| <?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"> |
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| Copyright 2012 Lennart Poettering |
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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 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>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> |
| </para> |
| </refsect1> |
| |
| </refentry> |