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92 lines
3.1 KiB
XML
92 lines
3.1 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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<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>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</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>May be 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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</sect1>
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