man: introduce bootup(7)

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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+  This file is part of systemd.
+
+  Copyright 2012 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+-->
+
+<refentry id="bootup">
+
+        <refentryinfo>
+                <title>bootup</title>
+                <productname>systemd</productname>
+
+                <authorgroup>
+                        <author>
+                                <contrib>Developer</contrib>
+                                <firstname>Lennart</firstname>
+                                <surname>Poettering</surname>
+                                <email>lennart@poettering.net</email>
+                        </author>
+                </authorgroup>
+        </refentryinfo>
+
+        <refmeta>
+                <refentrytitle>bootup</refentrytitle>
+                <manvolnum>7</manvolnum>
+        </refmeta>
+
+        <refnamediv>
+                <refname>bootup</refname>
+                <refpurpose>The System Bootup Process</refpurpose>
+        </refnamediv>
+
+        <refsect1>
+                <title>Description</title>
+
+                <para>A number of different components are involved in the
+                system boot. Immediately after power-up, the system
+                BIOS will do minimal hardware initialization, and hand
+                control over to a boot loader stored on a persistant
+                storage device. This boot loader will then invoke an
+                OS kernel from disk (or the network). In the Linux
+                case this kernel now (optionally) extracts and
+                executes an initial RAM disk image (initrd) such as
+                <citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+                which looks for the root file system. After the root
+                file system is found and mounted the initrd hands over
+                control to the system manager (such as
+                <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
+                stored on the OS image which is then responsible for
+                probing all remaining hardware, mounting all necessary
+                file systems and spawning all configured
+                services.</para>
+
+                <para>On shutdown the system manager stops all
+                services, unmounts all file systems (detaching the
+                storage technologies backing them), and then
+                (optionally) jumps back into the initrd code which
+                unmounts/detaches the root file system and the storage
+                it resides on. As last step the system powered down.</para>
+
+                <para>Additional information about the system boot
+                process may be found in
+                <citerefentry><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>System Manager Bootup</title>
+
+                <para>At boot, the system manager on the OS image is
+                responsible for initializing the required file
+                systems, services and drivers that are necessary for
+                operation of the system. On
+                <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+                systems this process is split up in various discrete
+                steps which are exposed as target units. (See
+                <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+                for detailed information about target units.) The
+                boot-up process is highly parallelized so that the
+                order in which specific target units are reached is not
+                deterministic, but still adheres to a limited amount
+                of ordering structure.</para>
+
+                <para>When systemd starts up the system it will
+                activate all units that are dependencies of
+                <filename>default.target</filename> (as well as
+                recursively all dependencies of these
+                dependencies). Usually
+                <filename>default.target</filename> is simply an alias
+                of <filename>graphical.target</filename> or
+                <filename>multi-user.target</filename> depending on
+                whether the system is configured for a graphical UI or
+                only for a text console. To enforce minimal ordering
+                between the units pulled in a number of well-known
+                target units are available, as listed on
+                <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
+
+                <para>The follow chart is a structural overview of
+                these well-known units and their position in the
+                boot-up logic. The arrows describe which units are
+                pulled in and ordered before which other units. Units
+                near the top are started before units nearer to the
+                bottom of the chart.</para>
+
+<programlisting>local-fs-pre.target
+         |
+         v
+(various mounts and   (various swap   (various cryptsetup
+ fsck services...)     devices...)        devices...)       (various low-level   (various low-level
+         |                  |                  |             services: udevd,     API VFS mounts:
+         v                  v                  v             tmpfiles, random     mqueue, configfs,
+  local-fs.target      swap.target     cryptsetup.target    seed, sysctl, ...)      debugfs, ...)
+         |                  |                  |                    |                    |
+         \__________________|_________________ | ___________________|____________________/
+                                              \|/
+                                               v
+                                        sysinit.target
+                                               |
+                             _________________/|\___________________
+                            /                  |                    \
+                            |                  |                    |
+                            v                  |                    v
+                        (various               |              rescue.service
+                       sockets...)             |                    |
+                            |                  |                    v
+                            v                  |              <emphasis>rescue.target</emphasis>
+                     sockets.target            |
+                            |                  |
+                            \_________________ |
+                                              \|
+                                               v
+                                         basic.target
+                                               |
+                             _________________/|                                 emergency.service
+                            /                  |                                         |
+                            |                  |                                         v
+                            v                  v                                 <emphasis>emergency.target</emphasis>
+                     (various system    (various system
+                        services           services)
+                      required for             |
+                     graphical UIs)            v
+                            |           <emphasis>multi-user.target</emphasis>
+                            |                  |
+                            | _________________/
+                            |/
+                            v
+                    <emphasis>graphical.target</emphasis></programlisting>
+
+                <para>Target units that are commonly used as boot
+                targets are <emphasis>emphasized</emphasis>. These
+                units are good choices as goal targets, for
+                example by passing them to the
+                <varname>systemd.unit=</varname> kernel command line
+                option (see
+                <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
+                or by symlinking <filename>default.target</filename>
+                to them.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>System Manager Shutdown</title>
+
+                <para>System shutdown also consists of various target
+                units with some minimal ordering structure
+                applied:</para>
+
+
+
+
+<programlisting>                   (conflicts with  (conflicts with
+                     all system     all file system
+                      services)     mounts, swaps,
+                          |           cryptsetup
+                          |          devices, ...)
+                          |                |
+                          v                v
+                   shutdown.target    umount.target
+                          |                |
+                          \_______   ______/
+                                  \ /
+                                   v
+                          (various low-level
+                               services)
+                                   |
+                                   v
+                             final.target
+                                   |
+        __________________________/ \_____________________
+       /                  |                |              \
+       |                  |                |              |
+       v                  v                v              v
+reboot.service    poweroff.service   halt.service   kexec.service
+       |                  |                |              |
+       v                  v                v              v
+<emphasis>reboot.target</emphasis>     <emphasis>poweroff.target</emphasis>    <emphasis>halt.target</emphasis>    <emphasis>kexec.target</emphasis></programlisting>
+
+                <para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>See Also</title>
+                <para>
+                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+                </para>
+        </refsect1>
+
+</refentry>