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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
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<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 now (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. After the root
file system is found and mounted the initrd hands over
control to the 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
(various | rescue.service
sockets...) | |
| | v
v | <emphasis>rescue.target</emphasis>
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>Initrd System Manager Bootup</title>
<para>The default target in the initrd is the initrd.target. The bootup process is
identical to the System Manager Bootup until it reaches basic.target. From there,
systemd approaches the special initrd.target. If the root device can be mounted at
/sysroot, the sysroot.mount unit becomes active and the initrd-root-fs.target is
reached.
The initrd-parse-etc.service scans /sysroot/etc/fstab for a possible /usr mountpoint
and additional entries marked with the <emphasis>x-initrd.mount</emphasis> option.
All entries found are mounted below /sysroot, and the initrd-fs.target is reached.
The initrd-cleanup.service isolates to the initrd-switch-root.target, where cleanup
services can run. As the very last step, the initrd-switch-root.service is activated,
which will cause the system to switch its root to /sysroot.
</para>
<programlisting>
(identical to System Manager Bootup)
:
:
v
basic.target
| emergency.service
______________________/| |
/ | v
| sysroot.mount <emphasis>emergency.target</emphasis>
| |
| v
| initrd-root-fs.target
| |
| 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
______________________/|
/ |
| initrd-udevadm-cleanup-db.service
| |
(custom initrd services) |
| |
\______________________ |
\|
v
initrd-switch-root.target
|
v
initrd-switch-root.service
|
v
switch-root
</programlisting>
</refsect1>
<refsect1>
<title>System Manager Shutdown</title>
<para>System shutdown 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>
|
