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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
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(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
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You should have received a copy of the GNU Lesser General Public License
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-->
<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
<citerefentry><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 last step the system is powered down.</para>
<para>Additional information about the system boot
process may be found in
<citerefentry><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
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>
</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>. 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
| sysroot.mount <emphasis>emergency.target</emphasis>
| |
| 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><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><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
|
