| <?xml version='1.0'?> <!--*-nxml-*--> |
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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 (optionally) extracts and |
| executes an initial RAM disk image (initrd), such as |
| generated by |
| <citerefentry><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><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><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> |