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<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN">
<HTML
><HEAD
><TITLE
>Using Disks and Other Storage Media</TITLE
><META
NAME="GENERATOR"
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TITLE="Hard disks"
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><BODY
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><TABLE
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>The Linux System Administrators' Guide</TH
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><HR
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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="AEN701"
>Chapter 4. Using Disks and Other Storage Media</A
></H1
><DIV
CLASS="TOC"
><DL
><DT
><B
>Table of Contents</B
></DT
><DT
><A
HREF="c701.html#AEN721"
>Two kinds of devices</A
></DT
><DT
><A
HREF="x747.html"
>Hard disks</A
></DT
><DT
><A
HREF="x787.html"
>Floppies</A
></DT
><DT
><A
HREF="x801.html"
>CD-ROM's</A
></DT
><DT
><A
HREF="x811.html"
>Tapes</A
></DT
><DT
><A
HREF="x817.html"
>Formatting</A
></DT
><DT
><A
HREF="x876.html"
>Partitions</A
></DT
><DT
><A
HREF="x1029.html"
>Filesystems</A
></DT
><DT
><A
HREF="x1366.html"
>Disks without filesystems</A
></DT
><DT
><A
HREF="x1384.html"
>Allocating disk space</A
></DT
></DL
></DIV
><BLOCKQUOTE
><P
>“On a clear disk you can seek forever.
”</P
></BLOCKQUOTE
><P
>When you install or upgrade your system, you need to do a
fair amount of work on your disks. 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.</P
><P
>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.</P
><P
> </P
><P
>The basic tasks in administering disks are:
<P
></P
><UL
><LI
><P
> 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.)</P
></LI
><LI
><P
> 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.
</P
></LI
><LI
><P
> 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.
</P
></LI
><LI
><P
> 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.)
</P
></LI
></UL
>
</P
><P
><A
HREF="c1450.html"
>Chapter 5</A
> contains information
about virtual memory and disk caching, of which you also need
to be aware when using disks.</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN721"
>Two kinds of devices</A
></H1
><P
>UNIX, and therefore Linux, recognizes two different
kinds of device: random-access block devices (such as disks),
and character devices (such as tapes and serial lines),
some of which may be serial, and some random-access. Each
supported device is represented in the filesystem as a
<I
CLASS="GLOSSTERM"
>device file</I
>.
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
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>cat filename > /dev/lp1</B
></TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
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 <B
CLASS="COMMAND"
>lpr</B
>).
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.</P
><P
>Since devices show up as files in the filesystem (in the
<TT
CLASS="FILENAME"
>/dev</TT
> directory), it is easy
to see just what device files exist, using <B
CLASS="COMMAND"
>ls</B
> or
another suitable command. In the output of <B
CLASS="COMMAND"
>ls -l</B
>, the
first column contains the type of the file and its
permissions. For example, inspecting a serial device
gives on my system
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>ls -l /dev/cua0</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>crw-rw-rw- 1 root uucp 5, 64 Nov 30 1993 /dev/cua0</TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
The first character in the first column, i.e.,
`<TT
CLASS="LITERAL"
>c</TT
>' in <TT
CLASS="LITERAL"
>crw-rw-rw-</TT
>
above, tells an informed user the type of the file, in this
case a character device. For ordinary files, the first
character is `<TT
CLASS="LITERAL"
>-</TT
>', for directories
it is `<TT
CLASS="LITERAL"
>d</TT
>', and for block devices
`<TT
CLASS="LITERAL"
>b</TT
>'; see the <B
CLASS="COMMAND"
>ls</B
> man page
for further information.</P
><P
>Note that usually all device files exist even though the
device itself might be not be installed. So just because you
have a file <TT
CLASS="FILENAME"
>/dev/sda</TT
>, 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).</P
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