systemd/man/bootup.xml

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<refentry id="bootup">

  <refentryinfo>
    <title>bootup</title>
    <productname>systemd</productname>
  </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 boot of a Linux system. Immediately after
    power-up, the system firmware will do minimal hardware initialization, and hand control over to a boot
    loader (e.g.
    <citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or
    <ulink url="https://www.gnu.org/software/grub/">GRUB</ulink>) stored on a persistent storage device. This
    boot loader will then invoke an OS kernel from disk (or the network). On systems using EFI or other types
    of firmware, this firmware may also load the kernel directly.</para>

    <para>The kernel (optionally) mounts an in-memory file system, often generated by <citerefentry
    project='man-pages'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>, which
    looks for the root file system. Nowadays this is implemented as an "initramfs" — a compressed CPIO
    archive that the kernel extracts into a tmpfs. In the past normal file systems using an in-memory block
    device (ramdisk) were used, and the name "initrd" is still used to describe both concepts. It's the boot
    loader or the firmware that loads both the kernel and initrd/initramfs images into memory, but the kernel
    which interprets it as a file system.
    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> may be used
    to manage services in the initrd, similarly to the real system.</para>

    <para>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 in
    the root file system, 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>

    <!-- note: do not use unicode ellipsis here, because docbook will replace that
         with three dots anyway, messing up alignment -->
<programlisting>                             cryptsetup-pre.target veritysetup-pre.target
                                                  |
(various low-level                                v
 API VFS mounts:             (various cryptsetup/veritysetup devices...)
 mqueue, configfs,                                |    |
 debugfs, ...)                                    v    |
 |                                  cryptsetup.target  |
 |  (various swap                                 |    |    remote-fs-pre.target
 |   devices...)                                  |    |     |        |
 |    |                                           |    |     |        v
 |    v                       local-fs-pre.target |    |     |  (network file systems)
 |  swap.target                       |           |    v     v                 |
 |    |                               v           |  remote-cryptsetup.target  |
 |    |  (various low-level  (various mounts and  |  remote-veritysetup.target |
 |    |   services: udevd,    fsck services...)   |             |              |
 |    |   tmpfiles, random            |           |             |    remote-fs.target
 |    |   seed, sysctl, ...)          v           |             |              |
 |    |      |                 local-fs.target    |             | _____________/
 |    |      |                        |           |             |/
 \____|______|_______________   ______|___________/             |
                             \ /                                |
                              v                                 |
                       sysinit.target                           |
                              |                                 |
       ______________________/|\_____________________           |
      /              |        |      |               \          |
      |              |        |      |               |          |
      v              v        |      v               |          |
 (various       (various      |  (various            |          |
  timers...)      paths...)   |   sockets...)        |          |
      |              |        |      |               |          |
      v              v        |      v               |          |
timers.target  paths.target   |  sockets.target      |          |
      |              |        |      |               v          |
      v              \_______ | _____/         rescue.service   |
                             \|/                     |          |
                              v                      v          |
                          basic.target         <emphasis>rescue.target</emphasis>    |
                              |                                 |
                      ________v____________________             |
                     /              |              \            |
                     |              |              |            |
                     v              v              v            |
                 display-    (various system   (various system  |
             manager.service     services        services)      |
                     |         required for        |            |
                     |        graphical UIs)       v            v
                     |              |            <emphasis>multi-user.target</emphasis>
emergency.service    |              |              |
        |            \_____________ | _____________/
        v                          \|/
<emphasis>emergency.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>User manager startup</title>

    <para>The system manager starts the <filename>user@<replaceable>uid</replaceable>.service</filename> unit
    for each user, which launches a separate unprivileged instance of <command>systemd</command> for each
    user — the user manager. Similarly to the system manager, the user manager starts units which are pulled
    in by <filename>default.target</filename>. The following chart is a structural overview of the well-known
    user units. For non-graphical sessions, <filename>default.target</filename> is used. Whenever the user
    logs into a graphical session, the login manager will start the
    <filename>graphical-session.target</filename> target that is used to pull in units required for the
    graphical session. A number of targets (shown on the right side) are started when specific hardware is
    available to the user.</para>

<programlisting>
   (various           (various         (various
    timers...)         paths...)        sockets...)    (sound devices)
        |                  |                 |               |
        v                  v                 v               v
  timers.target      paths.target     sockets.target    sound.target
        |                  |                 |
        \______________   _|_________________/         (bluetooth devices)
                       \ /                                   |
                        V                                    v
                  basic.target                          bluetooth.target
                        |
             __________/ \_______                      (smartcard devices)
            /                    \                           |
            |                    |                           v
            |                    v                      smartcard.target
            v            graphical-session-pre.target
(various user services)          |                       (printers)
            |                    v                           |
            |       (services for the graphical session)     v
            |                    |                       printer.target
            v                    v
     <emphasis>default.target</emphasis>      graphical-session.target</programlisting>

 </refsect1>

  <refsect1>
    <title>Bootup in the initrd</title>

    <para>Systemd can be used in the initrd as well. It detects the initrd environment by checking for the
    <filename>/etc/initrd-release</filename> file. The default target in the initrd is
    <filename>initrd.target</filename>. The bootup process is identical to the system manager bootup until
    the target <filename>basic.target</filename>. After that, systemd executes the special target
    <filename>initrd.target</filename>.

    Before any file systems are mounted, the manager will determine whether the system shall resume from
    hibernation or proceed with normal boot. This is accomplished by
    <filename>systemd-hibernate-resume@.service</filename> which must be finished before
    <filename>local-fs-pre.target</filename>, so no filesystems can be mounted before the check is complete.

    When the root device becomes available,
    <filename>initrd-root-device.target</filename> is reached.
    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
       |            initrd-root-device.target          <emphasis>emergency.target</emphasis>
       |                       |
       |                       v
       |                  sysroot.mount
       |                       |
       |                       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/
                               |           veritysetup
                               |          devices, ...)
                               |                |
                               v                v
                        shutdown.target    umount.target
                               |                |
                               \_______   ______/
                                       \ /
                                        v
                               (various low-level
                                    services)
                                        |
                                        v
                                  final.target
                                        |
            ___________________________/ \_________________
           /               |               |               \
           |               |               |               |
           v               |               |               |
systemd-reboot.service     |               |               |
           |               v               |               |
           |    systemd-poweroff.service   |               |
           v               |               v               |
     <emphasis>reboot.target</emphasis>         |      systemd-halt.service     |
                           v               |               v
                   <emphasis>poweroff.target</emphasis>         |    systemd-kexec.service
                                           v               |
                                      <emphasis>halt.target</emphasis>          |
                                                           v
                                                     <emphasis>kexec.target</emphasis></programlisting>

    <para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>

    <para>Note that
    <citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
    <filename>systemd-reboot.service</filename>, <filename>systemd-poweroff.service</filename> and
    <filename>systemd-kexec.service</filename> will transition the system and server manager (PID 1) into the second
    phase of system shutdown (implemented in the <filename>systemd-shutdown</filename> binary), which will unmount any
    remaining file systems, kill any remaining processes and release any other remaining resources, in a simple and
    robust fashion, without taking any service or unit concept into account anymore. At that point, regular
    applications and resources are generally terminated and released already, the second phase hence operates only as
    safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based
    shutdown phase described above.</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><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>