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543e6bd4c2
Mention that initramfs is used, not initrd, even though we still call it that. Also add links and clarify who loads the initramfs.
306 lines
17 KiB
XML
306 lines
17 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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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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</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 boot of a Linux system. Immediately after
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power-up, the system firmware will do minimal hardware initialization, and hand control over to a boot
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loader (e.g.
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<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or
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<ulink url="https://www.gnu.org/software/grub/">GRUB</ulink>) stored on a persistent storage device. This
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boot loader will then invoke an OS kernel from disk (or the network). On systems using EFI or other types
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of firmware, this firmware may also load the kernel directly.</para>
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<para>The kernel (optionally) mounts an in-memory file system, often 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. Nowadays this is usually implemented as an initramfs — a compressed
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archive which is extracted when the kernel boots up into a lightweight in-memory file system based on
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tmpfs, but in the past normal file systems using an in-memory block device (ramdisk) were used, and the
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name "initrd" is still used to describe both concepts. It's the boot loader or the firmware that loads
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both the kernel and initrd/initramfs images into memory, but the kernel which interprets it as a file
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system. <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> may
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be used to manage services in the initrd, similarly to the real system.</para>
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<para>After the root file system is found and mounted, the initrd hands over control to the host's system
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manager (such as
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>) stored in
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the root file system, which is then responsible for probing all remaining hardware, mounting all
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necessary file systems and 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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| | | |
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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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| |
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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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| |
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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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