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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 |_________________ | ___________________/

\|/

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>

<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>.

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>