8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster<?xml version='1.0'?> <!--*-nxml-*-->

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8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster<refentry id="bootup">

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <refentryinfo>

0c9594d96d580b0cba488fa7d01802fbb49d8a3eCraig McDonnell <title>bootup</title>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <productname>systemd</productname>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <authorgroup>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <author>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <contrib>Developer</contrib>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <firstname>Lennart</firstname>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <surname>Poettering</surname>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <email>lennart@poettering.net</email>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster </author>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster </authorgroup>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster </refentryinfo>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <refmeta>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <refentrytitle>bootup</refentrytitle>

0c9594d96d580b0cba488fa7d01802fbb49d8a3eCraig McDonnell <manvolnum>7</manvolnum>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster </refmeta>

0c9594d96d580b0cba488fa7d01802fbb49d8a3eCraig McDonnell

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <refnamediv>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <refname>bootup</refname>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <refpurpose>System bootup process</refpurpose>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster </refnamediv>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <refsect1>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <title>Description</title>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <para>A number of different components are involved in

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster the system boot. Immediately after power-up, the

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster system BIOS will do minimal hardware initialization,

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster and hand control over to a boot loader stored on a

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster persistent storage device. This boot loader will then

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster invoke an OS kernel from disk (or the network). In the

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster Linux case this kernel (optionally) extracts and

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster executes an initial RAM disk image (initrd) such as

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <citerefentry><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster which looks for the root file system (possibly using

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster for this). After the root file system is found and

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster mounted the initrd hands over control to the host's

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster system manager (such as

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster stored on the OS image which is then responsible for

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster probing all remaining hardware, mounting all necessary

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster file systems and spawning all configured

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster services.</para>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <para>On shutdown the system manager stops all

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster services, unmounts all file systems (detaching the

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster storage technologies backing them), and then

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster (optionally) jumps back into the initrd code which

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster unmounts/detaches the root file system and the storage

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster it resides on. As last step the system is powered down.</para>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <para>Additional information about the system boot

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster process may be found in

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <citerefentry><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster </refsect1>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <refsect1>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <title>System Manager Bootup</title>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <para>At boot, the system manager on the OS image is

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster responsible for initializing the required file

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster systems, services and drivers that are necessary for

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster operation of the system. On

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster systems this process is split up in various discrete

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster steps which are exposed as target units. (See

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster for detailed information about target units.) The

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster boot-up process is highly parallelized so that the

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster order in which specific target units are reached is not

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster deterministic, but still adheres to a limited amount

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster of ordering structure.</para>

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster <para>When systemd starts up the system it will

8af80418ba1ec431c8027fa9668e5678658d3611Allan Foster 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>