[thirdparty/systemd.git] / man / bootup.xml

<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1+ -->

<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 usually implemented as an initramfs — a compressed
    archive which is extracted when the kernel boots up into a lightweight in-memory file system based on
    tmpfs, but 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
                                                  |
(various low-level                                v
 API VFS mounts:                 (various cryptsetup 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  |             |              |
 |    |   services: udevd,    fsck services...)   |             |    remote-fs.target
 |    |   tmpfiles, random            |           |             |             /
 |    |   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 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>systemd detects that it is run within an initrd by checking
    for the file <filename>/etc/initrd-release</filename>.
    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>.

    Before any file systems are mounted, it must be determined whether
    the system will 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
                                         |          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>

    <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>
Commit	Line	Data
013d8a39	1	<?xml version='1.0'?> <!---nxml--->
3a54a157	2	<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
12b42c76	3	"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
0307f791	4	<!-- SPDX-License-Identifier: LGPL-2.1+ -->
013d8a39 LP	5
	6	<refentry id="bootup">
	7
798d3a52 ZJS	8	<refentryinfo>
	9	<title>bootup</title>
	10	<productname>systemd</productname>
798d3a52 ZJS	11	</refentryinfo>
	12
	13	<refmeta>
	14	<refentrytitle>bootup</refentrytitle>
	15	<manvolnum>7</manvolnum>
	16	</refmeta>
	17
	18	<refnamediv>
	19	<refname>bootup</refname>
	20	<refpurpose>System bootup process</refpurpose>
	21	</refnamediv>
	22
	23	<refsect1>
	24	<title>Description</title>
	25
543e6bd4 ZJS	26	<para>A number of different components are involved in the boot of a Linux system. Immediately after
	27	power-up, the system firmware will do minimal hardware initialization, and hand control over to a boot
	28	loader (e.g.
	29	<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or
	30	<ulink url="https://www.gnu.org/software/grub/">GRUB</ulink>) stored on a persistent storage device. This
	31	boot loader will then invoke an OS kernel from disk (or the network). On systems using EFI or other types
	32	of firmware, this firmware may also load the kernel directly.</para>
	33
	34	<para>The kernel (optionally) mounts an in-memory file system, often generated by
b7a47345	35	<citerefentry project='man-pages'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
543e6bd4 ZJS	36	which looks for the root file system. Nowadays this is usually implemented as an initramfs — a compressed
	37	archive which is extracted when the kernel boots up into a lightweight in-memory file system based on
	38	tmpfs, but in the past normal file systems using an in-memory block device (ramdisk) were used, and the
	39	name "initrd" is still used to describe both concepts. It's the boot loader or the firmware that loads
	40	both the kernel and initrd/initramfs images into memory, but the kernel which interprets it as a file
	41	system. <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> may
	42	be used to manage services in the initrd, similarly to the real system.</para>
	43
	44	<para>After the root file system is found and mounted, the initrd hands over control to the host's system
	45	manager (such as
	46	<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>) stored in
	47	the root file system, which is then responsible for probing all remaining hardware, mounting all
	48	necessary file systems and spawning all configured services.</para>
798d3a52 ZJS	49
	50	<para>On shutdown, the system manager stops all services, unmounts
	51	all file systems (detaching the storage technologies backing
	52	them), and then (optionally) jumps back into the initrd code which
	53	unmounts/detaches the root file system and the storage it resides
	54	on. As a last step, the system is powered down.</para>
	55
	56	<para>Additional information about the system boot process may be
	57	found in
	58	<citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
	59	</refsect1>
	60
	61	<refsect1>
	62	<title>System Manager Bootup</title>
	63
	64	<para>At boot, the system manager on the OS image is responsible
	65	for initializing the required file systems, services and drivers
	66	that are necessary for operation of the system. On
	67	<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
	68	systems, this process is split up in various discrete steps which
	69	are exposed as target units. (See
	70	<citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
	71	for detailed information about target units.) The boot-up process
	72	is highly parallelized so that the order in which specific target
	73	units are reached is not deterministic, but still adheres to a
	74	limited amount of ordering structure.</para>
	75
	76	<para>When systemd starts up the system, it will activate all
	77	units that are dependencies of <filename>default.target</filename>
	78	(as well as recursively all dependencies of these dependencies).
	79	Usually, <filename>default.target</filename> is simply an alias of
	80	<filename>graphical.target</filename> or
	81	<filename>multi-user.target</filename>, depending on whether the
	82	system is configured for a graphical UI or only for a text
	83	console. To enforce minimal ordering between the units pulled in,
	84	a number of well-known target units are available, as listed on
	85	<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
	86
	87	<para>The following chart is a structural overview of these
	88	well-known units and their position in the boot-up logic. The
	89	arrows describe which units are pulled in and ordered before which
	90	other units. Units near the top are started before units nearer to
	91	the bottom of the chart.</para>
013d8a39	92
24c44fdf ZJS	93	<!-- note: do not use unicode ellipsis here, because docbook will replace that
24c44fdf ZJS	94	with three dots anyway, messing up alignment -->
f71502c4 ZJS	95	<programlisting> cryptsetup-pre.target
	96	\|
	97	(various low-level v
	98	API VFS mounts: (various cryptsetup devices...)
	99	mqueue, configfs, \| \|
	100	debugfs, ...) v \|
	101	\| cryptsetup.target \|
	102	\| (various swap \| \| remote-fs-pre.target
	103	\| devices...) \| \| \| \|
	104	\| \| \| \| \| v
	105	\| v local-fs-pre.target \| \| \| (network file systems)
	106	\| swap.target \| \| v v \|
	107	\| \| v \| remote-cryptsetup.target \|
	108	\| \| (various low-level (various mounts and \| \| \|
	109	\| \| services: udevd, fsck services...) \| \| remote-fs.target
	110	\| \| tmpfiles, random \| \| \| /
	111	\| \| seed, sysctl, ...) v \| \| /
	112	\| \| \| local-fs.target \| \| /
	113	\| \| \| \| \| \| /
	114	\____\|______\|_______________ ______\|___________/ \| /
	115	\ / \| /
	116	v \| /
	117	sysinit.target \| /
	118	\| \| /
	119	______________________/\|\_____________________ \| /
	120	/ \| \| \| \ \| /
	121	\| \| \| \| \| \| /
	122	v v \| v \| \| /
	123	(various (various \| (various \| \|/
	124	timers...) paths...) \| sockets...) \| \|
	125	\| \| \| \| \| \|
	126	v v \| v \| \|
	127	timers.target paths.target \| sockets.target \| \|
	128	\| \| \| \| v \|
	129	v \_______ \| _____/ rescue.service \|
	130	\\|/ \| \|
	131	v v \|
	132	basic.target <emphasis>rescue.target</emphasis> \|
	133	\| \|
	134	________v____________________ \|
	135	/ \| \ \|
	136	\| \| \| \|
	137	v v v \|
	138	display- (various system (various system \|
	139	manager.service services services) \|
	140	\| required for \| \|
	141	\| graphical UIs) v v
	142	\| \| <emphasis>multi-user.target</emphasis>
	143	emergency.service \| \| \|
	144	\| \_____________ \| _____________/
	145	v \\|/
	146	<emphasis>emergency.target</emphasis> v
	147	<emphasis>graphical.target</emphasis></programlisting>
798d3a52 ZJS	148
	149	<para>Target units that are commonly used as boot targets are
	150	<emphasis>emphasized</emphasis>. These units are good choices as
	151	goal targets, for example by passing them to the
	152	<varname>systemd.unit=</varname> kernel command line option (see
	153	<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
72b7d998 ZJS	154	or by symlinking <filename>default.target</filename> to them.
72b7d998 ZJS	155	</para>
798d3a52 ZJS	156
	157	<para><filename>timers.target</filename> is pulled-in by
	158	<filename>basic.target</filename> asynchronously. This allows
	159	timers units to depend on services which become only available
	160	later in boot.</para>
	161	</refsect1>
	162
277519db ZJS	163	<refsect1>
	164	<title>User manager startup</title>
	165
	166	<para>The system manager starts the <filename>user@<replaceable>uid</replaceable>.service</filename> unit
	167	for each user, which launches a separate unprivileged instance of <command>systemd</command> for each
	168	user — the user manager. Similarly to the system manager, the user manager starts units which are pulled
	169	in by <filename>default.target</filename>. The following chart is a structural overview of the well-known
	170	user units. For non-graphical sessions, <filename>default.target</filename> is used. Whenever the user
	171	logs into a graphical session, the login manager will start the
	172	<filename>graphical-session.target</filename> target that is used to pull in units required for the
86b52a39	173	graphical session. A number of targets (shown on the right side) are started when specific hardware is
277519db ZJS	174	available to the user.</para>
	175
	176	<programlisting>
	177	(various (various (various
	178	timers...) paths...) sockets...) (sound devices)
	179	\| \| \| \|
	180	v v v v
	181	timers.target paths.target sockets.target sound.target
	182	\| \| \|
	183	\______________ _\|_________________/ (bluetooth devices)
	184	\ / \|
	185	V v
	186	basic.target bluetooth.target
	187	\|
	188	__________/ \_______ (smartcard devices)
	189	/ \ \|
	190	\| \| v
	191	\| v smartcard.target
	192	v graphical-session-pre.target
	193	(various user services) \| (printers)
	194	\| v \|
a51a3247	195	\| (services for the graphical session) v
277519db ZJS	196	\| \| printer.target
	197	v v
	198	<emphasis>default.target</emphasis> graphical-session.target</programlisting>
	199
	200	</refsect1>
	201
798d3a52 ZJS	202	<refsect1>
	203	<title>Bootup in the Initial RAM Disk (initrd)</title>
	204	<para>The initial RAM disk implementation (initrd) can be set up
	205	using systemd as well. In this case, boot up inside the initrd
	206	follows the following structure.</para>
	207
6e9d2465 JR	208	<para>systemd detects that it is run within an initrd by checking
	209	for the file <filename>/etc/initrd-release</filename>.
	210	The default target in the initrd is
798d3a52 ZJS	211	<filename>initrd.target</filename>. The bootup process begins
	212	identical to the system manager bootup (see above) until it
	213	reaches <filename>basic.target</filename>. From there, systemd
	214	approaches the special target <filename>initrd.target</filename>.
6d9e45e9 JH	215
	216	Before any file systems are mounted, it must be determined whether
	217	the system will resume from hibernation or proceed with normal boot.
	218	This is accomplished by <filename>systemd-hibernate-resume@.service</filename>
	219	which must be finished before <filename>local-fs-pre.target</filename>,
	220	so no filesystems can be mounted before the check is complete.
	221
7163e1ca	222	When the root device becomes available,
57680ddd	223	<filename>initrd-root-device.target</filename> is reached.
798d3a52 ZJS	224	If the root device can be mounted at
	225	<filename>/sysroot</filename>, the
	226	<filename>sysroot.mount</filename> unit becomes active and
	227	<filename>initrd-root-fs.target</filename> is reached. The service
	228	<filename>initrd-parse-etc.service</filename> scans
	229	<filename>/sysroot/etc/fstab</filename> for a possible
	230	<filename>/usr</filename> mount point and additional entries
	231	marked with the <emphasis>x-initrd.mount</emphasis> option. All
	232	entries found are mounted below <filename>/sysroot</filename>, and
	233	<filename>initrd-fs.target</filename> is reached. The service
	234	<filename>initrd-cleanup.service</filename> isolates to the
	235	<filename>initrd-switch-root.target</filename>, where cleanup
	236	services can run. As the very last step, the
	237	<filename>initrd-switch-root.service</filename> is activated,
	238	which will cause the system to switch its root to
	239	<filename>/sysroot</filename>.
	240	</para>
	241
72b7d998 ZJS	242	<programlisting> : (beginning identical to above)
	243	:
	244	v
	245	basic.target
	246	\| emergency.service
	247	______________________/\| \|
	248	/ \| v
7163e1ca DD	249	\| initrd-root-device.target <emphasis>emergency.target</emphasis>
	250	\| \|
	251	\| v
	252	\| sysroot.mount
72b7d998 ZJS	253	\| \|
	254	\| v
	255	\| initrd-root-fs.target
	256	\| \|
	257	\| v
	258	v initrd-parse-etc.service
	259	(custom initrd \|
	260	services...) v
	261	\| (sysroot-usr.mount and
	262	\| various mounts marked
	263	\| with fstab option
	264	\| x-initrd.mount...)
	265	\| \|
	266	\| v
	267	\| initrd-fs.target
	268	\______________________ \|
	269	\\|
	270	v
	271	initrd.target
	272	\|
	273	v
	274	initrd-cleanup.service
	275	isolates to
	276	initrd-switch-root.target
	277	\|
	278	v
	279	______________________/\|
	280	/ v
	281	\| initrd-udevadm-cleanup-db.service
	282	v \|
	283	(custom initrd \|
	284	services...) \|
	285	\______________________ \|
	286	\\|
	287	v
	288	initrd-switch-root.target
	289	\|
	290	v
	291	initrd-switch-root.service
	292	\|
	293	v
	294	Transition to Host OS</programlisting>
798d3a52 ZJS	295	</refsect1>
798d3a52 ZJS	296
798d3a52 ZJS	297	<refsect1>
	298	<title>System Manager Shutdown</title>
	299
	300	<para>System shutdown with systemd also consists of various target
	301	units with some minimal ordering structure applied:</para>
	302
72b7d998 ZJS	303	<programlisting> (conflicts with (conflicts with
	304	all system all file system
	305	services) mounts, swaps,
	306	\| cryptsetup
	307	\| devices, ...)
	308	\| \|
	309	v v
	310	shutdown.target umount.target
	311	\| \|
	312	\_______ ______/
	313	\ /
	314	v
	315	(various low-level
	316	services)
	317	\|
	318	v
	319	final.target
	320	\|
	321	_____________________________________/ \_________________________________
	322	/ \| \| \
	323	\| \| \| \|
	324	v v v v
0e0320e0	325	systemd-reboot.service systemd-poweroff.service systemd-halt.service systemd-kexec.service
72b7d998 ZJS	326	\| \| \| \|
	327	v v v v
	328	<emphasis>reboot.target</emphasis> <emphasis>poweroff.target</emphasis> <emphasis>halt.target</emphasis> <emphasis>kexec.target</emphasis></programlisting>
798d3a52	329
e799056e LP	330	<para>Commonly used system shutdown targets are <emphasis>emphasized</emphasis>.</para>
	331
	332	<para>Note that
39e4f5bc	333	<citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
e799056e LP	334	<filename>systemd-reboot.service</filename>, <filename>systemd-poweroff.service</filename> and
	335	<filename>systemd-kexec.service</filename> will transition the system and server manager (PID 1) into the second
	336	phase of system shutdown (implemented in the <filename>systemd-shutdown</filename> binary), which will unmount any
	337	remaining file systems, kill any remaining processes and release any other remaining resources, in a simple and
	338	robust fashion, without taking any service or unit concept into account anymore. At that point, regular
	339	applications and resources are generally terminated and released already, the second phase hence operates only as
	340	safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based
	341	shutdown phase described above.</para>
798d3a52 ZJS	342	</refsect1>
	343
	344	<refsect1>
	345	<title>See Also</title>
	346	<para>
	347	<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
	348	<citerefentry project='man-pages'><refentrytitle>boot</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
	349	<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
	350	<citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
e799056e	351	<citerefentry><refentrytitle>systemd-halt.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
b7a47345	352	<citerefentry project='man-pages'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>
798d3a52 ZJS	353	</para>
798d3a52 ZJS	354	</refsect1>
013d8a39 LP	355
013d8a39 LP	356	</refentry>