[fcc02767] | 1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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| 2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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| 3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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| 4 | <!ENTITY % general-entities SYSTEM "../general.ent">
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| 5 | %general-entities;
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| 6 | ]>
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| 7 |
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| 8 | <sect1 id="ch-config-introduction" revision="sysv">
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| 9 | <?dbhtml filename="introduction.html"?>
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| 10 |
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| 11 | <title>Introduction</title>
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| 12 |
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| 13 | <para>Booting a Linux system involves several tasks. The process must
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| 14 | mount both virtual and real file systems, initialize devices, activate swap,
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| 15 | check file systems for integrity, mount any swap partitions or files, set
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| 16 | the system clock, bring up networking, start any daemons required by the
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| 17 | system, and accomplish any other custom tasks needed by the user. This
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| 18 | process must be organized to ensure the tasks are performed in the correct
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| 19 | order but, at the same time, be executed as fast as possible.</para>
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| 20 |
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| 21 | <sect2 id='sysv-desc'>
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| 22 | <title>System V</title>
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| 23 |
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| 24 | <para>System V is the classic boot process that has been used in Unix and
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| 25 | Unix-like systems such as Linux since about 1983. It consists of a small
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| 26 | program, <command>init</command>, that sets up basic programs such as
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| 27 | <command>login</command> (via getty) and runs a script. This script,
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| 28 | usually named <command>rc</command>, controls the execution of a set of
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| 29 | additional scripts that perform the tasks required to initialize the
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| 30 | system.</para>
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| 31 |
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| 32 | <para>The <command>init</command> program is controlled by the
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| 33 | <filename>/etc/inittab</filename> file and is organized into run levels that
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| 34 | can be run by the user:</para>
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| 35 |
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[a3d0817] | 36 | <literallayout>0 — halt
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[fcc02767] | 37 | 1 — Single user mode
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| 38 | 2 — Multiuser, without networking
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| 39 | 3 — Full multiuser mode
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| 40 | 4 — User definable
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| 41 | 5 — Full multiuser mode with display manager
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[a3d0817] | 42 | 6 — reboot</literallayout>
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[fcc02767] | 43 |
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| 44 | <para>The usual default run level is 3 or 5.</para>
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| 45 |
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| 46 | <bridgehead renderas="sect3">Advantages</bridgehead>
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| 47 |
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| 48 | <itemizedlist>
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| 49 | <listitem>
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| 50 | <para>Established, well understood system.</para>
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| 51 | </listitem>
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| 52 |
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| 53 | <listitem>
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| 54 | <para>Easy to customize.</para>
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| 55 | </listitem>
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| 56 |
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| 57 | </itemizedlist>
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| 58 |
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| 59 |
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| 60 | <bridgehead renderas="sect3">Disadvantages</bridgehead>
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| 61 |
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| 62 | <itemizedlist>
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| 63 | <listitem>
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[a3d0817] | 64 | <para>May be slower to boot. A medium speed base LFS system
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[fcc02767] | 65 | takes 8-12 seconds where the boot time is measured from the
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| 66 | first kernel message to the login prompt. Network
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| 67 | connectivity is typically established about 2 seconds
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| 68 | after the login prompt.</para>
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| 69 | </listitem>
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| 70 |
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| 71 | <listitem>
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| 72 | <para>Serial processing of boot tasks. This is related to the previous
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| 73 | point. A delay in any process such as a file system check, will
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| 74 | delay the entire boot process.</para>
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| 75 | </listitem>
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| 76 |
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| 77 | <listitem>
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| 78 | <para>Does not directly support advanced features like
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| 79 | control groups (cgroups), and per-user fair share scheduling.</para>
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| 80 | </listitem>
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| 81 |
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| 82 | <listitem>
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| 83 | <para>Adding scripts requires manual, static sequencing decisions.</para>
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| 84 | </listitem>
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| 85 |
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| 86 | </itemizedlist>
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| 87 |
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| 88 | </sect2>
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| 89 |
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| 90 | </sect1>
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[a3d0817] | 91 |
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