source: chapter07/introduction.xml@ 5af3f9e

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<sect1 id="ch-scripts-introduction">
  <?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>

    <para>In the packages that were installed in Chapter 6, there were two
    different boot systems installed.  LFS provides the ability to easily
    select which system the user wants to use and to compare and contrast the
    two systems by actually running each system on the local computer.  The
    advantages and disadvantages of these systems is presented below.</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>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>

  <sect2 id='sysd-desc'>
    <title>Systemd</title>

    <para>Systemd is a group of interconnected programs that handles system and
    individual process requests.  It provides a dependency system between
    various entities called "units".  It automatically addresses dependencies
    between units and can execute several startup tasks in parallel.  It
    provides login, inetd, logging, time, and networking services. </para>

    <bridgehead renderas="sect3">Advantages</bridgehead>

    <itemizedlist>
      <listitem>
          <para>Used on many established distributions by default.</para>
      </listitem>

      <listitem>
          <para>There is extensive documentation. 
          See <ulink url="http://www.freedesktop.org/wiki/Software/systemd/"/>.</para>
      </listitem>

      <listitem>
          <para>Parallel execution of boot processes. A medium speed
          base LFS system takes 6-10 seconds from kernel start to a 
          login prompt.  Network connectivity is typically established 
          about 2 seconds after the login prompt.  More complex startup
          procedures may show a greater speedup when compared to System V.</para>
      </listitem>

      <listitem>
          <para>Implements advanced features such as control groups to 
          manage related processes.</para>
      </listitem>

      <listitem>
          <para>Maintains backward compatibility with System V programs 
          and scripts.</para>
      </listitem>
    </itemizedlist>

    <bridgehead renderas="sect3">Disadvantages</bridgehead>

    <itemizedlist>
      <listitem>
          <para>There is a substantial learning curve.</para>
      </listitem>

      <listitem>
          <para>Some advanced features such as dbus or cgroups cannot be
          disabled if they are not otherwise needed.</para>
      </listitem>

      <listitem>
          <para>Although implemented as several executable programs
          the user cannot choose to implement only the portions desired.</para>
      </listitem>

      <listitem>
          <para>Due to the nature of using compiled programs, systemd is
          more difficult to debug.</para>
      </listitem>

      <listitem>
          <para>Logging is done in a binary format.  Extra tools must
          be used to process logs or additional processes must be implemented
          to duplicate traditional logging programs.</para>
      </listitem>

    </itemizedlist>

  </sect2>

  <sect2 id='sysv'>
    <title>Selecting a Boot Method</title>

    <para>Selecting a boot method in LFS is relatively easy.  
    Both systems are installed side-by-side.  The only task needed is to
    ensure the files that are needed by the system have the correct names.
    The following scripts do that.</para>

<screen><userinput remap="install">cat &gt; /usr/sbin/set-systemd &lt;&lt; "EOF"
#! /bin/bash

ln -svfn init-systemd   /sbin/init
ln -svfn init.d-systemd /etc/init.d

for tool in halt poweroff reboot runlevel shutdown telinit; do
  ln -sfvn  ${tool}-systemd   /sbin/${tool}
  ln -svfn  ${tool}-systemd.8 /usr/share/man/man8/${tool}.8
done

echo "Now reboot with /sbin/reboot-sysv"
EOF

chmod 0744 /usr/sbin/set-systemd

cat &gt; /usr/sbin/set-sysv &lt;&lt; "EOF"
#! /bin/bash

ln -sfvn init-sysv    /sbin/init
ln -svfn init.d-sysv  /etc/init.d

for tool in halt poweroff reboot runlevel shutdown telinit; do
  ln -sfvn  ${tool}-sysv   /sbin/${tool}
  ln -svfn  ${tool}-sysv.8 /usr/share/man/man8/${tool}.8
done

echo "Now reboot with /sbin/reboot-systemd"
EOF

chmod 0744 /usr/sbin/set-sysv</userinput></screen>

  <note><para>The comment about the correct command to reboot in the 
  above scripts is correct.  The reboot command for the current boot
  system must be used after the script changes the default reboot command.
  </para></note>

  <para>Now set the desired boot system.  The default is System V:</para>

<screen><userinput remap="install">/usr/sbin/set-sysv</userinput></screen>

  <para>Changing the boot system can be done at any time by running the 
  appropriate script above and rebooting.</para>

  </sect2>

</sect1>