315 lines
17 KiB
XML
315 lines
17 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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SPDX-License-Identifier: LGPL-2.1+
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This file is part of systemd.
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Copyright 2012 Lennart Poettering
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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 system
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boot. Immediately after power-up, the system BIOS will do minimal
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hardware initialization, and hand control over to a boot loader
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stored on a persistent storage device. This boot loader will then
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invoke an OS kernel from disk (or the network). In the Linux case,
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this kernel (optionally) extracts and executes an initial RAM disk
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image (initrd), such as generated by
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<citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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which looks for the root file system (possibly using
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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for this). After the root file system is found and mounted, the
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initrd hands over control to the host's 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 probing all
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remaining hardware, mounting all necessary file systems and
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spawning all configured services.</para>
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<para>On shutdown, the system manager stops all services, unmounts
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all file systems (detaching the storage technologies backing
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them), and then (optionally) jumps back into the initrd code which
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unmounts/detaches the root file system and the storage it resides
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on. As a last step, the system is powered down.</para>
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<para>Additional information about the system boot process may be
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found in
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<citerefentry project='man-pages'><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 responsible
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for initializing the required file systems, services and drivers
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that are necessary for 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 steps which
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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 boot-up process
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is highly parallelized so that the order in which specific target
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units are reached is not deterministic, but still adheres to a
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limited amount of ordering structure.</para>
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<para>When systemd starts up the system, it will activate all
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units that are dependencies of <filename>default.target</filename>
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(as well as recursively all dependencies of these dependencies).
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Usually, <filename>default.target</filename> is simply an alias of
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<filename>graphical.target</filename> or
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<filename>multi-user.target</filename>, depending on whether the
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system is configured for a graphical UI or only for a text
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console. To enforce minimal ordering between the units pulled in,
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a number of well-known 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 these
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well-known units and their position in the boot-up logic. The
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arrows describe which units are pulled in and ordered before which
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other units. Units near the top are started before units nearer to
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the bottom of the chart.</para>
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<!-- note: do not use unicode ellipsis here, because docbook will replace that
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with three dots anyway, messing up alignment -->
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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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| | | | |
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v v | v v
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(various (various | (various rescue.service
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timers...) paths...) | sockets...) |
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| | | | v
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v v | v <emphasis>rescue.target</emphasis>
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timers.target paths.target | sockets.target
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v \_________________ | ___________________/
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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 targets are
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<emphasis>emphasized</emphasis>. These units are good choices as
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goal targets, for example by passing them to the
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<varname>systemd.unit=</varname> kernel command line 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> to them.
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</para>
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<para><filename>timers.target</filename> is pulled-in by
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<filename>basic.target</filename> asynchronously. This allows
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timers units to depend on services which become only available
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later in boot.</para>
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</refsect1>
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<refsect1>
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<title>Bootup in the Initial RAM Disk (initrd)</title>
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<para>The initial RAM disk implementation (initrd) can be set up
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using systemd as well. In this case, boot up inside the initrd
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follows the following structure.</para>
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<para>The default target in the initrd is
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<filename>initrd.target</filename>. The bootup process begins
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identical to the system manager bootup (see above) until it
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reaches <filename>basic.target</filename>. From there, systemd
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approaches the special target <filename>initrd.target</filename>.
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Before any file systems are mounted, it must be determined whether
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the system will resume from hibernation or proceed with normal boot.
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This is accomplished by <filename>systemd-hibernate-resume@.service</filename>
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which must be finished before <filename>local-fs-pre.target</filename>,
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so no filesystems can be mounted before the check is complete.
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When the root device becomes available,
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<filename>initd-root-device.target</filename> is reached.
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If the root device can be mounted at
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<filename>/sysroot</filename>, the
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<filename>sysroot.mount</filename> unit becomes active and
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<filename>initrd-root-fs.target</filename> is reached. The service
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<filename>initrd-parse-etc.service</filename> scans
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<filename>/sysroot/etc/fstab</filename> for a possible
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<filename>/usr</filename> mount point and additional entries
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marked with the <emphasis>x-initrd.mount</emphasis> option. All
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entries found are mounted below <filename>/sysroot</filename>, and
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<filename>initrd-fs.target</filename> is reached. The service
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<filename>initrd-cleanup.service</filename> isolates to the
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<filename>initrd-switch-root.target</filename>, where cleanup
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services can run. As the very last step, the
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<filename>initrd-switch-root.service</filename> is activated,
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which will cause the system to switch its root to
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<filename>/sysroot</filename>.
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</para>
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<programlisting> : (beginning identical to above)
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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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| initrd-root-device.target <emphasis>emergency.target</emphasis>
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| v
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| sysroot.mount
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| v
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| initrd-root-fs.target
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| v
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v initrd-parse-etc.service
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(custom initrd |
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services...) 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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| 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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/ v
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| initrd-udevadm-cleanup-db.service
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v |
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(custom initrd |
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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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Transition to Host OS</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 with systemd also consists of various target
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units with some minimal ordering structure applied:</para>
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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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<para>Note that
|
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<citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<filename>systemd-reboot.service</filename>, <filename>systemd-poweroff.service</filename> and
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<filename>systemd-kexec.service</filename> will transition the system and server manager (PID 1) into the second
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phase of system shutdown (implemented in the <filename>systemd-shutdown</filename> binary), which will unmount any
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remaining file systems, kill any remaining processes and release any other remaining resources, in a simple and
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robust fashion, without taking any service or unit concept into account anymore. At that point, regular
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applications and resources are generally terminated and released already, the second phase hence operates only as
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safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based
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shutdown phase described above.</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 project='man-pages'><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>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry project='die-net'><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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