source: chapter09/introduction.xml@ a3d0817

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Last change on this file since a3d0817 was a3d0817, checked in by Bruce Dubbs <bdubbs@…>, 18 months ago

Text updated for cross2 chapter 9

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1<?xml version="1.0" encoding="ISO-8859-1"?>
2<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
3 "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
4 <!ENTITY % general-entities SYSTEM "../general.ent">
5 %general-entities;
6]>
7
8<sect1 id="ch-config-introduction" revision="sysv">
9 <?dbhtml filename="introduction.html"?>
10
11 <title>Introduction</title>
12
13 <para>Booting a Linux system involves several tasks. The process must
14 mount both virtual and real file systems, initialize devices, activate swap,
15 check file systems for integrity, mount any swap partitions or files, set
16 the system clock, bring up networking, start any daemons required by the
17 system, and accomplish any other custom tasks needed by the user. This
18 process must be organized to ensure the tasks are performed in the correct
19 order but, at the same time, be executed as fast as possible.</para>
20
21 <sect2 id='sysv-desc'>
22 <title>System V</title>
23
24 <para>System V is the classic boot process that has been used in Unix and
25 Unix-like systems such as Linux since about 1983. It consists of a small
26 program, <command>init</command>, that sets up basic programs such as
27 <command>login</command> (via getty) and runs a script. This script,
28 usually named <command>rc</command>, controls the execution of a set of
29 additional scripts that perform the tasks required to initialize the
30 system.</para>
31
32 <para>The <command>init</command> program is controlled by the
33 <filename>/etc/inittab</filename> file and is organized into run levels that
34 can be run by the user:</para>
35
36<literallayout>0 &mdash; halt
371 &mdash; Single user mode
382 &mdash; Multiuser, without networking
393 &mdash; Full multiuser mode
404 &mdash; User definable
415 &mdash; Full multiuser mode with display manager
426 &mdash; reboot</literallayout>
43
44 <para>The usual default run level is 3 or 5.</para>
45
46 <bridgehead renderas="sect3">Advantages</bridgehead>
47
48 <itemizedlist>
49 <listitem>
50 <para>Established, well understood system.</para>
51 </listitem>
52
53 <listitem>
54 <para>Easy to customize.</para>
55 </listitem>
56
57 </itemizedlist>
58
59
60 <bridgehead renderas="sect3">Disadvantages</bridgehead>
61
62 <itemizedlist>
63 <listitem>
64 <para>May be slower to boot. A medium speed base LFS system
65 takes 8-12 seconds where the boot time is measured from the
66 first kernel message to the login prompt. Network
67 connectivity is typically established about 2 seconds
68 after the login prompt.</para>
69 </listitem>
70
71 <listitem>
72 <para>Serial processing of boot tasks. This is related to the previous
73 point. A delay in any process such as a file system check, will
74 delay the entire boot process.</para>
75 </listitem>
76
77 <listitem>
78 <para>Does not directly support advanced features like
79 control groups (cgroups), and per-user fair share scheduling.</para>
80 </listitem>
81
82 <listitem>
83 <para>Adding scripts requires manual, static sequencing decisions.</para>
84 </listitem>
85
86 </itemizedlist>
87
88 </sect2>
89
90</sect1>
91
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