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This file is part of systemd.
Copyright 2012 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
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<refentry id= "bootup" >
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<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>
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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
| | | services: udevd, API VFS mounts:
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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| | | | |
\__________________|_________________ | ___________________|____________________/
\|/
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
| | | |
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v \_________________ | ___________________/
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\|/
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>
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<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> )
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or by symlinking <filename > default.target</filename> to them.
</para>
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<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> .
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Before any file systems are mounted, it must be determined whether
the system will resume from hibernation or proceed with normal boot.
This is accomplished by <filename > systemd-hibernate-resume@.service</filename>
which must be finished before <filename > local-fs-pre.target</filename> ,
so no filesystems can be mounted before the check is complete.
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When the root device becomes available,
<filename > initd-root-device.target</filename> is reached.
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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>
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<programlisting > : (beginning identical to above)
:
v
basic.target
| emergency.service
______________________/| |
/ | v
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| initrd-root-device.target <emphasis > emergency.target</emphasis>
| |
| v
| sysroot.mount
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| |
| 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>
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</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>
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<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
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systemd-reboot.service systemd-poweroff.service systemd-halt.service systemd-kexec.service
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| | | |
v v v v
<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>
</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>
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</refentry>
