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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36 | <literallayout>0 — halt
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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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42 | 6 — reboot</literallayout>
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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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64 | <para>May be slower to boot. A medium speed base LFS system
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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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91 |
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