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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>
