source: chapter09/introduction.xml@ 9e7475a

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<sect1 id="ch-config-introduction" revision="sysv">
  <?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>

  <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 &mdash; halt
1 &mdash; Single user mode
2 &mdash; Multiuser, without networking
3 &mdash; Full multiuser mode
4 &mdash; User definable
5 &mdash; Full multiuser mode with display manager
6 &mdash; 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>May be 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>

</sect1>