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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

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>

<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 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>

</refentry>