./tools/notsd-move

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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>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 persistent storage device. This boot loader will then
+    invoke an OS kernel from disk (or the network). In the Linux case,
+    this kernel (optionally) extracts and executes an initial RAM disk
+    image (initrd), such as generated by
+    <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+    which looks for the root file system (possibly using
+    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+    for this). After the root file system is found and mounted, the
+    initrd hands over control to the host's 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 a last step, the system is powered down.</para>
+
+    <para>Additional information about the system boot process may be
+    found in
+    <citerefentry project='man-pages'><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 following 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                  |                    v                    v
+     (various           (various               |                (various          rescue.service
+    timers...)          paths...)              |               sockets...)               |
+         |                  |                  |                    |                    v
+         v                  v                  |                    v              <emphasis>rescue.target</emphasis>
+   timers.target      paths.target             |             sockets.target
+         |                  |                  |                    |
+         v                  \_________________ | ___________________/
+                                              \|/
+                                               v
+                                         basic.target
+                                               |
+          ____________________________________/|                                 emergency.service
+         /                  |                  |                                         |
+         |                  |                  |                                         v
+         v                  v                  v                                 <emphasis>emergency.target</emphasis>
+     display-        (various system    (various system
+ manager.service         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>
+
+    <para><filename>timers.target</filename> is pulled-in by
+    <filename>basic.target</filename> asynchronously. This allows
+    timers units to depend on services which become only available
+    later in boot.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Bootup in the Initial RAM Disk (initrd)</title>
+    <para>The initial RAM disk implementation (initrd) can be set up
+    using systemd as well. In this case, boot up inside the initrd
+    follows the following structure.</para>
+
+    <para>The default target in the initrd is
+    <filename>initrd.target</filename>. The bootup process begins
+    identical to the system manager bootup (see above) until it
+    reaches <filename>basic.target</filename>. From there, systemd
+    approaches the special target <filename>initrd.target</filename>.
+    When the root device becomes available,
+    <filename>initd-root-device.target</filename> is reached.
+    If the root device can be mounted at
+    <filename>/sysroot</filename>, the
+    <filename>sysroot.mount</filename> unit becomes active and
+    <filename>initrd-root-fs.target</filename> is reached. The service
+    <filename>initrd-parse-etc.service</filename> scans
+    <filename>/sysroot/etc/fstab</filename> for a possible
+    <filename>/usr</filename> mount point and additional entries
+    marked with the <emphasis>x-initrd.mount</emphasis> option. All
+    entries found are mounted below <filename>/sysroot</filename>, and
+    <filename>initrd-fs.target</filename> is reached. The service
+    <filename>initrd-cleanup.service</filename> isolates to the
+    <filename>initrd-switch-root.target</filename>, where cleanup
+    services can run. As the very last step, the
+    <filename>initrd-switch-root.service</filename> is activated,
+    which will cause the system to switch its root to
+    <filename>/sysroot</filename>.
+    </para>
+
+<programlisting>                                               : (beginning identical to above)
+                                               :
+                                               v
+                                         basic.target
+                                               |                                 emergency.service
+                        ______________________/|                                         |
+                       /                       |                                         v
+                       |            initrd-root-device.target                    <emphasis>emergency.target</emphasis>
+                       |                       |
+                       |                       v
+                       |                  sysroot.mount
+                       |                       |
+                       |                       v
+                       |             initrd-root-fs.target
+                       |                       |
+                       |                       v
+                       v            initrd-parse-etc.service
+                (custom initrd                 |
+                 services...)                  v
+                       |            (sysroot-usr.mount and
+                       |             various mounts marked
+                       |               with fstab option
+                       |              x-initrd.mount...)
+                       |                       |
+                       |                       v
+                       |                initrd-fs.target
+                       \______________________ |
+                                              \|
+                                               v
+                                          initrd.target
+                                               |
+                                               v
+                                     initrd-cleanup.service
+                                          isolates to
+                                    initrd-switch-root.target
+                                               |
+                                               v
+                        ______________________/|
+                       /                       v
+                       |        initrd-udevadm-cleanup-db.service
+                       v                       |
+                (custom initrd                 |
+                 services...)                  |
+                       \______________________ |
+                                              \|
+                                               v
+                                   initrd-switch-root.target
+                                               |
+                                               v
+                                   initrd-switch-root.service
+                                               |
+                                               v
+                                     Transition to Host OS</programlisting>
+  </refsect1>
+
+  <refsect1>
+    <title>System Manager Shutdown</title>
+
+    <para>System shutdown with systemd 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
+systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-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 project='man-pages'><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>,
+      <citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>