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