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git-svn-id: http://svn.linuxfromscratch.org/LFS/branches/cross2@11897 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689
226 lines
7.5 KiB
XML
226 lines
7.5 KiB
XML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY % general-entities SYSTEM "../general.ent">
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%general-entities;
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]>
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<sect1 id="ch-config-introduction" revision="sysv">
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<?dbhtml filename="introduction.html"?>
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<title>Introduction</title>
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<para>Booting a Linux system involves several tasks. The process must
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mount both virtual and real file systems, initialize devices, activate swap,
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check file systems for integrity, mount any swap partitions or files, set
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the system clock, bring up networking, start any daemons required by the
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system, and accomplish any other custom tasks needed by the user. This
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process must be organized to ensure the tasks are performed in the correct
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order but, at the same time, be executed as fast as possible.</para>
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<!-- <para>In the packages that were installed in Chapter 6, there were two
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different boot systems installed. LFS provides the ability to easily
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select which system the user wants to use and to compare and contrast the
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two systems by actually running each system on the local computer. The
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advantages and disadvantages of these systems is presented below.</para>-->
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<sect2 id='sysv-desc'>
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<title>System V</title>
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<para>System V is the classic boot process that has been used in Unix and
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Unix-like systems such as Linux since about 1983. It consists of a small
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program, <command>init</command>, that sets up basic programs such as
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<command>login</command> (via getty) and runs a script. This script,
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usually named <command>rc</command>, controls the execution of a set of
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additional scripts that perform the tasks required to initialize the
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system.</para>
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<para>The <command>init</command> program is controlled by the
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<filename>/etc/inittab</filename> file and is organized into run levels that
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can be run by the user:</para>
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<literallayout>
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0 — halt
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1 — Single user mode
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2 — Multiuser, without networking
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3 — Full multiuser mode
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4 — User definable
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5 — Full multiuser mode with display manager
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6 — reboot
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</literallayout>
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<para>The usual default run level is 3 or 5.</para>
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<bridgehead renderas="sect3">Advantages</bridgehead>
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<itemizedlist>
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<listitem>
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<para>Established, well understood system.</para>
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</listitem>
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<listitem>
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<para>Easy to customize.</para>
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</listitem>
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</itemizedlist>
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<bridgehead renderas="sect3">Disadvantages</bridgehead>
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<itemizedlist>
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<listitem>
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<para>Slower to boot. A medium speed base LFS system
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takes 8-12 seconds where the boot time is measured from the
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first kernel message to the login prompt. Network
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connectivity is typically established about 2 seconds
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after the login prompt.</para>
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</listitem>
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<listitem>
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<para>Serial processing of boot tasks. This is related to the previous
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point. A delay in any process such as a file system check, will
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delay the entire boot process.</para>
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</listitem>
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<listitem>
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<para>Does not directly support advanced features like
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control groups (cgroups), and per-user fair share scheduling.</para>
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</listitem>
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<listitem>
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<para>Adding scripts requires manual, static sequencing decisions.</para>
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</listitem>
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</itemizedlist>
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</sect2>
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<!--
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<sect2 id='sysd-desc'>
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<title>Systemd</title>
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<para>Systemd is a group of interconnected programs that handles system and
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individual process requests. It provides a dependency system between
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various entities called "units". It automatically addresses dependencies
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between units and can execute several startup tasks in parallel. It
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provides login, inetd, logging, time, and networking services. </para>
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<bridgehead renderas="sect3">Advantages</bridgehead>
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<itemizedlist>
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<listitem>
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<para>Used on many established distributions by default.</para>
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</listitem>
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<listitem>
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<para>There is extensive documentation.
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See <ulink url="http://www.freedesktop.org/wiki/Software/systemd/"/>.</para>
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</listitem>
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<listitem>
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<para>Parallel execution of boot processes. A medium speed
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base LFS system takes 6-10 seconds from kernel start to a
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login prompt. Network connectivity is typically established
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about 2 seconds after the login prompt. More complex startup
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procedures may show a greater speedup when compared to System V.</para>
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</listitem>
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<listitem>
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<para>Implements advanced features such as control groups to
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manage related processes.</para>
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</listitem>
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<listitem>
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<para>Maintains backward compatibility with System V programs
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and scripts.</para>
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</listitem>
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</itemizedlist>
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<bridgehead renderas="sect3">Disadvantages</bridgehead>
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<itemizedlist>
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<listitem>
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<para>There is a substantial learning curve.</para>
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</listitem>
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<listitem>
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<para>Some advanced features such as dbus or cgroups cannot be
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disabled if they are not otherwise needed.</para>
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</listitem>
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<listitem>
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<para>Although implemented as several executable programs
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the user cannot choose to implement only the portions desired.</para>
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</listitem>
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<listitem>
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<para>Due to the nature of using compiled programs, systemd is
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more difficult to debug.</para>
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</listitem>
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<listitem>
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<para>Logging is done in a binary format. Extra tools must
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be used to process logs or additional processes must be implemented
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to duplicate traditional logging programs.</para>
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</listitem>
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</itemizedlist>
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</sect2>
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-->
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<!--
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<sect2 id='sysv'>
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<title>Selecting a Boot Method</title>
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<para>Selecting a boot method in LFS is relatively easy.
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Both systems are installed side-by-side. The only task needed is to
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ensure the files that are needed by the system have the correct names.
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The following scripts do that.</para>
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<screen><userinput remap="install">cat > /usr/sbin/set-systemd << "EOF"
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#! /bin/bash
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ln -svfn init-systemd /sbin/init
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ln -svfn init.d-systemd /etc/init.d
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for tool in halt poweroff reboot runlevel shutdown telinit; do
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ln -sfvn ${tool}-systemd /sbin/${tool}
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ln -svfn ${tool}-systemd.8 /usr/share/man/man8/${tool}.8
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done
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echo "Now reboot with /sbin/reboot-sysv"
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EOF
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chmod 0744 /usr/sbin/set-systemd
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cat > /usr/sbin/set-sysv << "EOF"
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#! /bin/bash
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ln -sfvn init-sysv /sbin/init
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ln -svfn init.d-sysv /etc/init.d
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for tool in halt poweroff reboot runlevel shutdown telinit; do
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ln -sfvn ${tool}-sysv /sbin/${tool}
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ln -svfn ${tool}-sysv.8 /usr/share/man/man8/${tool}.8
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done
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echo "Now reboot with /sbin/reboot-systemd"
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EOF
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chmod 0744 /usr/sbin/set-sysv</userinput></screen>
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<note><para>The comment about the correct command to reboot in the
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above scripts is correct. The reboot command for the current boot
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system must be used after the script changes the default reboot command.
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</para></note>
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<para>Now set the desired boot system. The default is System V:</para>
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<screen><userinput remap="install">/usr/sbin/set-sysv</userinput></screen>
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<para>Changing the boot system can be done at any time by running the
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appropriate script above and rebooting.</para>
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</sect2>
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-->
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</sect1>
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