P
><P
>There is a program for MS-DOS, called
<B
CLASS="COMMAND"
>fips</B
>, which resizes an MS-DOS partition without
requiring the backup and restore, but for other filesystems it
is still necessary.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN1024"
>Device files and partitions</A
></H2
><P
>Each partition and extended partition has its own
device file. The naming convention for these files is that a
partition's number is appended after the name of the whole disk,
with the convention that 1-4 are primary partitions (regardless
of how many primary partitions there are) and 5-8 are logical
partitions (regardless of within which primary partition
they reside). For example, <TT
CLASS="FILENAME"
>/dev/hda1</TT
>
is the first primary partition on the first IDE hard disk, and
<TT
CLASS="FILENAME"
>/dev/sdb7</TT
> is the third extended partition on
the second SCSI hard disk. The device list in XXX (device list)
gives more information.</P
></DIV
></DIV
><H3
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
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