source: chapter09/introduction.xml@ fcc02767

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Last change on this file since fcc02767 was fcc02767, checked in by Bruce Dubbs <bdubbs@…>, 18 months ago

Initial commit of alternative cross LFS

git-svn-id: http://svn.linuxfromscratch.org/LFS/branches/cross2@11897 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

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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<!-- <para>In the packages that were installed in Chapter&nbsp;6, there were two
22 different boot systems installed. LFS provides the ability to easily
23 select which system the user wants to use and to compare and contrast the
24 two systems by actually running each system on the local computer. The
25 advantages and disadvantages of these systems is presented below.</para>-->
26
27 <sect2 id='sysv-desc'>
28 <title>System V</title>
29
30 <para>System V is the classic boot process that has been used in Unix and
31 Unix-like systems such as Linux since about 1983. It consists of a small
32 program, <command>init</command>, that sets up basic programs such as
33 <command>login</command> (via getty) and runs a script. This script,
34 usually named <command>rc</command>, controls the execution of a set of
35 additional scripts that perform the tasks required to initialize the
36 system.</para>
37
38 <para>The <command>init</command> program is controlled by the
39 <filename>/etc/inittab</filename> file and is organized into run levels that
40 can be run by the user:</para>
41
42<literallayout>
430 &mdash; halt
441 &mdash; Single user mode
452 &mdash; Multiuser, without networking
463 &mdash; Full multiuser mode
474 &mdash; User definable
485 &mdash; Full multiuser mode with display manager
496 &mdash; reboot
50</literallayout>
51
52 <para>The usual default run level is 3 or 5.</para>
53
54 <bridgehead renderas="sect3">Advantages</bridgehead>
55
56 <itemizedlist>
57 <listitem>
58 <para>Established, well understood system.</para>
59 </listitem>
60
61 <listitem>
62 <para>Easy to customize.</para>
63 </listitem>
64
65 </itemizedlist>
66
67
68 <bridgehead renderas="sect3">Disadvantages</bridgehead>
69
70 <itemizedlist>
71 <listitem>
72 <para>Slower to boot. A medium speed base LFS system
73 takes 8-12 seconds where the boot time is measured from the
74 first kernel message to the login prompt. Network
75 connectivity is typically established about 2 seconds
76 after the login prompt.</para>
77 </listitem>
78
79 <listitem>
80 <para>Serial processing of boot tasks. This is related to the previous
81 point. A delay in any process such as a file system check, will
82 delay the entire boot process.</para>
83 </listitem>
84
85 <listitem>
86 <para>Does not directly support advanced features like
87 control groups (cgroups), and per-user fair share scheduling.</para>
88 </listitem>
89
90 <listitem>
91 <para>Adding scripts requires manual, static sequencing decisions.</para>
92 </listitem>
93
94 </itemizedlist>
95
96 </sect2>
97<!--
98 <sect2 id='sysd-desc'>
99 <title>Systemd</title>
100
101 <para>Systemd is a group of interconnected programs that handles system and
102 individual process requests. It provides a dependency system between
103 various entities called "units". It automatically addresses dependencies
104 between units and can execute several startup tasks in parallel. It
105 provides login, inetd, logging, time, and networking services. </para>
106
107 <bridgehead renderas="sect3">Advantages</bridgehead>
108
109 <itemizedlist>
110 <listitem>
111 <para>Used on many established distributions by default.</para>
112 </listitem>
113
114 <listitem>
115 <para>There is extensive documentation.
116 See <ulink url="http://www.freedesktop.org/wiki/Software/systemd/"/>.</para>
117 </listitem>
118
119 <listitem>
120 <para>Parallel execution of boot processes. A medium speed
121 base LFS system takes 6-10 seconds from kernel start to a
122 login prompt. Network connectivity is typically established
123 about 2 seconds after the login prompt. More complex startup
124 procedures may show a greater speedup when compared to System V.</para>
125 </listitem>
126
127 <listitem>
128 <para>Implements advanced features such as control groups to
129 manage related processes.</para>
130 </listitem>
131
132 <listitem>
133 <para>Maintains backward compatibility with System V programs
134 and scripts.</para>
135 </listitem>
136 </itemizedlist>
137
138 <bridgehead renderas="sect3">Disadvantages</bridgehead>
139
140 <itemizedlist>
141 <listitem>
142 <para>There is a substantial learning curve.</para>
143 </listitem>
144
145 <listitem>
146 <para>Some advanced features such as dbus or cgroups cannot be
147 disabled if they are not otherwise needed.</para>
148 </listitem>
149
150 <listitem>
151 <para>Although implemented as several executable programs
152 the user cannot choose to implement only the portions desired.</para>
153 </listitem>
154
155 <listitem>
156 <para>Due to the nature of using compiled programs, systemd is
157 more difficult to debug.</para>
158 </listitem>
159
160 <listitem>
161 <para>Logging is done in a binary format. Extra tools must
162 be used to process logs or additional processes must be implemented
163 to duplicate traditional logging programs.</para>
164 </listitem>
165
166 </itemizedlist>
167
168 </sect2>
169-->
170<!--
171 <sect2 id='sysv'>
172 <title>Selecting a Boot Method</title>
173
174 <para>Selecting a boot method in LFS is relatively easy.
175 Both systems are installed side-by-side. The only task needed is to
176 ensure the files that are needed by the system have the correct names.
177 The following scripts do that.</para>
178
179<screen><userinput remap="install">cat &gt; /usr/sbin/set-systemd &lt;&lt; "EOF"
180#! /bin/bash
181
182ln -svfn init-systemd /sbin/init
183ln -svfn init.d-systemd /etc/init.d
184
185for tool in halt poweroff reboot runlevel shutdown telinit; do
186 ln -sfvn ${tool}-systemd /sbin/${tool}
187 ln -svfn ${tool}-systemd.8 /usr/share/man/man8/${tool}.8
188done
189
190echo "Now reboot with /sbin/reboot-sysv"
191EOF
192
193chmod 0744 /usr/sbin/set-systemd
194
195cat &gt; /usr/sbin/set-sysv &lt;&lt; "EOF"
196#! /bin/bash
197
198ln -sfvn init-sysv /sbin/init
199ln -svfn init.d-sysv /etc/init.d
200
201for tool in halt poweroff reboot runlevel shutdown telinit; do
202 ln -sfvn ${tool}-sysv /sbin/${tool}
203 ln -svfn ${tool}-sysv.8 /usr/share/man/man8/${tool}.8
204done
205
206echo "Now reboot with /sbin/reboot-systemd"
207EOF
208
209chmod 0744 /usr/sbin/set-sysv</userinput></screen>
210
211 <note><para>The comment about the correct command to reboot in the
212 above scripts is correct. The reboot command for the current boot
213 system must be used after the script changes the default reboot command.
214 </para></note>
215
216 <para>Now set the desired boot system. The default is System V:</para>
217
218<screen><userinput remap="install">/usr/sbin/set-sysv</userinput></screen>
219
220 <para>Changing the boot system can be done at any time by running the
221 appropriate script above and rebooting.</para>
222
223 </sect2>
224-->
225</sect1>
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