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<chapter id="disk-usage">
<title>Using Disks and Other Storage Media</title>
<blockquote><para><quote>On a clear disk you can seek forever.
</quote></para></blockquote>
<!--
% the following metas need too much work for the next version
%
% \meta copying a directory/disk verbatim
%
% \meta disaster recovery: program to scan for ext2 superblocks
%
% \meta explain lost+found; how to fix a filesystem; what to do when
% there is a bad block; identifying the file that has the bad block
%
% \meta chart that shows characteristics of various fs: max size,
% max file size, usable as root, max name length, speed, support
%
% \meta
% Recovering from a bad MBR or super block.
% Manually remounting (ro->w, rw->ro, when, why)
% automounting
% MD patches
% Why does Linux read/write disk in background?
% how to mount a dos disk so that everyone can access it?
% supermount
% ide disks map away bad sectors (until they're too many, then
% use badblocks)
% mounting: mountee root becomes mount point, e.g.
permissions/ownership
-->
<para>When you install or upgrade your system, you need to do a
fair amount of work on your disks<indexterm id="ch05-disks">
<primary>disks</primary></indexterm>. You have to make filesystems on
your disks so that files can be stored on them and reserve
space for the different parts of your system.</para>
<para>This chapter explains all these initial activities. Usually,
once you get your system set up, you won't have to go through the
work again, except for using floppies. You'll need to come back to
this chapter if you add a new disk or want to fine-tune your disk
usage.<para>
<para>The basic tasks in administering disks are:
<itemizedlist>
<listitem><para>
Format your disk. This does various things to prepare it for use,
such as checking for bad sectors. (Formatting is nowadays
not necessary for most hard disks.)</para></listitem>
<listitem><para>
Partition a hard disk, if you want to use it for several activities
that aren't supposed to interfere with one another. One reason for
partitioning is to store different operating systems on the same
disk. Another reason is to keep user files separate from system
files, which simplifies back-ups and helps protect the system files
from corruption.
</para></listitem>
<listitem><para>
Make a filesystem (of a suitable type) on each disk or partition.
The disk means nothing to Linux until you make a filesystem; then
files can be created and accessed on it.
</para></listitem>
<listitem><para>
Mount different filesystems to form a single tree structure, either
automatically, or manually as needed. (Manually mounted filesystems
usually need to be unmounted manually as well.)
</para></listitem>
</itemizedlist>
<para><xref linkend="memory-management"> contains information
about virtual memory and disk caching, of which you also need
to be aware when using disks.</para>
<sect1 id="block-char-dev">
<title>Two kinds of devices</title>
<para>UNIX, and therefore Linux, recognizes two different
kinds of device: random-access block devices (such as disks),
<indexterm id="ch05-block"><primary>devices</primary>
<secondary>block</secondary></indexterm> and
character devices (such as tapes and serial lines)
<indexterm id="ch05-char"><primary>devices</primary>
<secondary>character</secondary></indexterm>, some of which
may be serial, and some random-access. Each supported device is
represented in the filesystem as a <glossterm>device file</glossterm>.
When you read or write a device file, the data
comes from or goes to the device it represents. This way no special
programs (and no special application programming methodology, such
as catching interrupts or polling a serial port) are necessary to
access devices; for example, to send a file to the printer, one
could just say
<screen>
<prompt>$</prompt> <userinput>cat filename &gt; /dev/lp1</userinput>
<prompt>$</prompt>
</screen>
and the contents of the file are printed (the file must, of course,
be in a form that the printer understands). However, since it is
not a good idea to have several people cat their files to the
printer at the same time, one usually uses a special program to send
the files to be printed (usually <command>lpr</command>
<indexterm id="ch05-lpr"><primary>commands</primary>
<secondary>lpr</secondary></indexterm>). This
program makes sure that only one file is being printed at a time,
and will automatically send files to the printer as soon as it
finishes with the previous file. Something similar is needed for
most devices. In fact, one seldom needs to worry
about device files at all.</para>
<para>Since devices show up as files in the filesystem (in the
<filename>/dev</filename><indexterm id="ch05-dev">
<primary>filesystem</primary><secondary>/dev</secondary></indexterm>
directory), it is easy to see just what
device files exist, using <command>ls</command><indexterm id="ch05-ls">
<primary>commands</primary><secondary>ls</secondary></indexterm> or another suitable
command. In the output of <command>ls -l</command>, the first
column contains the type of the file and its permissions. For
example, inspecting a serial device might give
<screen>
<prompt>$</prompt> <userinput>ls -l /dev/ttyS0</userinput>
<computeroutput>
crw-rw-r-- 1 root dialout 4, 64 Aug 19 18:56 /dev/ttyS0
</computeroutput>
<prompt>$</prompt>
</screen>
The first character in the first column, i.e.,
`<literal>c</literal>' in <literal>crw-rw-rw-</literal> above, tells
an informed user the type of the file, in this case a character
device. For ordinary files, the first character is
`<literal>-</literal>', for directories it is
`<literal>d</literal>', and for block devices
`<literal>b</literal>'; see the <command>ls</command> man page
for further information.</para>
<para>Note that usually all device files exist even though the
device itself might be not be installed. So just because you have a
file <filename>/dev/sda</filename><indexterm id="ch05-sda">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/sda</tertiary></indexterm>, it doesn't mean that you really
do have an SCSI hard disk. Having all the device files makes the
installation programs simpler, and makes it easier to add new
hardware (there is no need to find out the correct parameters
for and create the device files for the new device).</para>
</sect1>
<sect1 id="hard-disk">
<title>Hard disks</title>
<para>This subsection introduces terminology related to hard
disks. If you already know the terms and concepts, you can skip
this subsection.</para>
<para>See <xref linkend="hd-schematic"> for a schematic picture
of the important parts in a hard disk. A hard disk consists of one
or more circular aluminum <glossterm>platters</glossterm>\
<indexterm id="ch05-disk-internals">
<primary>disks</primary><secondary>components</secondary></indexterm>,
of which either or both <glossterm>surfaces</glossterm> are coated
with a magnetic substance used for recording the data. For each
surface, there is a <glossterm>read-write head</glossterm> that
examines or alters the recorded data. The platters rotate on a
common axis; typical rotation speed is 5400 or 7200 rotations per
minute, although high-performance hard disks have higher speeds and
older disks may have lower speeds. The heads move along the radius
of the platters; this movement combined with the rotation of the
platters allows the head to access all parts of the surfaces.</para>
<para>The processor (CPU)<indexterm id="ch05-cpu">
<primary>hardware</primary><secondary>Central Processing Unit (CPU)</secondary>
</indexterm> and the actual disk communicate through a
<glossterm>disk controller</glossterm><indexterm id="ch05-disk-cont">
<primary>hardware</primary><secondary>disk controller</secondary>
</indexterm>
. This relieves the rest of
the computer from knowing how to use the drive, since the
controllers for different types of disks can be made to use the same
interface towards the rest of the computer. Therefore, the computer
can say just ``hey disk, give me what I want'', instead of a long
and complex series of electric signals to move the head to the
proper location and waiting for the correct position to come under
the head and doing all the other unpleasant stuff necessary. (In
reality, the interface to the controller is still complex, but much
less so than it would otherwise be.) The controller may also do
other things, such as caching, or automatic bad sector
replacement.</para>
<para>The above is usually all one needs to understand about the
hardware. There are also other things, such as the motor that
rotates the platters and moves the heads, and the electronics that
control the operation of the mechanical parts, but they are mostly
not relevant for understanding the working principles of a hard
disk.</para>
<para>The surfaces are usually divided into concentric rings,
called <glossterm>tracks</glossterm><indexterm id="ch05-tracks">
<primary>disks</primary><secondary>tracks</secondary>
</indexterm>, and these in turn are divided
into <glossterm>sectors</glossterm><indexterm id="ch05-sectors">
<primary>disks</primary><secondary>sectors</secondary>
</indexterm>. This division is used to
specify locations on the hard disk and to allocate disk space to
files. To find a given place on the hard disk, one might say
``surface 3, track 5, sector 7''. Usually the number of sectors is
the same for all tracks, but some hard disks put more sectors in
outer tracks (all sectors are of the same physical size, so more of
them fit in the longer outer tracks). Typically, a sector will hold
512 bytes of data. The disk itself
can't handle smaller amounts of data than one sector.</para>
<figure id="hd-schematic" float="1">
<title>A schematic picture of a hard disk.</title>
<graphic fileref="hd-schematic.png">
</figure>
<para>Each surface is divided into tracks (and sectors) in
the same way. This means that when the head for one surface is on a
track, the heads for the other surfaces are also on the
corresponding tracks. All the corresponding tracks taken together
are called a <glossterm>cylinder</glossterm><indexterm id="ch05-cylinders">
<primary>disks</primary><secondary>cylinders</secondary>
</indexterm>. It takes time to
move the heads from one track (cylinder) to another, so by placing
the data that is often accessed together (say, a file) so that it is
within one cylinder, it is not necessary to move the heads to read
all of it. This improves performance. It is not always possible to
place files like this; files that are stored in several places on
the disk are called
<glossterm>fragmented</glossterm>.</para>
<para>The number of surfaces (or heads, which is the same thing),
cylinders, and sectors vary a lot; the specification of the number
of each is called the <glossterm>geometry</glossterm><indexterm id="ch05-geometry">
<primary>disks</primary><secondary>geometry</secondary>
</indexterm> of a hard
disk. The geometry is usually stored in a special, battery-powered
memory location called the <glossterm>CMOS RAM</glossterm>
<indexterm id="ch05-tracks2"><primary>CMOS</primary>
</indexterm>, from
where the operating system can fetch it during bootup or driver
initialization.</para>
<para>Unfortunately, the BIOS<indexterm id="ch05-bios2">
<primary>BIOS</primary></indexterm>
has a design limitation, which makes it impossible to specify a
track number that is larger than 1024 in the CMOS RAM, which is too
little for a large hard disk. To overcome this, the hard disk
controller lies about the geometry<indexterm id="ch05-geometry2">
<primary>disks</primary><secondary>geometry</secondary>
</indexterm>, and <glossterm>translates the
addresses</glossterm> given by the computer into something that fits
reality. For example, a hard disk might have 8 heads, 2048 tracks,
and 35 sectors per track.
Its controller could lie to the computer and claim that it has 16
heads, 1024 tracks, and 35 sectors per track, thus not exceeding the
limit on tracks, and translates the address that the computer gives
it by halving the head number, and doubling the track number. The
mathematics can be more complicated in reality, because the numbers
are not as nice as here (but again, the details are not relevant for
understanding the principle). This translation distorts the
operating system's view of how the disk is organized, thus making it
impractical to use the all-data-on-one-cylinder trick to boost
performance.</para>
<para>The translation is only a problem for IDE disks. SCSI disks
use a sequential sector number (i.e., the controller translates a
sequential sector number to a head, cylinder, and sector triplet),
and a completely different method for the CPU to talk with the
controller, so they are insulated from the problem. Note, however,
that the computer might not know the real geometry of an SCSI disk
either.</para>
<para>Since Linux often will not know the real geometry of a disk,
its filesystems don't even try to keep files within a single
cylinder. Instead, it tries to assign sequentially numbered sectors
to files, which almost always gives similar performance. The issue
is further complicated by on-controller caches, and automatic
prefetches done by the controller.</para>
<para>Each hard disk is represented by a separate device
file. There can (usually) be only two or four IDE hard disks. These
are known as <filename>/dev/hda</filename><indexterm id="ch05-hda">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/hda</tertiary></indexterm>,
<filename>/dev/hdb</filename><indexterm id="ch05-hdb">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/hdb</tertiary></indexterm>,
<filename>/dev/hdc</filename><indexterm id="ch05-hdc">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/hdc</tertiary></indexterm>, and
<filename>/dev/hdd</filename><indexterm id="ch05-hdd">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/hdd</tertiary></indexterm>,
respectively. SCSI hard disks are
known as <filename>/dev/sda</filename><indexterm id="ch05-sda22">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/sda</tertiary></indexterm>,
<filename>/dev/sdb</filename><indexterm id="ch05-sdb">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/sdb</tertiary></indexterm>, and so on. Similar naming
conventions exist for other hard disk types; see <xref
linkend="device-list"> for more information. Note that the device
files for the hard disks give access to the entire disk, with no
regard to partitions (which will be discussed below), and it's easy
to mess up the partitions or the data in them if you aren't careful.
The disks' device files are usually used only to get access to the
master boot record (which will also be discussed below).</para>
</sect1>
<sect1 id="san">
<title>Storage Area Networks<indexterm id="ch05-san-chapt">
<primary>Storage Area Network (SAN)</primary></indexterm> - Draft</title>
<para>A SAN<indexterm id="ch05-san1">
<primary>Storage Area Network (SAN)</primary></indexterm>
is a dedicated storage network that provides block level
access to LUNs. A LUN<indexterm id="ch05-san2">
<primary>Storage Area Network (SAN)</primary><secondary>LUN</secondary>
</indexterm>,
or logical unit number, is a virtual disk
provided by the SAN.
The system administrator the same access and rights to the LUN as if
it were a disk directly attached to it. The administrator can partition, and
format the disk in any means he or she chooses.</para>
<para>Two networking protocols commonly used in a SAN are
<glossterm>fibre channel</glossterm>
<indexterm id="ch05-FC"><primary>hardware</primary><secondary>fibre channel</secondary></indexterm>
<indexterm id="ch05-FC2"><primary>fibre channel</primary></indexterm> and
<glossterm>iSCSI</glossterm>
<indexterm id="ch05-iSCSI"><primary>iSCSI</primary></indexterm>.
A fibre channel network is very fast and is not
burdened by the other network traffic in a company's LAN. However, it's
very expensive. Fibre channel cards cost around $1000.00 USD each. They
also require special fibre channel switches. </para>
<para>iSCSI is a newer technology that sends SCSI commands over a TCP/IP
network. While this method may not be as fast as a Fibre Channel network,
it does save money by using less expensive network hardware.</para>
<para>More To Be Added</para>
</sect1>
<sect1 id="net-attached">
<title>Network Attached Storage<indexterm id="ch05-nas-chapt">
<primary>Network Attached Storage (NAS)</primary></indexterm> - Draft</title>
<para>A NAS<indexterm id="ch05-nas1">
<primary>Network Attached Storage (NAS)</primary></indexterm>
uses your companies existing Ethernet network to allow
access to shared disks. This is filesystem level access.
The system administrator does not have the ability to partition or format
the disks since they are potentially shared by multiple computers.
This technology is commonly used to provide multiple workstations
access to the same data.</para>
<para>Similar to a SAN<indexterm id="ch05-san3">
<primary>Storage Area Network (SAN)</primary></indexterm>,
a NAS need to make use of a protocol to allow access
to it's disks. With a NAS this is either CIFS/Samba
<indexterm id="ch05-cifs2"><primary>Common Internet File System (CIFS)</primary>
</indexterm><indexterm id="ch05-samba2"><primary>Samba</primary>
</indexterm>, or NFS<indexterm id="ch05-nfs2">
<primary>Network File System (NFS)</primary>
</indexterm>.</para>
<para>Traditionally CIFS was used with Microsoft Windows networks, and
NFS was used with UNIX & Linux networks. However, with Samba, Linux
machines can also make use of CIFS shares.</para>
<para>Does this mean that your Windows 2003 server or your Linux box
are NAS servers because they provide access to shared drives over your
network? Yes, they are. You could also purchase a NAS device from a
number of manufacturers. These devices are specifically designed to
provide high speed access to data.</para>
<para>More To Be Added</para>
<sect2 id="net-file-sys">
<title>NFS<indexterm id="ch05-nfs"><primary>Network File System (NFS)</primary>
</indexterm></title>
<para>TO BE ADDED</para>
</sect2>
<sect2 id="cifs">
<title>CIFS<indexterm id="ch05-cifs"><primary>Common Internet File System (CIFS)</primary>
</indexterm><indexterm id="ch05-samba"><primary>Samba</primary>
</indexterm></title>
<para>TO BE ADDED</para>
</sect2>
</sect1>
<sect1 id="floppies">
<title>Floppies</title>
<para>A floppy disk<indexterm id="ch05-floppy"><primary>hardware</primary>
<secondary>floppy disk</secondary></indexterm>
consists of a flexible membrane covered on one
or both sides with similar magnetic substance as a hard disk. The
floppy disk itself doesn't have a read-write head, that is included
in the drive. A floppy corresponds to one platter in a hard disk,
but is removable and one drive can be used to access different
floppies, and the same floppy can be read by many drives, whereas
the hard disk is one indivisible unit.</para>
<para>Like a hard disk, a floppy is divided into tracks and sectors
(and the two corresponding tracks on either side of a floppy
form a cylinder), but there are many fewer of them than on a
hard disk.</para>
<para>A floppy drive can usually use several different types of disks;
for example, a 3.5 inch drive can use both 720 KB and 1.44 MB disks.
Since the drive has to operate a bit differently and the operating
system must know how big the disk is, there are many device files
for floppy drives, one per combination of drive and disk type.
Therefore, <filename>/dev/fd0H1440</filename> is the first floppy
drive (fd0), which must be a 3.5 inch drive, using a 3.5 inch, high
density disk (H) of size 1440 KB (1440), i.e., a normal 3.5 inch HD
floppy.
</para>
<para>The names for floppy drives are complex, however, and Linux
therefore has a special floppy device type that automatically
detects the type of the disk in the drive. It works by trying to
read the first sector of a newly inserted floppy using different
floppy types until it finds the correct one. This naturally requires
that the floppy is formatted first. The automatic devices are called
<filename>/dev/fd0</filename><indexterm id="ch05-fd0">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/fd0</tertiary></indexterm>,
<filename>/dev/fd1</filename><indexterm id="ch05-fd1">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/fd1</tertiary></indexterm>, and so
on.</para>
<para>The parameters the automatic device uses to access a disk can
also be set using the program <command>setfdprm</command>
<indexterm id="ch05-setfdparm"><primary>commands</primary>
<secondary>setfdparm</secondary></indexterm>. This can
be useful if you need to use disks that do not follow any usual
floppy sizes, e.g., if they have an unusual number of sectors, or if
the autodetecting for some reason fails and the proper device file is
missing.</para>
<para>Linux can handle many nonstandard floppy disk formats
in addition to all the standard ones. Some of these require using
special formatting programs. We'll skip these disk types for now,
but in the mean time you can examine the
<filename>/etc/fdprm</filename><indexterm id="ch05-fdprm">
<primary>filesystem</primary><secondary>/etc</secondary>
<tertiary>/etc/fdprm</tertiary></indexterm> file. It specifies the settings
that <command>setfdprm</command><indexterm id="ch05-setfdparm2">
<primary>commands</primary><secondary>setfdparm</secondary>
</indexterm> recognizes.</para>
<para>The operating system must know when a disk has been changed in
a floppy drive, for example, in order to avoid using cached data
from the previous disk. Unfortunately, the signal line that is used
for this is sometimes broken, and worse, this won't always be
noticeable when using the drive from within MS-DOS. If you are
experiencing weird problems using floppies, this might be the
reason. The only way to correct it is to repair the floppy drive.</para>
</sect1>
<sect1 id="cdrom">
<title>CD-ROMs</title>
<para>A CD-ROM<indexterm id="ch05-cdrom"><primary>hardware</primary>
<secondary>CD-ROM</secondary></indexterm> drive uses an
optically read, plastic coated disk.
The information is recorded on the surface of the disk
in small `holes' aligned along a spiral from the center to the edge.
The drive directs a laser beam along the spiral to read the disk.
When the laser hits a hole, the laser is reflected in one way; when
it hits smooth surface, it is reflected in another way. This makes
it easy to code bits, and therefore information. The rest is easy,
mere mechanics.</para>
<para>CD-ROM drives are slow compared to hard disks. Whereas a
typical hard disk will have an average seek time less than 15
milliseconds, a fast CD-ROM drive can use tenths of a second for
seeks. The actual data transfer rate is fairly high at hundreds of
kilobytes per second. The slowness means that CD-ROM drives are not
as pleasant to use as hard disks (some Linux distributions provide
`live' filesystems on CD-ROMs, making it unnecessary to copy the
files to the hard disk, making installation easier and saving a lot
of hard disk space), although it is still possible. For installing
new software, CD-ROMs are very good, since maximum speed is not
essential during installation.</para>
<para>There are several ways to arrange data on a CD-ROM. The most
popular one is specified by the international standard ISO 9660
<indexterm id="ch05-iso9660"><primary>ISO 9660</primary></indexterm>.
This standard specifies a very minimal filesystem, which is even
more crude than the one MS-DOS uses. On the other hand, it is so
minimal that every operating system should be able to map it to its
native system.</para>
<para>For normal UNIX use, the ISO 9660 filesystem is not usable, so
an extension to the standard has been developed, called the Rock
Ridge extension<indexterm id="ch05-rockridge"><primary>ISO 9660</primary>
<secondary>Rock Ridge extensions</secondary></indexterm>. Rock Ridge
allows longer filenames, symbolic
links, and a lot of other goodies, making a CD-ROM look more or less
like any contemporary UNIX filesystem. Even better, a Rock Ridge
filesystem is still a valid ISO 9660 filesystem, making it usable by
non-UNIX systems as well. Linux supports both ISO 9660 and the Rock
Ridge extensions; the extensions are recognized and used
automatically.</para>
<para>The filesystem is only half the battle, however. Most CD-ROMs
contain data that requires a special program to access, and most of
these programs do not run under Linux (except, possibly, under
dosemu, the Linux MS-DOS emulator, or wine, the Windows emulator.
</para>
<para>Ironically perhaps, wine actually stands for ``Wine Is Not an
Emulator''<indexterm id="ch05-wine"><primary>WINE</primary></indexterm>.
Wine, more strictly, is an API (Application Program
Interface) replacement. Please see the wine documentation at
<ulink url="http://www.winehq.com">http://www.winehq.com</ulink>
for more information.</para>
<para>There is also VMWare<indexterm id="ch05-vmware">
<primary>VMWare</primary></indexterm>, a commercial product, which emulates
an entire x86 machine in software. See the VMWare website,
<ulink url="http://www.vmware.com">http://www.vmware.com</ulink>
for more information.</para>
<para>A CD-ROM drive is accessed via the corresponding device file.
There are several ways to connect a CD-ROM drive to the computer:
via SCSI, via a sound card, or via EIDE. The hardware hacking
needed to do this is outside the scope of this book, but the
type of connection decides the device file.</para>
</sect1>
<sect1 id="tapes">
<title>Tapes</title>
<para>A tape drive<indexterm id="ch05-tapes"><primary>hardware</primary>
<secondary>tape drive</secondary></indexterm> uses a tape, similar
to cassettes used for music. A tape is serial in nature, which
means that in order to get to any given part of it, you first have
to go through all the parts in between. A disk can be accessed
randomly, i.e., you can jump directly to any place on the disk.
The serial access of tapes makes them slow.</para>
<para>On the other hand, tapes are relatively cheap to make,
since they do not need to be fast. They can also easily be made
quite long, and can therefore contain a large amount of data. This
makes tapes very suitable for things like archiving and backups,
which do not require large speeds, but benefit from
low costs and large storage capacities.</para>
</sect1>
<sect1 id="formatting">
<title>Formatting</title>
<para><glossterm>Formatting</glossterm><indexterm id="ch05-formatting">
<primary>disks</primary><secondary>formatting</secondary></indexterm>
is the process of writing marks
on the magnetic media that are used to mark tracks and sectors.
Before a disk is formatted, its magnetic surface is a complete mess
of magnetic signals. When it is formatted, some order is brought
into the chaos by essentially drawing lines where the tracks go, and
where they are divided into sectors. The actual details are not
quite exactly like this, but that is irrelevant. What is important
is that a disk cannot be used unless it has been formatted.</para>
<para>The terminology is a bit confusing here: in MS-DOS and MS
Windows, the word formatting is used to cover also the process of
creating a filesystem (which will be discussed below). There, the
two processes are often combined, especially for floppies. When the
distinction needs to be made, the real formatting is called
<glossterm>low-level formatting</glossterm><indexterm id="ch05-formatting2">
<primary>disks</primary><secondary>formatting</secondary>
<tertiary>low-level</tertiary></indexterm>, while making the
filesystem is called <glossterm>high-level formatting</glossterm>
<indexterm id="ch05-formatting333"><primary>disks</primary>
<secondary>formatting</secondary><tertiary>high-level</tertiary>
</indexterm>.
In UNIX circles, the two are called formatting and making a
filesystem, so that's what is used in this book as well.</para>
<para>For IDE and some SCSI disks the formatting is actually
done at the factory and doesn't need to be repeated; hence most
people rarely need to worry about it. In fact, formatting a hard
disk can cause it to work less well, for example because a disk
might need to be formatted in some very special way to
allow automatic bad sector replacement to work.</para>
<para>Disks that need to be or can be formatted often require a
special program anyway, because the interface to the formatting
logic inside the drive is different from drive to drive. The
formatting program is often either on the controller BIOS, or is
supplied as an MS-DOS program; neither of these can easily
be used from within Linux.</para>
<para>During formatting one might encounter bad spots on the
disk, called <glossterm>bad blocks</glossterm><indexterm id="ch05-bblocks">
<primary>disks</primary><secondary>bad blocks</secondary></indexterm>
or <glossterm>bad sectors</glossterm><indexterm id="ch05-bsect">
<primary>disks</primary><secondary>bad sectors</secondary></indexterm>.
These are sometimes handled by the drive
itself, but even then, if more of them develop, something needs to
be done to avoid using those parts of the disk. The logic to do
this is built into the filesystem; how to add the information into
the filesystem is described below. Alternatively, one might create
a small partition that covers just the bad part of the disk; this
approach might be a good idea if the bad spot is very large, since
filesystems can sometimes have trouble with very large bad areas.</para>
<para>Floppies are formatted with <command>fdformat</command>
<indexterm id="ch05-fdformat"><primary>commands</primary>
<secondary>fdformat</secondary></indexterm>. The
floppy device file to use is given as the parameter. For example,
the following command would format a high density, 3.5 inch floppy
in the first floppy drive:
<screen>
<prompt>$</prompt> <userinput>fdformat /dev/fd0H1440</userinput>
<computeroutput>Double-sided, 80 tracks, 18 sec/track. Total capacity
1440 kB.</computeroutput>
<computeroutput>Formatting ... done</computeroutput>
<computeroutput>Verifying ... done</computeroutput>
<prompt>$</prompt>
</screen>
Note that if you want to use an autodetecting device (e.g.,
<filename>/dev/fd0</filename>)<indexterm id="ch05-fd02">
<primary>filesystem</primary><secondary>/dev</secondary>
<tertiary>/dev/fd0</tertiary></indexterm>,
you <emphasis>must</emphasis> set
the parameters of the device with <command>setfdprm</command>
<indexterm id="ch05-setfdprm"><primary>commands</primary>
<secondary>setfdprm</secondary></indexterm> first.
To achieve the same effect as above, one would have to do the
following:
<screen>
<prompt>$</prompt> <userinput>setfdprm /dev/fd0 1440/1440</userinput>
<prompt>$</prompt> <userinput>fdformat /dev/fd0</userinput>
<computeroutput>Double-sided, 80 tracks, 18 sec/track. Total capacity
1440 KB.</computeroutput>
<computeroutput>Formatting ... done</computeroutput>
<computeroutput>Verifying ... done</computeroutput>
<prompt>$</prompt>
</screen>
It is usually more convenient to choose the correct device file that
matches the type of the floppy. Note that it is unwise to format
floppies to contain more information than what they are
designed for.</para>
<para><command>fdformat</command><indexterm id="ch05-fdformat2">
<primary>commands</primary><secondary>fdformat</secondary>
</indexterm>also validate the floppy,
i.e., check it for bad blocks. It will try a bad block several
times (you can usually hear this, the drive noise changes
dramatically). If the floppy is only marginally bad (due to dirt on
the read/write head, some errors are false signals),
<command>fdformat</command> won't complain, but a real error will
abort the validation process. The kernel will print log messages for
each I/O error it finds; these will go to the console or, if
<command>syslog</command><indexterm id="ch05-syslog2">
<primary>commands</primary><secondary>syslog</secondary>
</indexterm> is being used, to the file
<filename>/var/log/messages</filename><indexterm id="ch05-messages">
<primary>logs</primary><secondary>/var/log/messages</secondary>.
<command>fdformat</command>
itself won't tell where the error is (one usually doesn't care,
floppies are cheap enough that a bad one is automatically thrown
away).
<screen>
<prompt>$</prompt> <userinput>fdformat /dev/fd0H1440</userinput>
<computeroutput>Double-sided, 80 tracks, 18 sec/track. Total capacity
1440 KB.</computeroutput>
<computeroutput>Formatting ... done</computeroutput>
<computeroutput>Verifying ... read: Unknown error</computeroutput>
<prompt>$</prompt>
</screen>
The <command>badblocks</command><indexterm id="ch05-badblockcmd">
<primary>commands</primary><secondary>badblocks</secondary>
</indexterm> command can be used to search any
disk or partition for bad blocks (including a floppy). It does not
format the disk, so it can be used to check even existing
filesystems. The example below checks a 3.5 inch floppy with two
bad blocks.
<screen>
<prompt>$</prompt> <userinput>badblocks /dev/fd0H1440 1440</userinput>
<computeroutput>718</computeroutput>
<computeroutput>719</computeroutput>
<prompt>$</prompt>
</screen>
<command>badblocks</command> outputs the block numbers of the bad
blocks it finds. Most filesystems can avoid such bad blocks. They
maintain a list of known bad blocks, which is initialized when the
filesystem is made, and can be modified later. The initial search
for bad blocks can be done by the <command>mkfs</command>
<indexterm id="ch05-mkfs">
<primary>commands</primary><secondary>mkfs</secondary>
</indexterm> command
(which initializes the filesystem), but later checks should be done
with <command>badblocks</command><indexterm id="ch05-badblockscmd2">
<primary>commands</primary><secondary>badblocks</secondary>
</indexterm> and the new blocks should be added
with <command>fsck</command><indexterm id="ch05-fsck">
<primary>commands</primary><secondary>fsck</secondary>
</indexterm>. We'll describe
<command>mkfs</command>
and <command>fsck</command> later.</para>
<para>Many modern disks automatically notice bad blocks, and attempt
to fix them by using a special, reserved good block instead. This is
invisible to the operating system. This feature should be
documented in the disk's manual, if you're curious if it is
happening. Even such disks can fail, if the number of bad blocks
grows too large, although chances are that by then the disk
will be so rotten as to be unusable.</para>
</sect1>
<sect1 id="partitions">
<title>Partitions</title>
<para>A hard disk can be divided into several
<glossterm>partitions</glossterm><indexterm id="ch05-diskpart">
<primary>disks</primary><secondary>partitions</secondary>
</indexterm>. 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 ones. 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.</para>
<para>Floppies are not usually partitioned. There is no technical reason
against this, but since they're so small, partitions would be useful
only very rarely. CD-ROMs 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.</para>
<sect2 id="mbr">
<title>The MBR<indexterm id="ch05-mbr">
<primary>disks</primary><secondary>MBR</secondary>
</indexterm>, boot sectors<indexterm id="ch05-bootsect">
<primary>disks</primary><secondary>boot sectors</secondary>
</indexterm> and partition table<indexterm id="ch05-parttbl">
<primary>disks</primary><secondary>partition table</secondary>
</indexterm></title>
<para>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
<glossterm>master boot record</glossterm> (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 <glossterm>boot sector</glossterm> (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.</para>
<para>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.</para>
<para>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 <command>fdisk</command><indexterm id="ch05-fdisk2">
<primary>commands</primary><secondary>fdisk</secondary></indexterm>).
The relevant information is
given by the <command>fdisk -l</command> command:
<screen>
<prompt>$</prompt> <userinput>fdisk -l /dev/hda</userinput>
<computeroutput></computeroutput>
<computeroutput>Disk /dev/hda: 15 heads, 57 sectors, 790 cylinders</computeroutput>
<computeroutput>Units = cylinders of 855 * 512 bytes</computeroutput>
<computeroutput></computeroutput>
<computeroutput> Device Boot Begin Start End Blocks Id System</computeroutput>
<computeroutput>/dev/hda1 1 1 24 10231+ 82 Linux swap</computeroutput>
<computeroutput>/dev/hda2 25 25 48 10260 83 Linux native</computeroutput>
<computeroutput>/dev/hda3 49 49 408 153900 83 Linux native</computeroutput>
<computeroutput>/dev/hda4 409 409 790 163305 5 Extended</computeroutput>
<computeroutput>/dev/hda5 409 409 744 143611+ 83 Linux native</computeroutput>
<computeroutput>/dev/hda6 745 745 790 19636+ 83 Linux native</computeroutput>
<prompt>$</prompt>
</screen>
</sect2>
<sect2 id="extended-logical-part">
<title>Extended and logical partitions</title>
<para>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).</para>
<para>To overcome this design problem, <glossterm>extended
partitions</glossterm><indexterm id="ch05-extendedpart">
<primary>disks</primary><secondary>extended partition</secondary>
</indexterm> were invented. This trick allows
partitioning a <glossterm>primary partition</glossterm>
<indexterm id="ch05-extendedpart2"><primary>disks</primary>
<secondary>extended partition</secondary></indexterm> into
sub-partitions. The primary partition thus subdivided is the
<glossterm>extended partition</glossterm>; the sub-partitions are
<glossterm>logical partitions</glossterm>. They behave like primary
partitions, but are created differently. There is no speed
difference between them. By using an extended partition you can now
have up to 15 partitions per disk.</para>
<para>The partition structure of a hard disk might look like that
in <xref linkend="hard-disk-layout">. 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.</para>
<figure id="hard-disk-layout" float="1">
<title>A sample hard disk partitioning.</title>
<graphic fileref="hd-layout.png">
</figure>
</sect2>
<sect2 id="part-types">
<title>Partition types</title>
<para>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<indexterm id="ch05-parttype">
<primary>disks</primary><secondary>partition type</secondary>
</indexterm>. 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.</para>
<para>There is no standardization agency to specify what each byte
value means, but as far as Linux is concerned, here is a list of partition
types as per the <command>fdisk</command><indexterm id="ch05-fdisk3">
<primary>commands</primary><secondary>fdisk</secondary>
</indexterm> program.</para>
<screen>
<computeroutput> 0 Empty 1c Hidden Win95 FA 70 DiskSecure Mult bb Boot Wizard hid
1 FAT12 1e Hidden Win95 FA 75 PC/IX be Solaris boot
2 XENIX root 24 NEC DOS 80 Old Minix c1 DRDOS/sec (FAT-
3 XENIX usr 39 Plan 9 81 Minix / old Lin c4 DRDOS/sec (FAT-
4 FAT16 <32M 3c PartitionMagic 82 Linux swap c6 DRDOS/sec (FAT-
5 Extended 40 Venix 80286 83 Linux c7 Syrinx
6 FAT16 41 PPC PReP Boot 84 OS/2 hidden C: da Non-FS data
7 HPFS/NTFS 42 SFS 85 Linux extended db CP/M / CTOS / .
8 AIX 4d QNX4.x 86 NTFS volume set de Dell Utility
9 AIX bootable 4e QNX4.x 2nd part 87 NTFS volume set df BootIt
a OS/2 Boot Manag 4f QNX4.x 3rd part 8e Linux LVM e1 DOS access
b Win95 FAT32 50 OnTrack DM 93 Amoeba e3 DOS R/O
c Win95 FAT32 (LB 51 OnTrack DM6 Aux 94 Amoeba BBT e4 SpeedStor
e Win95 FAT16 (LB 52 CP/M 9f BSD/OS eb BeOS fs
f Win95 Ext'd (LB 53 OnTrack DM6 Aux a0 IBM Thinkpad hi ee EFI GPT
10 OPUS 54 OnTrackDM6 a5 FreeBSD ef EFI (FAT-12/16/
11 Hidden FAT12 55 EZ-Drive a6 OpenBSD f0 Linux/PA-RISC b
12 Compaq diagnost 56 Golden Bow a7 NeXTSTEP f1 SpeedStor
14 Hidden FAT16 <3 5c Priam Edisk a8 Darwin UFS f4 SpeedStor
16 Hidden FAT16 61 SpeedStor a9 NetBSD f2 DOS secondary
17 Hidden HPFS/NTF 63 GNU HURD or Sys ab Darwin boot fd Linux raid auto
18 AST SmartSleep 64 Novell Netware b7 BSDI fs fe LANstep
1b Hidden Win95 FA 65 Novell Netware b8 BSDI swap ff BBT
</computeroutput>
</screen>
<indexterm id="pt1"><primary>partition types</primary>
<secondary>FAT32</secondary></indexterm>
<indexterm id="pt2"><primary>partition types</primary>
<secondary>FAT16</secondary></indexterm>
<indexterm id="pt3"><primary>partition types</primary>
<secondary>AIX</secondary></indexterm>
<indexterm id="pt4"><primary>partition types</primary>
<secondary>NTFS</secondary></indexterm>
<indexterm id="pt5"><primary>partition types</primary>
<secondary>HPFS</secondary></indexterm>
<indexterm id="pt6"><primary>partition types</primary>
<secondary>Minix</secondary></indexterm>
<indexterm id="pt7"><primary>partition types</primary>
<secondary>Linux Swap</secondary></indexterm>
<indexterm id="pt8"><primary>partition types</primary>
<secondary>Linux</secondary></indexterm>
<indexterm id="pt9"><primary>partition types</primary>
<secondary>Linux LVM</secondary></indexterm>
<indexterm id="pt10"><primary>partition types</primary>
<secondary>FreeBSD</secondary></indexterm>
<indexterm id="pt11"><primary>partition types</primary>
<secondary>NetBSD</secondary></indexterm>
</sect2>
<sect2 id="part-hd">
<title>Partitioning a hard disk</title>
<para>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 <command>fdisk</command><indexterm id="ch05-fdisk4">
<primary>commands</primary><secondary>fdisk</secondary>
</indexterm>, including the Linux one, or
variations thereof. Details on using the Linux
<command>fdisk</command> given on its man page. The
<command>cfdisk</command><indexterm id="ch05-cfdisk">
<primary>commands</primary><secondary>cfdisk</secondary>
</indexterm> command is similar to
<command>fdisk</command>, but has a nicer (full screen) user
interface.</para>
<para>When using IDE disks<indexterm id="ch05-IDE">
<primary>disks</primary><secondary>IDE</secondary></indexterm>,
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 <emphasis>a very bad idea</emphasis> 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.</para>
<para>However, this may no longer be true with newer versions of
LILO<indexterm id="ch05-lilo"><primary>LILO</primary></indexterm>
that support LBA (Logical Block Addressing)<indexterm id="ch05-lba">
<primary>disks</primary><secondary>Logical Block Addressing (LBA)</secondary>
</indexterm>. Consult the
documentation for your distribution to see if it has a version
of LILO where LBA is supported.</para>
<para>Some newer versions of the BIOS<indexterm id="ch05-bios">
<primary>BIOS</primary></indexterm> 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.</para>
<para>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 <command>fdisk</command> will warn about it.</para>
<para>Changing a partition's size<indexterm id="ch05-partsize">
<primary>disks</primary><secondary>changing partition size</secondary>
</indexterm> 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.</para>
<para>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.</para>
<para>There is a program for MS-DOS, called <command>fips</command>
<indexterm id="ch05-fips"><primary>commands</primary>
<secondary>fips</secondary></indexterm>,
which resizes an MS-DOS partition without requiring the backup and
restore, but for other filesystems it is still necessary.</para>
<para>The <command>fips</command> program is included in most Linux
distributions. The commercial partition manager ``Partition Magic''
also has a similar facility but with a nicer interface. Please do
remember that partitioning is dangerous. Make
<emphasis>sure</emphasis> you have a recent backup of any important
data before you try changing partition sizes ``on the fly''. The
program <command>parted</command><indexterm id="ch05-parted">
<primary>commands</primary><secondary>parted</secondary>
</indexterm> can resize other types of partitions
as well as MS-DOS, but sometimes in a limited manner. Consult the
<command>parted</command> documentation before using it, better safe
than sorry.
</para>
</sect2>
<sect2 id="dev-files-parts">
<title>Device files and partitions</title>
<para>Each partition and extended partition<indexterm id="ch05-extendedpart3">
<primary>disks</primary><secondary>extended partition</secondary>
</indexterm> 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 number greater than 5 are
logical partitions (regardless of within which primary partition
they reside). For example, <filename>/dev/hda1</filename> is the
first primary partition on the first IDE hard disk, and
<filename>/dev/sdb7</filename> is the third extended partition on
the second SCSI hard disk.</para>
</sect2>
</sect1>
<sect1 id="filesystems">
<title>Filesystems</title>
<sect2 id="fs-intro">
<title>What are filesystems?</title>
<para>A <glossterm>filesystem</glossterm><indexterm id="ch05-fs">
<primary>disks</primary><secondary>filesystem</secondary>
</indexterm> is the methods and
data structures that an operating system uses to keep track of files
on a disk or partition; that is, the way the files are organized on
the disk. The word is also used to refer to a partition or disk
that is used to store the files or the type of the filesystem.
Thus, one might say ``I have two filesystems'' meaning one has two
partitions on which one stores files, or that one is using the
``extended filesystem'', meaning the type of the filesystem.</para>
<para>The difference between a disk or partition and the
filesystem it contains is important. A few programs (including,
reasonably enough, programs that create filesystems) operate
directly on the raw sectors of a disk or partition; if there is an
existing file system there it will be destroyed or seriously
corrupted. Most programs operate on a filesystem, and therefore
won't work on a partition that doesn't contain one (or that contains
one of the wrong type).</para>
<para>Before a partition or disk can be used as a filesystem, it
needs to be initialized, and the bookkeeping data structures need to
be written to the disk. This process is called
<glossterm>making a filesystem</glossterm>.</para>
<para>Most UNIX filesystem types have a similar general
structure, although the exact details vary quite a bit. The central
concepts are <glossterm>superblock</glossterm><indexterm id="ch05-sblock">
<primary>disks</primary><secondary>filesystem</secondary>
<tertiary>superblock</tertiary>, <glossterm>inode</glossterm>
<indexterm id="ch05-inode"><primary>disks</primary>
<secondary>filesystem</secondary><tertiary>inode</tertiary>
</indexterm>, <glossterm>data block</glossterm><indexterm id="ch05-dblock">
<primary>disks</primary><secondary>filesystem</secondary>
<tertiary>data block</tertiary></indexterm>,
<glossterm>directory block</glossterm> <indexterm id="ch05-dirblock">
<primary>disks</primary><secondary>filesystem</secondary>
<tertiary>directory block</tertiary></indexterm>, and <glossterm>indirection
block</glossterm><indexterm id="ch05-indirblock"><primary>disks</primary>
<secondary>filesystem</secondary><tertiary>indirection block</tertiary>
</indexterm>. The superblock contains information about the
filesystem as a whole, such as its size (the exact information here
depends on the filesystem). An inode contains all information about
a file, except its name. The name is stored in the directory,
together with the number of the inode. A directory entry consists of
a filename and the number of the inode which represents the file.
The inode contains the numbers of several data blocks, which are
used to store the data in the file. There is space only for a few
data block numbers in the inode, however, and if more are needed,
more space for pointers to the data blocks is allocated dynamically.
These dynamically allocated blocks are indirect blocks; the name
indicates that in order to find the data block, one has to find
its number in the indirect block first.</para>
<para>UNIX filesystems usually allow one to create a
<glossterm>hole</glossterm> in a file (this is done with the
<function>lseek()</function> system call; check the manual page),
which means that the filesystem just pretends that at a particular
place in the file there is just zero bytes, but no actual disk
sectors are reserved for that place in the file (this means that the
file will use a bit less disk space). This happens especially often
for small binaries, Linux shared libraries, some databases, and a
few other special cases. (Holes are implemented by storing a
special value as the address of the data block in the indirect block
or inode. This special address means that no data block is
allocated for that part of the file, ergo, there is a hole in the
file.)</para>
</sect2>
<sect2 id="fs-galore">
<title>Filesystems galore</title>
<para>Linux supports several types of filesystems. As of this
writing the most important ones are:
<glosslist>
<glossentry>
<glossterm>minix<indexterm id="ch05-fs01">
<primary>filesystem types</primary><secondary>minix</secondary>
</indexterm></glossterm>
<glossdef><para>The oldest, presumed to be the most
reliable, but quite limited in features (some time stamps
are missing, at most 30 character filenames) and restricted
in capabilities (at most 64 MB per filesystem).
</para></glossdef></glossentry>
<glossentry>
<glossterm>xia<indexterm id="ch05-fs02">
<primary>filesystem types</primary><secondary>xia</secondary>
</indexterm></glossterm>
<glossdef><para>A modified version of the minix filesystem
that lifts the limits on the filenames and filesystem sizes,
but does not otherwise introduce new features. It is not
very popular, but is reported to work very well.
</para></glossdef></glossentry>
<glossentry>
<glossterm>ext3<indexterm id="ch05-fs03">
<primary>filesystem types</primary><secondary>ext3</secondary>
</indexterm></glossterm>
<glossdef><para>The ext3 filesystem has all the features of
the ext2 filesystem. The difference is, journaling has been
added. This improves performance and recovery time in case
of a system crash. This has become more popular than ext2.
</para></glossdef></glossentry>
<glossentry>
<glossterm>ext2<indexterm id="ch05-fs04">
<primary>filesystem types</primary><secondary>ext2</secondary>
</indexterm></glossterm>
<glossdef><para>The most featureful of the native Linux
filesystems. It is designed to be easily upwards compatible,
so that new versions of the filesystem code do not require
re-making the existing filesystems.</para></glossdef>
</glossentry>
<glossentry>
<glossterm>ext<indexterm id="ch05-fs05">
<primary>filesystem types</primary><secondary>ext</secondary>
</indexterm></glossterm>
<glossdef><para>An older version of ext2 that wasn't upwards
compatible. It is hardly ever used in new installations any
more, and most people have converted to ext2.
</para></glossdef></glossentry>
<glossentry>
<glossterm>reiserfs<indexterm id="ch05-fs06">
<primary>filesystem types</primary><secondary>reiserfs</secondary>
</indexterm></glossterm>
<glossdef><para>A more robust filesystem. Journaling is
used which makes data loss less likely. Journaling is a
mechanism whereby a record is kept of transaction which are
to be performed, or which have been performed. This allows
the filesystem to reconstruct itself fairly easily after
damage caused by, for example, improper
shutdowns.</para></glossdef></glossentry>
<glossentry>
<glossterm>jfs<indexterm id="ch05-fs07">
<primary>filesystem types</primary><secondary>jfs</secondary>
</indexterm></glossterm>
<glossdef><para>JFS is a journaled filesystem designed
by IBM to to work in high performance environments></para>
</glossdef></glossentry>
<glossentry>
<glossterm>xfs<indexterm id="ch05-fs08">
<primary>filesystem types</primary><secondary>xfs</secondary>
</indexterm></glossterm>
<glossdef><para>XFS was originally designed by Silicon Graphics
to work as a 64-bit journaled filesystem. XFS was also designed
to maintain high performance with large files and filesystems.
</para></glossdef></glossentry>
</para>
<para>In addition, support for several foreign filesystems exists,
to make it easier to exchange files with other operating systems.
These foreign filesystems work just like native ones, except that
they may be lacking in some usual UNIX features, or have curious
limitations, or other oddities.
<glosslist>
<glossentry>
<glossterm>msdos<indexterm id="ch05-fs09">
<primary>filesystem types</primary><secondary>msdos</secondary>
</indexterm></glossterm>
<glossdef><para>Compatibility with MS-DOS (and OS/2 and
Windows NT) FAT filesystems.</para></glossdef></glossentry>
<glossentry>
<glossterm>umsdos<indexterm id="ch05-fs10">
<primary>filesystem types</primary><secondary>umsdos</secondary>
</indexterm></glossterm>
<glossdef><para>Extends the msdos filesystem driver under
Linux to get long filenames, owners, permissions, links, and
device files. This allows a normal msdos filesystem to be
used as if it were a Linux one, thus removing the need for a
separate partition for Linux.</para></glossdef></glossentry>
<glossentry>
<glossterm>vfat<indexterm id="ch05-fs11">
<primary>filesystem types</primary><secondary>vfat</secondary>
</indexterm></glossterm>
<glossdef><para>This is an extension of the FAT filesystem
known as FAT32. It supports larger disk sizes than FAT.
Most MS Windows disks are vfat.</para></glossdef>
</glossentry>
<glossentry>
<glossterm>iso9660<indexterm id="ch05-fs12">
<primary>filesystem types</primary><secondary>iso9660</secondary>
</indexterm></glossterm>
<glossdef><para>The standard CD-ROM filesystem; the popular
Rock Ridge extension to the CD-ROM standard that allows
longer file names is supported automatically.
</para></glossdef></glossentry>
<glossentry>
<glossterm>nfs<indexterm id="ch05-fs13">
<primary>filesystem types</primary><secondary>nfs</secondary>
</indexterm></glossterm>
<glossdef><para>A networked filesystem that allows sharing a
filesystem between many computers to allow easy access to
the files from all of them.</para></glossdef></glossentry>
<glossentry>
<glossterm>smbfs<indexterm id="ch05-fs14">
<primary>filesystem types</primary><secondary>smbfs</secondary>
</indexterm></glossterm>
<glossdef><para>A networks filesystem which allows sharing
of a filesystem with an MS Windows computer. It is
compatible with the Windows file sharing protocols.
</para></glossdef></glossentry>
<glossentry>
<glossterm>hpfs<indexterm id="ch05-fs15">
<primary>filesystem types</primary><secondary>hpfs</secondary>
</indexterm></glossterm>
<glossdef><para>The OS/2 filesystem.
</para></glossdef></glossentry>
<glossentry>
<glossterm>sysv<indexterm id="ch05-fs16">
<primary>filesystem types</primary><secondary>sysv</secondary>
</indexterm></glossterm>
<glossdef><para>SystemV/386, Coherent, and Xenix filesystems.
</para></glossdef></glossentry>
<glossentry>
<glossterm>NTFS<indexterm id="ch05-fs17">
<primary>filesystem types</primary><secondary>ntfs</secondary>
</indexterm></glossterm>
<glossdef><para>The most advanced Microsoft journaled filesystem
providing faster file access and stability over previous
Microsoft filesystems.
</para></glossdef></glossentry>
</glosslist>
</para>
<para>The choice of filesystem to use depends on the situation. If
compatibility or other reasons make one of the non-native
filesystems necessary, then that one must be used. If one can
choose freely, then it is probably wisest to use ext3, since it has
all the features of ext2, and is a journaled filesystem. For more
information on filesystems, see <xref linkend="fs-compare">. You
can also read the Filesystems HOWTO located at
<ulink url="http://www.tldp.org/HOWTO/Filesystems-HOWTO.html">
http://www.tldp.org/HOWTO/Filesystems-HOWTO.html</ulink></para>
<para>There is also the proc filesystem, usually accessible as
the <filename>/proc</filename><indexterm id="ch05-proc">
<primary>filesystem</primary><secondary>/proc</secondary>
</indexterm> directory, which is not really a
filesystem at all, even though it looks like one. The proc
filesystem makes it easy to access certain kernel data structures,
such as the process list (hence the name). It makes these data
structures look like a filesystem, and that filesystem can be
manipulated with all the usual file tools. For example, to get a
listing of all processes one might use the command
<screen>
<prompt>$</prompt> <userinput>ls -l /proc</userinput>
<computeroutput>total 0
dr-xr-xr-x 4 root root 0 Jan 31 20:37 1
dr-xr-xr-x 4 liw users 0 Jan 31 20:37 63
dr-xr-xr-x 4 liw users 0 Jan 31 20:37 94
dr-xr-xr-x 4 liw users 0 Jan 31 20:37 95
dr-xr-xr-x 4 root users 0 Jan 31 20:37 98
dr-xr-xr-x 4 liw users 0 Jan 31 20:37 99
-r--r--r-- 1 root root 0 Jan 31 20:37 devices
-r--r--r-- 1 root root 0 Jan 31 20:37 dma
-r--r--r-- 1 root root 0 Jan 31 20:37 filesystems
-r--r--r-- 1 root root 0 Jan 31 20:37 interrupts
-r-------- 1 root root 8654848 Jan 31 20:37 kcore
-r--r--r-- 1 root root 0 Jan 31 11:50 kmsg
-r--r--r-- 1 root root 0 Jan 31 20:37 ksyms
-r--r--r-- 1 root root 0 Jan 31 11:51 loadavg
-r--r--r-- 1 root root 0 Jan 31 20:37 meminfo
-r--r--r-- 1 root root 0 Jan 31 20:37 modules
dr-xr-xr-x 2 root root 0 Jan 31 20:37 net
dr-xr-xr-x 4 root root 0 Jan 31 20:37 self
-r--r--r-- 1 root root 0 Jan 31 20:37 stat
-r--r--r-- 1 root root 0 Jan 31 20:37 uptime
-r--r--r-- 1 root root 0 Jan 31 20:37
version</computeroutput>
<prompt>$</prompt>
</screen>
(There will be a few extra files that don't correspond to
processes, though. The above example has been shortened.)</para>
<para>Note that even though it is called a filesystem, no part of
the proc filesystem touches any disk. It exists only in the
kernel's imagination. Whenever anyone tries to look at any part of
the proc filesystem, the kernel makes it look as if the part existed
somewhere, even though it doesn't. So, even though there is a
multi-megabyte <filename>/proc/kcore</filename><indexterm id="ch05-kcore">
<primary>filesystem</primary><secondary>/proc</secondary>
<tertiary>/proc/kcore</tertiary></indexterm> file, it doesn't
take any disk space. </sect2>
<!-- INDEX -->
<sect2 id="use-which-fs">
<title>Which filesystem should be used?</title>
<para>There is usually little point in using many different
filesystems. Currently, ext3 is the most popular filesystem, because
it is a journaled filesystem. Currently it is probably the wisest
choice. Reiserfs is another popular choice because it to is journaled.
Depending on the overhead for bookkeeping structures, speed, (perceived)
reliability, compatibility, and various other reasons, it may be
advisable to use another file system. This needs to be decided on a
case-by-case basis.</para>
<para> A filesystem that uses journaling is also called a journaled
filesystem. A journaled filesystem maintains a log, or journal, of
what has happened on a filesystem. In the event of a system crash, or
if your 2 year old son hits the power button like mine loves to do, a
journaled filesystem is designed to use the filesystem's logs to recreate
unsaved and lost data. This makes data loss much less likely and
will likely become a standard feature in Linux filesystems. However,
do not get a false sense of security from this. Like everything
else, errors can arise. Always make sure to back up your data in the
event of an emergency.
</para>
<para>See <xref linkend="fs-compare"> for more details about the
features of the different filesystem types.</para>
</sect2>
<sect2 id="create-fs">
<title>Creating a filesystem</title>
<para>Filesystems are created, i.e., initialized, with the
<command>mkfs</command> command. There is actually a separate
program for each filesystem type. <command>mkfs</command> is just a
front end that runs the appropriate program depending on the desired
filesystem type. The type is selected with the
<option>-t fstype</option> option.</para>
<para>The programs called by <command>mkfs</command> have slightly
different command line interfaces. The common and most important
options are summarized below; see the manual pages for more.
<glosslist>
<glossentry>
<glossterm><option>-t fstype</option></glossterm>
<glossdef><para>
Select the type of the filesystem.
</para></glossdef></glossentry>
<glossentry>
<glossterm><option>-c</option></glossterm>
<glossdef><para>
Search for bad blocks and initialize the bad
block list accordingly.
</para></glossdef></glossentry>
<glossentry>
<glossterm>-l filename</glossterm>
<glossdef><para>
Read the initial bad block list from the name file.
</para></glossdef></glossentry>
</glosslist>
</para>
<para>There are also many programs written to add specific options
when creating a specific filesystem. For example
<command>mkfs.ext3</command> adds a <command>-b</command> option to
allow the administrator to specify what block size should be used.
Be sure to find out if there is a specific program available for the
filesystem type you want to use. For more information on determining
what block size to use please see
<xref linkend="fs-block-size">.</para>
<para>To create an ext2 filesystem on a floppy, one would give the
following commands:
<screen>
<prompt>$</prompt> <userinput>fdformat -n /dev/fd0H1440</userinput>
<computeroutput>Double-sided, 80 tracks, 18 sec/track. Total capacity
1440 KB.
Formatting ... done</computeroutput>
<prompt>$</prompt> <userinput>badblocks /dev/fd0H1440 1440 $>$
bad-blocks</userinput>
<prompt>$</prompt> <userinput>mkfs.ext2 -l bad-blocks
/dev/fd0H1440</userinput>
<computeroutput>mke2fs 0.5a, 5-Apr-94 for EXT2 FS 0.5, 94/03/10
360 inodes, 1440 blocks
72 blocks (5.00%) reserved for the super user
First data block=1
Block size=1024 (log=0)
Fragment size=1024 (log=0)
1 block group
8192 blocks per group, 8192 fragments per group
360 inodes per group
Writing inode tables: done
Writing superblocks and filesystem accounting information:
done</computeroutput>
<prompt>$</prompt>
</screen>
First, the floppy was formatted (the <option>-n</option> option
prevents validation, i.e., bad block checking). Then bad blocks
were searched with <command>badblocks</command>, with the output
redirected to a file, <filename>bad-blocks</filename>. Finally, the
filesystem was created, with the bad block list initialized
by whatever <command>badblocks</command> found.</para>
<para>The <option>-c</option> option could have been used with
<command>mkfs</command> instead of <command>badblocks</command>
and a separate file. The example below does that.
<screen>
<prompt>$</prompt> <userinput>mkfs.ext2 -c
/dev/fd0H1440</userinput>
<computeroutput>mke2fs 0.5a, 5-Apr-94 for EXT2 FS 0.5, 94/03/10
360 inodes, 1440 blocks
72 blocks (5.00%) reserved for the super user
First data block=1
Block size=1024 (log=0)
Fragment size=1024 (log=0)
1 block group
8192 blocks per group, 8192 fragments per group
360 inodes per group
Checking for bad blocks (read-only test): done
Writing inode tables: done
Writing superblocks and filesystem accounting information:
done</computeroutput>
<prompt>$</prompt>
</screen>
The <option>-c</option> option is more convenient than a separate
use of <command>badblocks</command>, but
<command>badblocks</command> is necessary for checking
after the filesystem has been created.</para>
<para>The process to prepare filesystems on hard disks or
partitions is the same as for floppies, except that the formatting
isn't needed.</para>
</sect2>
<sect2 id="fs-block-size">
<title>Filesystem block size</title>
<para>The block size specifies size that the filesystem will use
to read and write data. Larger block sizes will help improve disk
I/O performance when using large files, such as databases. This
happens because the disk can read or write data for a longer period
of time before having to search for the next block.</para>
<para>On the downside, if you are going to have a lot of smaller
files on that filesystem, like the <filename>/etc</filename>, there
the potential for a lot of wasted disk space.</para>
<para>For example, if you set your block size to 4096, or 4K, and
you create a file that is 256 bytes in size, it will still consume
4K of space on your harddrive. For one file that may seem trivial,
but when your filesystem contains hundreds or thousands of files,
this can add up.</para>
<para>Block size can also effect the maximum supported file size
on some filesystems. This is because many modern filesystem are
limited not by block size or file size, but by the number of blocks.
Therefore you would be using a
"block size * max # of blocks = max block size" formula.</para>
</sect2>
<sect2 id="fs-compare">
<title>Filesystem comparison</title>
<para>
<table>
<title>Comparing Filesystem Features</title>
<tgroup cols="6">
<colspec colname="FS Name">
<colspec colname="Year">
<colspec colname="Origional OS">
<colspec colname="Max File Size">
<colspec colname="Max FS Size">
<colspec colname="Journaling">
<thead>
<row>
<entry>FS Name</entry>
<entry>Year Introduced</entry>
<entry>Original OS</entry>
<entry>Max File Size</entry>
<entry>Max FS Size</entry>
<entry>Journaling</entry>
</row>
</thead>
<tbody>
<row>
<entry>FAT16<indexterm id="ch05-fst1">
<primary>filesystem types</primary>
<secondary>fat16</secondary></indexterm></entry>
<entry>1983</entry>
<entry>MSDOS V2</entry>
<entry>4GB</entry>
<entry>16MB to 8GB</entry>
<entry>N</entry>
</row>
<row>
<entry>FAT32<indexterm id="ch05-fst2">
<primary>filesystem types</primary>
<secondary>fat32</secondary></indexterm></entry>
<entry>1997</entry>
<entry>Windows 95</entry>
<entry>4GB</entry>
<entry>8GB to 2TB</entry>
<entry>N</entry>
</row>
<row>
<entry>HPFS<indexterm id="ch05-fst3">
<primary>filesystem types</primary>
<secondary>hpfs</secondary></indexterm></entry>
<entry>1988</entry>
<entry>OS/2</entry>
<entry>4GB</entry>
<entry>2TB</entry>
<entry>N</entry>
</row>
<row>
<entry>NTFS<indexterm id="ch05-fst4">
<primary>filesystem types</primary>
<secondary>ntfs</secondary></indexterm></entry>
<entry>1993</entry>
<entry>Windows NT</entry>
<entry>16EB</entry>
<entry>16EB</entry>
<entry>Y</entry>
</row>
<row>
<entry>HFS+<indexterm id="ch05-fst5">
<primary>filesystem types</primary>
<secondary>hfs+</secondary></indexterm></entry>
<entry>1998</entry>
<entry>Mac OS</entry>
<entry>8EB</entry>
<entry>?</entry>
<entry>N</entry>
</row>
<row>
<entry>UFS2<indexterm id="ch05-fst6">
<primary>filesystem types</primary>
<secondary>ufs2</secondary></indexterm>
<entry>2002</entry>
<entry>FreeBSD</entry>
<entry>512GB to 32PB</entry>
<entry>1YB </entry>
<entry>N</entry>
</row>
<row>
<entry>ext2<indexterm id="ch05-fst7">
<primary>filesystem types</primary>
<secondary>ext2</secondary></indexterm></entry>
<entry>1993</entry>
<entry>Linux</entry>
<entry>16GB to 2TB4</entry>
<entry>2TB to 32TB</entry>
<entry>N</entry>
</row>
<row>
<entry>ext3<indexterm id="ch05-fst8">
<primary>filesystem types</primary>
<secondary>ext3</secondary></indexterm></entry>
<entry>1999</entry>
<entry>Linux</entry>
<entry>16GB to 2TB4</entry>
<entry>2TB to 32TB</entry>
<entry>Y</entry>
</row>
<row>
<entry>ReiserFS3<indexterm id="ch05-fst9">
<primary>filesystem types</primary>
<secondary>reiserfs</secondary></indexterm>
<entry>2001</entry>
<entry>Linux</entry>
<entry>8TB8</entry>
<entry>16TB</entry>
<entry>Y</entry>
</row>
<row>
<entry>ReiserFS4<indexterm id="ch05-fst10">
<primary>filesystem types</primary>
<secondary>reiserfs</secondary></indexterm></entry>
<entry>2005</entry>
<entry>Linux</entry>
<entry>?</entry>
<entry>?</entry>
<entry>Y</entry>
</row>
<row>
<entry>XFS<indexterm id="ch05-fst11">
<primary>filesystem types</primary>
<secondary>fat16</secondary></indexterm></entry>
<entry>1994</entry>
<entry>IRIX</entry>
<entry>9EB</entry>
<entry>9EB</entry>
<entry>Y</entry>
</row>
<row>
<entry>JFS<indexterm id="ch05-fst12">
<primary>filesystem types</primary>
<secondary>jfs</secondary></indexterm></entry>
<entry>?</entry>
<entry>AIX</entry>
<entry>8EB</entry>
<entry>512TB to 4PB</entry>
<entry>Y</entry>
</row>
<row>
<entry>VxFS<indexterm id="ch05-fst13">
<primary>filesystem types</primary>
<secondary>vxfs</secondary></indexterm>
<entry>1991</entry>
<entry>SVR4.0</entry>
<entry>16EB</entry>
<entry>?</entry>
<entry>Y</entry>
</row>
<row>
<entry>ZFS<indexterm id="ch05-fst14">
<primary>filesystem types</primary>
<secondary>zfs</secondary></indexterm></entry>
<entry>2004</entry>
<entry>Solaris 10</entry>
<entry>1YB</entry>
<entry>16EB</entry>
<entry>N</entry>
</row>
</tbody>
</tgroup>
</table>
</para>
<para>Legend
<table>
<title>Sizes</title>
<tgroup cols=2>
<tbody>
<row> <entry>Kilobyte - KB</entry> <entry>1024 Bytes</entry> </row>
<row> <entry>Megabyte - MB</entry> <entry>1024 KBs</entry> </row>
<row> <entry>Gigabyte - GB</entry> <entry>1024 MBs</entry> </row>
<row> <entry>Terabyte - TB</entry> <entry>1024 GBs</entry> </row>
<row> <entry>Petabyte - PB</entry> <entry>1024 TBs</entry> </row>
<row> <entry>Exabyte - EB</entry> <entry>1024 PBs</entry> </row>
<row> <entry>Zettabyte - ZB</entry> <entry>1024 EBs</entry> </row>
<row> <entry>Yottabyte - YB</entry> <entry>1024 ZBs</entry> </row>
</tbody>
</tgroup>
</table>
</para>
<para>It should be noted that Exabytes, Zettabytes, and Yottabytes
are rarely encountered, if ever. There is a current estimate that
the worlds printed material is equal to 5 Exabytes. Therefore, some
of these filesystem limitations are considered by many as
theoretical. However, the filesystem software has been written
with these capabilities.</para>
<para>For more detailed information you can visit
<ulink url="http://en.wikipedia.org/wiki/Comparison_of_file_systems">
http://en.wikipedia.org/wiki/Comparison_of_file_systems</ulink>.
</para>
</sect2>
<sect2 id="mount-and-umount">
<title>Mounting and unmounting</title>
<para>Before one can use a filesystem, it has to be
<glossterm>mounted</glossterm>. The operating system then does
various bookkeeping things to make sure that everything works. Since
all files in UNIX are in a single directory tree, the mount
operation will make it look like the contents of the new filesystem
are the contents of an existing subdirectory in some already mounted
filesystem.</para>
<para>For example, <xref linkend="hd-mount-root"> shows three
separate filesystems, each with their own root directory. When the
last two filesystems are mounted below <filename>/home</filename>
and <filename>/usr</filename>, respectively, on the first
filesystem, we can get a single directory tree, as in
<xref linkend="hd-mount-all">.</para>
<figure id="hd-mount-root" float="1">
<title>Three separate filesystems.</title>
<graphic fileref="hd-mount-separate.png">
</figure>
<figure id="hd-mount-all" float="1">
<title><filename>/home</filename> and <filename>/usr</filename>
have been
mounted.</title>
<graphic fileref="hd-mount-mounted.png">
</figure>
<para>The mounts could be done as in the following example:
<screen>
<prompt>$</prompt> <userinput>mount /dev/hda2 /home</userinput>
<prompt>$</prompt> <userinput>mount /dev/hda3 /usr</userinput>
<prompt>$</prompt>
</screen>
The <command>mount</command> command takes two arguments. The first
one is the device file corresponding to the disk or partition
containing the filesystem. The second one is the directory below
which it will be mounted. After these commands the contents of the
two filesystems look just like the contents of the
<filename>/home</filename> and <filename>/usr</filename>
directories, respectively. One would then say that
<filename>/dev/hda2</filename> <glossterm>is mounted
on</glossterm> <filename>/home</filename>'', and similarly for
<filename>/usr</filename>. To look at either filesystem, one would
look at the contents of the directory on which it has been mounted,
just as if it were any other directory. Note the difference between
the device file, <filename>/dev/hda2</filename>, and the mounted-on
directory, <filename>/home</filename>. The device file gives access
to the raw contents of the disk, the mounted-on directory gives
access to the files on the disk. The mounted-on directory is called
the <glossterm>mount point</glossterm>.</para>
<para>Linux supports many filesystem types.
<command>mount</command> tries to guess the type of the filesystem.
You can also use the <option>-t fstype</option> option to specify
the type directly; this is sometimes necessary, since the heuristics
<command>mount</command> uses do not always work. For example, to
mount an MS-DOS floppy, you could use the following command:
<screen>
<prompt>$</prompt> <userinput>mount -t msdos /dev/fd0
/floppy</userinput>
<prompt>$</prompt>
</screen>
</para>
<para>The mounted-on directory need not be empty, although it
must exist. Any files in it, however, will be inaccessible by name
while the filesystem is mounted. (Any files that have already been
opened will still be accessible. Files that have hard links from
other directories can be accessed using those names.) There is no
harm done with this, and it can even be useful. For instance, some
people like to have <filename>/tmp</filename> and
<filename>/var/tmp</filename> synonymous, and make
<filename>/tmp</filename> be a symbolic link to
<filename>/var/tmp</filename>. When the system is booted, before
the <filename>/var</filename> filesystem is mounted, a
<filename>/var/tmp</filename> directory residing on the root
filesystem is used instead. When <filename>/var</filename> is
mounted, it will make the <filename>/var/tmp</filename> directory
on the root filesystem inaccessible. If
<filename>/var/tmp</filename> didn't exist on the root filesystem,
it would be impossible to use temporary files
before mounting <filename>/var</filename>.</para>
<para>If you don't intend to write anything to the filesystem, use
the <option>-r</option> switch for <command>mount</command> to do a
<glossterm>read-only mount</glossterm>. This will make the kernel
stop any attempts at writing to the filesystem, and will also stop
the kernel from updating file access times in the inodes. Read-only
mounts are necessary for unwritable media, e.g., CD-ROMs.</para>
<para>The alert reader has already noticed a slight
logistical problem. How is the first filesystem (called the
<glossterm>root filesystem</glossterm>, because it contains the root
directory) mounted, since it obviously can't be mounted on another
filesystem? Well, the answer is that it is done by magic.
The root filesystem is magically mounted at boot time, and one can
rely on it to always be mounted. If the root filesystem can't be
mounted, the system does not boot. The name of the filesystem that
is magically mounted as root is either compiled into the kernel, or
set using LILO or <command>rdev</command>.</para>
<para>For more information, see the kernel source or the Kernel
Hackers' Guide.</para>
<para>The root filesystem is usually first mounted read-only.
The startup scripts will then run <command>fsck</command> to verify
its validity, and if there are no problems, they will
<glossterm>re-mount</glossterm> it so that writes will also be
allowed. <command>fsck</command> must not be run on a mounted
filesystem, since any changes to the filesystem while
<command>fsck</command> is running <emphasis>will</emphasis> cause
trouble. Since the root filesystem is mounted read-only while
it is being checked, <command>fsck</command> can fix any problems
without worry, since the remount operation will flush
any metadata that the filesystem keeps in memory.</para>
<para>On many systems there are other filesystems that should
also be mounted automatically at boot time. These are specified
in the <filename>/etc/fstab</filename> file; see the fstab man
page for details on the format. The details of exactly when the
extra filesystems are mounted depend on many factors, and can be
configured by each administrator if need be; see
<xref linkend="boots-and-shutdowns">.</para>
<para>When a filesystem no longer needs to be mounted, it can be
unmounted with <command>umount</command>.
<command>umount</command> takes one argument:
either the device file or the mount point.
For example, to unmount the directories of
the previous example, one could use the commands
<screen>
<prompt>$</prompt> <userinput>umount /dev/hda2</userinput>
<prompt>$</prompt> <userinput>umount /usr</userinput>
<prompt>$</prompt>
</screen>
</para>
<para>See the man page for further instructions on how to
use the command. It is imperative that you always unmount a mounted
floppy. <emphasis>Don't just pop the floppy out of the
drive!</emphasis> Because of disk caching, the data is not
necessarily written to the floppy until you unmount it, so removing
the floppy from the drive too early might cause the contents to
become garbled. If you only read from the floppy, this is not very
likely, but if you write, even accidentally,
the result may be catastrophic.</para>
<para>Mounting and unmounting requires super user privileges, i.e.,
only root can do it. The reason for this is that if any user can
mount a floppy on any directory, then it is rather easy to create a
floppy with, say, a Trojan horse disguised as
<filename>/bin/sh</filename>, or any other often used program.
However, it is often necessary to allow users to use floppies, and
there are several ways to do this:
<itemizedlist>
<listitem><para>Give the users the root password. This is
obviously bad security, but is the easiest solution. It works well
if there is no need for security anyway, which is the case
on many non-networked, personal systems.</para></listitem>
<listitem><para>Use a program such as <command>sudo</command> to
allow users to use mount. This is still bad security, but doesn't
directly give super user privileges to everyone. It requires several
seconds of hard thinking on the users' behalf. Furthermore
<command>sudo</command> can be configured to only allow users to
execute certain commands. See the sudo(8), sudoers(5), and visudo(8)
manual pages.
</para></listitem>
<listitem><para>Make the users use <command>mtools</command>, a
package for manipulating MS-DOS filesystems, without mounting them.
This works well if MS-DOS floppies are all that is needed, but is
rather awkward otherwise.
</para></listitem>
<listitem><para>List the floppy devices and their allowable mount
points together with the suitable options in
<filename>/etc/fstab</filename>.
</itemizedlist>
The last alternative can be implemented by adding a line like the
following to the <filename>/etc/fstab</filename> file:
<screen>
/dev/fd0 /floppy msdos user,noauto 0 0
</screen>
The columns are: device file to mount, directory to mount on,
filesystem type, options, backup frequency (used by
<command>dump</command>), and <command>fsck</command> pass number
(to specify the order in which filesystems should be checked
upon boot; 0 means no check).</para>
<para>The <option>noauto</option> option stops this mount to be done
automatically when the system is started (i.e., it stops
<command>mount -a</command> from mounting it). The
<option>user</option> option allows any user to mount the
filesystem, and, because of security reasons, disallows execution of
programs (normal or setuid) and interpretation of device files from
the mounted filesystem. After this, any user can mount a floppy with
an msdos filesystem with the following command:
<screen>
<prompt>$</prompt> <userinput>mount /floppy</userinput>
<prompt>$</prompt>
</screen>
The floppy can (and needs to, of course) be unmounted with
the corresponding <command>umount</command> command.</para>
<para>If you want to provide access to several types of floppies,
you need to give several mount points. The settings can be
different for each mount point. For example, to give access to both
MS-DOS and ext2 floppies, you could have the following to lines in
<filename>/etc/fstab</filename>:
<screen>
/dev/fd0 /mnt/dosfloppy msdos user,noauto 0 0
/dev/fd0 /mnt/ext2floppy ext2 user,noauto 0 0
</screen>
The alternative is to just add one line similar to the following:
<screen>
/dev/fd0 /mnt/floppy auto user,noauto 0 0
</screen>
The "auto" option in the filesystem type column allows the mount command
to query the filesystem and try to determine what type it is itself. This
option won't work on all filesystem types, but works fine on the more common
ones.</para>
<para>For MS-DOS filesystems (not just floppies), you probably want to
restrict access to it by using the <option>uid</option>,
<option>gid</option>, and <option>umask</option> filesystem options,
described in detail on the <command>mount</command> manual page. If
you aren't careful, mounting an MS-DOS filesystem gives everyone at
least read access to the files in it, which
is not a good idea.</para>
</sect2>
<sect2>
<title>Filesystem Security</title>
<para>TO BE ADDED</para>
<para>This section will describe mount options and how to use them
in <filename>/etc/fstab</filename> to provide additional system
security.</para>
</sect2>
<sect2 id="fsck">
<title>Checking filesystem integrity with
<command>fsck</command></title>
<para>Filesystems are complex creatures, and as such, they
tend to be somewhat error-prone. A filesystem's correctness and
validity can be checked using the <command>fsck</command> command.
It can be instructed to repair any minor problems it finds, and to
alert the user if there any unrepairable problems. Fortunately, the
code to implement filesystems is debugged quite effectively, so
there are seldom any problems at all, and they are usually caused by
power failures, failing hardware, or operator errors;
for example, by not shutting down the system properly.</para>
<para>Most systems are setup to run <command>fsck</command>
automatically at boot time, so that any errors are detected (and
hopefully corrected) before the system is used. Use of a corrupted
filesystem tends to make things worse: if the data structures are
messed up, using the filesystem will probably mess them up even
more, resulting in more data loss. However, <command>fsck</command>
can take a while to run on big filesystems, and since errors almost
never occur if the system has been shut down properly, a couple of
tricks are used to avoid doing the checks in such cases. The first
is that if the file <filename>/etc/fastboot</filename> exists, no
checks are made. The second is that the ext2 filesystem has a
special marker in its superblock that tells whether the filesystem
was unmounted properly after the previous mount. This allows
<command>e2fsck</command> (the version of <command>fsck</command>
for the ext2 filesystem) to avoid checking the filesystem if the
flag indicates that the unmount was done (the assumption being that
a proper unmount indicates no problems). Whether the
<filename>/etc/fastboot</filename> trick works on your system
depends on your startup scripts, but the ext2 trick works every time
you use <command>e2fsck</command>. It has to be explicitly bypassed
with an option to <command>e2fsck</command> to be avoided. (See
the <command>e2fsck</command> man page for
details on how.)</para>
<para>The automatic checking only works for the
filesystems that are mounted automatically at boot time. Use
<command>fsck</command> manually to check other filesystems,
e.g., floppies.</para>
<para>If <command>fsck</command> finds unrepairable problems,
you need either in-depth knowledge of how filesystems work in
general, and the type of the corrupt filesystem in particular, or
good backups. The latter is easy (although sometimes tedious) to
arrange, the former can sometimes be arranged via a friend, the
Linux newsgroups and mailing lists, or some other source of support,
if you don't have the know-how yourself. I'd like to tell you more
about it, but my lack of education and experience in this regard
hinders me. The <command>debugfs</command>
program by Theodore Ts'o should be useful.</para>
<para><command>fsck</command> must only be run on unmounted
filesystems, never on mounted filesystems (with the exception of the
read-only root during startup). This is because it accesses the raw
disk, and can therefore modify the filesystem without the operating
system realizing it. There <emphasis>will</emphasis>
be trouble, if the operating system is confused.</para>
</sect2>
<sect2 id="badblocks">
<title>Checking for disk errors with <command>badblocks</command></title>
<para>It can be a good idea to periodically check for bad blocks.
This is done with the <command>badblocks</command> command. It
outputs a list of the numbers of all bad blocks it can find. This
list can be fed to <command>fsck</command> to be recorded in the
filesystem data structures so that the operating system won't try to
use the bad blocks for storing data. The following example will show
how this could be done.
<screen>
<prompt>$</prompt> <userinput>badblocks /dev/fd0H1440 1440 &gt;
bad-blocks</userinput>
<prompt>$</prompt> <userinput>fsck -t ext2 -l bad-blocks
/dev/fd0H1440</userinput>
<computeroutput>Parallelizing fsck version 0.5a (5-Apr-94)
e2fsck 0.5a, 5-Apr-94 for EXT2 FS 0.5, 94/03/10
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Check reference counts.
Pass 5: Checking group summary information.
/dev/fd0H1440: ***** FILE SYSTEM WAS MODIFIED *****
/dev/fd0H1440: 11/360 files, 63/1440 blocks</computeroutput>
<prompt>$</prompt>
</screen>
If badblocks reports a block that was already used,
<command>e2fsck</command> will try to move the block to another
place. If the block was really bad, not just marginal, the
contents of the file may be corrupted.</para>
</sect2>
<sect2 id="fragmentation">
<title>Fighting fragmentation?</title>
<para>When a file is written to disk, it can't always be written
in consecutive blocks. A file that is not stored in consecutive
blocks is <glossterm>fragmented</glossterm>. It takes longer to
read a fragmented file, since the disk's read-write head will have
to move more. It is desirable to avoid fragmentation, although it
is less of a problem in a system with a good buffer
cache with read-ahead.</para>
<para>Modern Linux filesystem keep fragmentation at a minimum
by keeping all blocks in a file close together, even if
they can't be stored in consecutive sectors. Some filesystems, like
ext3, effectively allocate the free block that is nearest to other blocks
in a file. Therefore it is not necessary to worry about fragmentation
in a Linux system.</para>
<para>In the earlier days of the ext2 filesystem, there was a concern
over file fragmentation that lead to the development of a
defragmentation program called, defrag. A copy of it can still be
downloaded at <ulink
url="http://www.go.dlr.de/linux/src/defrag-0.73.tar.gz">
http://www.go.dlr.de/linux/src/defrag-0.73.tar.gz</ulink>. However,
it is HIGHLY recommended that you NOT use it. It was designed for
an older version of ext2, and has not bee updated since 1998! I
only mention it here for references purposes.</para>
<para>There are many MS-DOS defragmentation programs that
move blocks around in the filesystem to remove fragmentation. For
other filesystems, defragmentation must be done by backing up the
filesystem, re-creating it, and restoring the files from backups.
Backing up a filesystem before defragmenting is a good idea for all
filesystems, since many things can go wrong during the
defragmentation.</para>
</sect2>
<sect2 id="fs-other-tools">
<title>Other tools for all filesystems</title>
<para>Some other tools are also useful for managing filesystems.
<command>df</command> shows the free disk space on one or more
filesystems; <command>du</command> shows how much disk space a
directory and all its files contain. These can be used to hunt down
disk space wasters. Both have manual pages which detail
the (many) options which can be used.</para>
<para><command>sync</command> forces all unwritten blocks
in the buffer cache (see <xref linkend="buffer-cache">) to be
written to disk. It is seldom necessary to do this by hand; the
daemon process <command>update</command> does this automatically.
It can be useful in catastrophes, for example if
<command>update</command> or its helper process
<command>bdflush</command> dies, or if you must turn off power
<emphasis>now</emphasis> and can't wait for
<command>update</command> to run. Again, there are manual pages.
The <command>man</command> is your very best friend in Linux. Its
cousin <command>apropos</command> is also very useful when you don't
know what the name of the command you want
is.</para>
</sect2>
<sect2 id="fs-ext3-tools">
<title>Other tools for the ext2/ext3 filesystem</title>
<para>In addition to the filesystem creator
(<command>mke2fs</command>) and checker (<command>e2fsck</command>)
accessible directly or via the filesystem type independent front
ends, the ext2
filesystem has some additional tools that can be useful.</para>
<para><command>tune2fs</command> adjusts filesystem parameters.
Some of the more interesting parameters are:
<itemizedlist>
<listitem><para>
A maximal mount count. <command>e2fsck</command> enforces a check
when filesystem has been mounted too many times, even if the clean
flag is set. For a system that is used for developing or testing
the system, it might be a good idea to reduce this limit.
</para></listitem>
<listitem><para>
A maximal time between checks. <command>e2fsck</command> can also
enforce a maximal time between two checks, even if the clean flag is
set, and the filesystem hasn't been mounted very often. This can be
disabled, however.
</para></listitem>
<listitem><para>
Number of blocks reserved for root. Ext2 reserves some blocks for
root so that if the filesystem fills up, it is still possible to do
system administration without having to delete anything. The
reserved amount is by default 5 percent, which on most disks isn't
enough to be wasteful. However, for floppies there is no point in
reserving any blocks.
</para></listitem>
</itemizedlist>
See the <command>tune2fs</command> manual page for more
information.</para>
<para><command>dumpe2fs</command> shows information about an ext2 or ext3
filesystem, mostly from the superblock. Below is a sample output. Some
of the information in the output is technical and requires understanding
of how the filesystem works, but much of it is readily understandable
even for lay-admins.</para>
<screen>
<prompt>#</prompt> <userinput>dumpe2fs</userinput>
<computeroutput>dumpe2fs 1.32 (09-Nov-2002)
Filesystem volume name: /
Last mounted on: not available
Filesystem UUID: 51603f82-68f3-4ae7-a755-b777ff9dc739
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal filetype needs_recovery sparse_super
Default mount options: (none)
Filesystem state: clean
Errors behavior: Continue
Filesystem OS type: Linux
Inode count: 3482976
Block count: 6960153
Reserved block count: 348007
Free blocks: 3873525
Free inodes: 3136573
First block: 0
Block size: 4096
Fragment size: 4096
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 16352
Inode blocks per group: 511
Filesystem created: Tue Aug 26 08:11:55 2003
Last mount time: Mon Dec 22 08:23:12 2003
Last write time: Mon Dec 22 08:23:12 2003
Mount count: 3
Maximum mount count: -1
Last checked: Mon Nov 3 11:27:38 2003
Check interval: 0 (none)
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)
First inode: 11
Inode size: 128
Journal UUID: none
Journal inode: 8
Journal device: 0x0000
First orphan inode: 655612
Group 0: (Blocks 0-32767)
Primary superblock at 0, Group descriptors at 1-2
Block bitmap at 3 (+3), Inode bitmap at 4 (+4)
Block bitmap at 3 (+3), Inode bitmap at 4 (+4)
Inode table at 5-515 (+5)
3734 free blocks, 16338 free inodes, 2 directories
</computeroutput>
</screen>
<para><command>debugfs</command> is a filesystem debugger.
It allows direct access to the filesystem data structures stored on
disk and can thus be used to repair a disk that is so broken that
<command>fsck</command> can't fix it automatically. It has also been
known to be used to recover deleted files. However,
<command>debugfs</command> very much requires that you understand
what you're doing; a failure to understand can
destroy all your data.</para>
<para><command>dump</command> and <command>restore</command> can be
used to back up an ext2 filesystem. They are ext2 specific versions
of the traditional UNIX backup tools. See <xref linkend="backups">
for more information on backups.</para>
</sect2>
</sect1>
<sect1 id="disk-no-fs">
<title>Disks without filesystems</title>
<para>Not all disks or partitions are used as filesystems.
A swap partition, for example, will not have a filesystem on it.
Many floppies are used in a tape-drive emulating fashion, so that a
<command>tar</command> (tape archive) or other file is written
directly on the raw disk, without a filesystem. Linux boot floppies
don't
contain a filesystem, only the raw kernel.</para>
<para>Avoiding a filesystem has the advantage of making more of
the disk usable, since a filesystem always has some bookkeeping
overhead. It also makes the disks more easily compatible with other
systems: for example, the <command>tar</command> file format is the
same on all systems, while filesystems are different on most
systems. You will quickly get used to disks without filesystems if
you need them. Bootable Linux floppies
also do not necessarily have a filesystem, although they may.</para>
<para>One reason to use raw disks is to make image copies of them.
For instance, if the disk contains a partially damaged filesystem,
it is a good idea to make an exact copy of it before trying to fix
it, since then you can start again if your fixing breaks things even
more. One way to do this is to use <command>dd</command>:
<screen>
<prompt>$</prompt> <userinput>dd if=/dev/fd0H1440
of=floppy-image</userinput>
<computeroutput>2880+0 records in
2880+0 records out</computeroutput>
<prompt>$</prompt> <userinput>dd if=floppy-image
of=/dev/fd0H1440</userinput>
<computeroutput>2880+0 records in
2880+0 records out</computeroutput>
<prompt>$</prompt>
</screen>
The first <command>dd</command> makes an exact image of the floppy
to the file <filename>floppy-image</filename>, the second one writes
the image to the floppy. (The user has presumably switched the
floppy before the second command. Otherwise the
command pair is of doubtful usefulness.)</para>
</sect1>
<sect1 id="alloc-disk">
<title>Allocating disk space</title>
<sect2 id="part-schemes">
<title>Partitioning schemes</title>
<para>When it comes to partitioning your machine,
there is no universally correct way to do it. There are
many factors that must be taken into account depending on
the purpose of the machine.</para>
<para>For a simple workstation with limited disk space, such
as a laptop, you may have as few a 3 partitions. A partition for
<filename>/</filename>, <filename>/boot</filename>, and swap.
However, for most users this is not a recommended solution.<para>
<para>The traditional way is to have a (relatively) small
root filesystem, and separate partitions for filesystems such as
<filename>/usr</filename> and <filename>/home></filename>.
Creating a separate root filesystem if the root filesystem is small and
not heavily used, it is less likely to become corrupt when the system
crashes, and therefore make it easier to recover a crashed system. The
reason is to prevent having the root filesystem get filled and cause a
system crash.</para>
<para>When creating your partitioning scheme, there are some things you
need to remember. You cannot create separate partitions for the following
directories: <filename>/bin</filename>, <filename>/etc</filename>,
<filename>/dev</filename>, <filename>/initrd</filename>,
<filename>/lib</filename>, and <filename>/sbin</filename>. The
contents of these directories are required at bootup and must
always be part of the <filename>/</filename> partition.</para>
<para>It is also recommended that you create separate partitions for
<filename>/var</filename> and <filename>/tmp</filename>. This is
because both directories typically have data that is constantly
changing. Not creating separate partitions for these filesystems
puts you at risk of having log file fill up our <filename>/</filename>
partition.</para>
<para>An example of a server partition is:
<screen><computeroutput>
Filesystem Size Used Avail Use% Mounted on
/dev/hda2 9.7G 1.3G 8.0G 14% /
/dev/hda1 128M 44M 82M 34% /boot
/dev/hda3 4.9G 4.0G 670M 86% /usr
/dev/hda5 4.9G 2.1G 2.5G 46% /var
/dev/hda7 31G 24G 5.6G 81% /home
/dev/hda8 4.9G 2.0G 670M 43% /opt
</computeroutput></screen></para>
<para>The problem with having many partitions is that it splits
the total amount of free disk space into many small pieces. One way
to avoid this problem is to use to create Logical Volumes.</para>
</sect2>
<sect2 id="lvm">
<title>Logical Volume Manager (LVM)</title>
<para>Using LVM allows administrators the flexibility to create
logical disks that can be expanded dynamically as more disk space
is required.</para>
<para>This is done first by creating partitions with as an
0x8e Linux LVM partition type. Then the <glossterm>Physical Partitions
</glossterm> are added to a <glossterm>Volume Group</glossterm> and
broken up into chunks, or <glossterm>Physical Extents</glossterm>
<glossterm>Volume Group</glossterm>. These extends can then be grouped
into <glossterm>Logical Volumes</glossterm>. These Logical Volumes
then can be formatted just like a physical partition. The big
difference is that they can be expanded by adding more extents to
them.</para>
<para>Right now, a full discussion of LVM is beyond the scope of this
guide. However, and excellent resource for learning more about LVM
can be found at <ulink url="http://www.tldp.org/HOWTO/LVM-HOWTO.html">
http://www.tldp.org/HOWTO/LVM-HOWTO.html</ulink>.</para>
<!--
% \meta more reasons for many partitions: users/temp files/spools
% can't fill up all disks, readonly partitions less likely to
corrupt,
% fsck is faster, limits losses a filesystem goes really wrong,
% logging must not be disturbed, boots from >1023 cylinders do not
% work on all BIOSs, /usr/local won't be disturbed by an upgrade,
% easy to divide backup on many tapes, spare (scratch) partition for
% experimentation (e.g., a new Linux distribution), scratch can
% also be used to backup root during upgrades
-->
</sect2>
<sect2 id="space-req">
<title>Space requirements</title>
<para>The Linux distribution you install will give some indication
of how much disk space you need for various configurations. Programs
installed separately may also do the same. This will help you plan
your disk space usage, but you should prepare for the future and
reserve some extra space for things you will
notice later that you need.</para>
<para>The amount you need for user files depends on what your
users wish to do. Most people seem to need as much space for their
files as possible, but the amount they will live happily with varies
a lot. Some people do only light text processing and will survive
nicely with a few megabytes, others do heavy
image processing and will need gigabytes.</para>
<para>By the way, when comparing file sizes given in
kilobytes or megabytes and disk space given in megabytes, it can be
important to know that the two units can be different. Some disk
manufacturers like to pretend that a kilobyte is 1000 bytes and a
megabyte is 1000 kilobytes, while all the rest of the computing
world uses 1024 for both factors. Therefore, a 345 MB hard disk
is really a 330 MB hard disk.</para>
<para>Swap space allocation is discussed in <xref
linkend="swap-allocation">.</para>
</sect2>
<sect2 id="alloc-space-ex">
<title>Examples of hard disk allocation</title>
<para>I used to have a 10 GB hard disk. Now I am using a 30 GB
hard disk. I'll explain how and why I partitioned those
disks.</para>
<para>First, I created a <filename>/boot</filename> partition
at 128 MG. This is larger than I will need, and big enough to
give me space if I need it. I created a separate
<filename>/boot</filename> partition to ensure that this
filesystem will never get filled up, and therefore will be
bootable. Then I created a 5 GB <filename>/var</filename>
partition. Since the <filename>/var</filename> filesystem is
where log files and email is stored I wanted to isolate it
from my root partition. (I have had log files grow overnight
and fill my root filesystem in the past.) Next, I created a 15 GB
<filename>/home</filename> partition. This is handy in the event
of a system crash. If I ever have to re-install Linux from scratch,
I can tell the installation program to not format this partition, and
instead remount it without the data being lost. Finally
since I had 512 MG of RAM I created a 1024 MG (or 1 GB) swap
partition. This left me with roughly a 9 GB root filesystem. I using
my old 10 GB hard drive, I created an 8 GB <filename>/usr</filename>
partition and left 2 GB unused. This is incase I need more space
in the future.</para>
<para>In the end, my partition tables looked like this:
<table>
<title>My Partitions</title>
<tgroup cols=2>
<tbody>
<row> <entry>9 GB</entry> <entry>root filesystem</entry> </row>
<row> <entry>1 GB</entry> <entry>swap partition</entry> </row>
<row> <entry>5 GB</entry> <entry><filename>/var</filename>
filesystem</entry>
</row>
<row> <entry>15 GB</entry> <entry><filename>/home</filename>
filesystem</entry>
</row>
<row> <entry> 8 GB</entry> <entry><filename>/usr</filename>
filesystem</entry> </row>
<row> <entry> 2 GB</entry> <entry>scratch partition</entry> </row>
</tbody>
</tgroup>
</table>
</para>
</sect2>
<sect2 id="adding-disk">
<title>Adding more disk space for Linux</title>
<para>Adding more disk space for Linux is easy, at least after the
hardware has been properly installed (the hardware installation
is outside the scope of this book). You format it if necessary,
then create the partitions and filesystem as described above,
and add the proper lines to <filename>/etc/fstab</filename>
<indexterm id="ch05-fstab"><primary>filesystems</primary>
<secondary>/etc</secondary><tertiary>/etc/fstab</tertiary>
</indexterm>
so that it is mounted automatically.</para>
</sect2>
<sect2 id="space-saving-tips">
<title>Tips for saving disk space</title>
<para>The best tip for saving disk space<indexterm id="ch05-savespc">
<primary>disks</primary>
<secondary>saving space</secondary></indexterm> is to avoid installing
unnecessary programs. Most Linux distributions have an
option to install only part of the packages they contain,
and by analyzing your needs you might notice that you don't
need most of them. This will help save a lot of disk space,
since many programs are quite large. Even if you do need a
particular package or program, you might not need all of it.
For example, some on-line documentation might be unnecessary,
as might some of the Elisp files for GNU Emacs, some of the
fonts for X11, or some of the libraries for programming.</para>
<para>If you cannot uninstall packages, you might look into
compression. Compression programs such as <command>gzip</command>
<indexterm id="ch05-gzip"><primary>commands</primary>
<secondary>gzip</secondary></indexterm>
or <command>zip</command><indexterm id="ch05-zip">
<primary>commands</primary><secondary>zip</secondary></indexterm>
will compress (and uncompress)
individual files or groups of files. The <command>gzexe</command>
<indexterm id="ch05-gzexe"><primary>commands</primary>
<secondary>gzexe</secondary></indexterm>
system will compress and uncompress programs invisibly to the
user (unused programs are compressed, then uncompressed as they
are used). The experimental DouBle system will compress all
files in a filesystem, invisibly to the programs that use them.
(If you are familiar with products such as Stacker for MS-DOS
or DriveSpace for Windows, the principle is the same.)</para>
<para>Another way to save space is to take special care when
formatting you partitions. Most modern filesystems will allow
you to specify the block size. The block size is chunk size that
the filesystem will use to read and write data. Larger block sizes
will help disk I/O performance when using large files, such as databases.
This happens because the disk can read or write data for a longer
period of time before having to search for the next block. The
</sect2>
</sect1>
</chapter>
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