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- <?xml version="1.0" encoding="ISO-8859-1"?>
- <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
- "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
- <!ENTITY % general-entities SYSTEM "../general.ent">
- %general-entities;
- ]>
- <sect1 id="ch-scripts-introduction">
- <?dbhtml filename="introduction.html"?>
- <title>Introduction</title>
- <para>Booting a Linux system involves several tasks. The process must
- mount both virtual and real file systems, initialize devices, activate swap,
- check file systems for integrity, mount any swap partitions or files, set
- the system clock, bring up networking, start any daemons required by the
- system, and accomplish any other custom tasks needed by the user. This
- process must be organized to ensure the tasks are performed in the correct
- order but, at the same time, be executed as fast as possible.</para>
- <!-- <para>In the packages that were installed in Chapter 6, there were two
- different boot systems installed. LFS provides the ability to easily
- select which system the user wants to use and to compare and contrast the
- two systems by actually running each system on the local computer. The
- advantages and disadvantages of these systems is presented below.</para>-->
- <sect2 id='sysv-desc'>
- <title>System V</title>
- <para>System V is the classic boot process that has been used in Unix and
- Unix-like systems such as Linux since about 1983. It consists of a small
- program, <command>init</command>, that sets up basic programs such as
- <command>login</command> (via getty) and runs a script. This script,
- usually named <command>rc</command>, controls the execution of a set of
- additional scripts that perform the tasks required to initialize the
- system.</para>
- <para>The <command>init</command> program is controlled by the
- <filename>/etc/inittab</filename> file and is organized into run levels that
- can be run by the user:</para>
- <literallayout>
- 0 — halt
- 1 — Single user mode
- 2 — Multiuser, without networking
- 3 — Full multiuser mode
- 4 — User definable
- 5 — Full multiuser mode with display manager
- 6 — reboot
- </literallayout>
- <para>The usual default run level is 3 or 5.</para>
- <bridgehead renderas="sect3">Advantages</bridgehead>
- <itemizedlist>
- <listitem>
- <para>Established, well understood system.</para>
- </listitem>
- <listitem>
- <para>Easy to customize.</para>
- </listitem>
- </itemizedlist>
- <bridgehead renderas="sect3">Disadvantages</bridgehead>
- <itemizedlist>
- <listitem>
- <para>Slower to boot. A medium speed base LFS system
- takes 8-12 seconds where the boot time is measured from the
- first kernel message to the login prompt. Network
- connectivity is typically established about 2 seconds
- after the login prompt.</para>
- </listitem>
- <listitem>
- <para>Serial processing of boot tasks. This is related to the previous
- point. A delay in any process such as a file system check, will
- delay the entire boot process.</para>
- </listitem>
- <listitem>
- <para>Does not directly support advanced features like
- control groups (cgroups), and per-user fair share scheduling.</para>
- </listitem>
- <listitem>
- <para>Adding scripts requires manual, static sequencing decisions.</para>
- </listitem>
- </itemizedlist>
- </sect2>
- <!--
- <sect2 id='sysd-desc'>
- <title>Systemd</title>
- <para>Systemd is a group of interconnected programs that handles system and
- individual process requests. It provides a dependency system between
- various entities called "units". It automatically addresses dependencies
- between units and can execute several startup tasks in parallel. It
- provides login, inetd, logging, time, and networking services. </para>
- <bridgehead renderas="sect3">Advantages</bridgehead>
- <itemizedlist>
- <listitem>
- <para>Used on many established distributions by default.</para>
- </listitem>
- <listitem>
- <para>There is extensive documentation.
- See <ulink url="http://www.freedesktop.org/wiki/Software/systemd/"/>.</para>
- </listitem>
- <listitem>
- <para>Parallel execution of boot processes. A medium speed
- base LFS system takes 6-10 seconds from kernel start to a
- login prompt. Network connectivity is typically established
- about 2 seconds after the login prompt. More complex startup
- procedures may show a greater speedup when compared to System V.</para>
- </listitem>
- <listitem>
- <para>Implements advanced features such as control groups to
- manage related processes.</para>
- </listitem>
- <listitem>
- <para>Maintains backward compatibility with System V programs
- and scripts.</para>
- </listitem>
- </itemizedlist>
- <bridgehead renderas="sect3">Disadvantages</bridgehead>
- <itemizedlist>
- <listitem>
- <para>There is a substantial learning curve.</para>
- </listitem>
- <listitem>
- <para>Some advanced features such as dbus or cgroups cannot be
- disabled if they are not otherwise needed.</para>
- </listitem>
- <listitem>
- <para>Although implemented as several executable programs
- the user cannot choose to implement only the portions desired.</para>
- </listitem>
- <listitem>
- <para>Due to the nature of using compiled programs, systemd is
- more difficult to debug.</para>
- </listitem>
- <listitem>
- <para>Logging is done in a binary format. Extra tools must
- be used to process logs or additional processes must be implemented
- to duplicate traditional logging programs.</para>
- </listitem>
- </itemizedlist>
- </sect2>
- -->
- <!--
- <sect2 id='sysv'>
- <title>Selecting a Boot Method</title>
- <para>Selecting a boot method in LFS is relatively easy.
- Both systems are installed side-by-side. The only task needed is to
- ensure the files that are needed by the system have the correct names.
- The following scripts do that.</para>
- <screen><userinput remap="install">cat > /usr/sbin/set-systemd << "EOF"
- #! /bin/bash
- ln -svfn init-systemd /sbin/init
- ln -svfn init.d-systemd /etc/init.d
- for tool in halt poweroff reboot runlevel shutdown telinit; do
- ln -sfvn ${tool}-systemd /sbin/${tool}
- ln -svfn ${tool}-systemd.8 /usr/share/man/man8/${tool}.8
- done
- echo "Now reboot with /sbin/reboot-sysv"
- EOF
- chmod 0744 /usr/sbin/set-systemd
- cat > /usr/sbin/set-sysv << "EOF"
- #! /bin/bash
- ln -sfvn init-sysv /sbin/init
- ln -svfn init.d-sysv /etc/init.d
- for tool in halt poweroff reboot runlevel shutdown telinit; do
- ln -sfvn ${tool}-sysv /sbin/${tool}
- ln -svfn ${tool}-sysv.8 /usr/share/man/man8/${tool}.8
- done
- echo "Now reboot with /sbin/reboot-systemd"
- EOF
- chmod 0744 /usr/sbin/set-sysv</userinput></screen>
- <note><para>The comment about the correct command to reboot in the
- above scripts is correct. The reboot command for the current boot
- system must be used after the script changes the default reboot command.
- </para></note>
- <para>Now set the desired boot system. The default is System V:</para>
- <screen><userinput remap="install">/usr/sbin/set-sysv</userinput></screen>
- <para>Changing the boot system can be done at any time by running the
- appropriate script above and rebooting.</para>
- </sect2>
- -->
- </sect1>
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