[systemd.git] / man / bootup.html

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  </style><a href="index.html">Index </a>·
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  <a href="../libudev/index.html">gudev </a><span style="float:right">systemd 214</span><hr><div class="refentry"><a name="bootup"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>bootup — System bootup process</p></div><div class="refsect1"><a name="idm214173222816"></a><h2 id="Description">Description<a class="headerlink" title="Permalink to this headline" href="#Description">¶</a></h2><p>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
                <a href="dracut.html"><span class="citerefentry"><span class="refentrytitle">dracut</span>(8)</span></a>,
                which looks for the root file system (possibly using
                <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>
                for this). After the root file system is found and
                mounted, the initrd hands over control to the host's
                system manager (such as
                <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>)
                stored on the OS image, which is then responsible for
                probing all remaining hardware, mounting all necessary
                file systems and spawning all configured
                services.</p><p>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.</p><p>Additional information about the system boot
                process may be found in
                <a href="boot.html"><span class="citerefentry"><span class="refentrytitle">boot</span>(7)</span></a>.</p></div><div class="refsect1"><a name="idm214169432720"></a><h2 id="System Manager Bootup">System Manager Bootup<a class="headerlink" title="Permalink to this headline" href="#System%20Manager%20Bootup">¶</a></h2><p>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
                <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>
                systems, this process is split up in various discrete
                steps which are exposed as target units. (See
                <a href="systemd.target.html"><span class="citerefentry"><span class="refentrytitle">systemd.target</span>(5)</span></a>
                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.</p><p>When systemd starts up the system, it will
                activate all units that are dependencies of
                <code class="filename">default.target</code> (as well as
                recursively all dependencies of these
                dependencies). Usually,
                <code class="filename">default.target</code> is simply an alias
                of <code class="filename">graphical.target</code> or
                <code class="filename">multi-user.target</code>, 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
                <a href="systemd.special.html"><span class="citerefentry"><span class="refentrytitle">systemd.special</span>(7)</span></a>.</p><p>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.</p><pre class="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              <span class="emphasis"><em>rescue.target</em></span>
   timers.target      paths.target             |             sockets.target
         |                  |                  |                    |
         \__________________|_________________ | ___________________/
                                              \|/
                                               v
                                         basic.target
                                               |
          ____________________________________/|                                 emergency.service
         /                  |                  |                                         |
         |                  |                  |                                         v
         v                  v                  v                                 <span class="emphasis"><em>emergency.target</em></span>
     display-        (various system    (various system
 manager.service         services           services)
         |             required for            |
         |            graphical UIs)           v
         |                  |           <span class="emphasis"><em>multi-user.target</em></span>
         |                  |                  |
         \_________________ | _________________/
                           \|/
                            v
                  <span class="emphasis"><em>graphical.target</em></span></pre><p>Target units that are commonly used as boot
                targets are <span class="emphasis"><em>emphasized</em></span>. These
                units are good choices as goal targets, for
                example by passing them to the
                <code class="varname">systemd.unit=</code> kernel command line
                option (see
                <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>)
                or by symlinking <code class="filename">default.target</code>
                to them.</p></div><div class="refsect1"><a name="idm214173316512"></a><h2 id="Bootup in the Initial RAM Disk (initrd)">Bootup in the Initial RAM Disk (initrd)<a class="headerlink" title="Permalink to this headline" href="#Bootup%20in%20the%20Initial%20RAM%20Disk%20(initrd)">¶</a></h2><p>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.</p><p>The default target in the initrd is
                <code class="filename">initrd.target</code>. The bootup process
                begins identical to the system manager bootup (see
                above) until it reaches
                <code class="filename">basic.target</code>. From there, systemd
                approaches the special target
                <code class="filename">initrd.target</code>. If the root device
                can be mounted at <code class="filename">/sysroot</code>, the
                <code class="filename">sysroot.mount</code> unit becomes active
                and <code class="filename">initrd-root-fs.target</code> is
                reached.  The service
                <code class="filename">initrd-parse-etc.service</code> scans
                <code class="filename">/sysroot/etc/fstab</code> for a possible
                <code class="filename">/usr</code> mount point and additional
                entries marked with the
                <span class="emphasis"><em>x-initrd.mount</em></span> option. All
                entries found are mounted below
                <code class="filename">/sysroot</code>, and
                <code class="filename">initrd-fs.target</code> is reached. The
                service <code class="filename">initrd-cleanup.service</code>
                isolates to the
                <code class="filename">initrd-switch-root.target</code>, where
                cleanup services can run. As the very last step, the
                <code class="filename">initrd-switch-root.service</code> is
                activated, which will cause the system to switch its
                root to <code class="filename">/sysroot</code>.
                </p><pre class="programlisting">                                               : (beginning identical to above)
                                               :
                                               v
                                         basic.target
                                               |                                 emergency.service
                        ______________________/|                                         |
                       /                       |                                         v
                       |                  sysroot.mount                          <span class="emphasis"><em>emergency.target</em></span>
                       |                       |
                       |                       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</pre></div><div class="refsect1"><a name="idm214173291536"></a><h2 id="System Manager Shutdown">System Manager Shutdown<a class="headerlink" title="Permalink to this headline" href="#System%20Manager%20Shutdown">¶</a></h2><p>System shutdown with systemd also consists of
                various target units with some minimal ordering
                structure applied:</p><pre class="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
    <span class="emphasis"><em>reboot.target</em></span>             <span class="emphasis"><em>poweroff.target</em></span>            <span class="emphasis"><em>halt.target</em></span>           <span class="emphasis"><em>kexec.target</em></span></pre><p>Commonly used system shutdown targets are <span class="emphasis"><em>emphasized</em></span>.</p></div><div class="refsect1"><a name="idm214173283648"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
                        <a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
                        <a href="boot.html"><span class="citerefentry"><span class="refentrytitle">boot</span>(7)</span></a>,
                        <a href="systemd.special.html"><span class="citerefentry"><span class="refentrytitle">systemd.special</span>(7)</span></a>,
                        <a href="systemd.target.html"><span class="citerefentry"><span class="refentrytitle">systemd.target</span>(5)</span></a>,
                        <a href="dracut.html"><span class="citerefentry"><span class="refentrytitle">dracut</span>(8)</span></a>
                </p></div></div></body></html>
Commit	Line	Data
663996b3 MS	1	<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>bootup</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><style>
	2	a.headerlink {
	3	color: #c60f0f;
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	5	padding: 0 4px 0 4px;
	6	text-decoration: none;
	7	visibility: hidden;
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	9
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	11	background-color: #c60f0f;
	12	color: white;
	13	}
	14
	15	h1:hover > a.headerlink, h2:hover > a.headerlink, h3:hover > a.headerlink, dt:hover > a.headerlink {
	16	visibility: visible;
	17	}
	18	</style><a href="index.html">Index </a>·
	19	<a href="systemd.directives.html">Directives </a>·
	20	<a href="../python-systemd/index.html">Python </a>·
	21	<a href="../libudev/index.html">libudev </a>·
60f067b4	22	<a href="../libudev/index.html">gudev </a><span style="float:right">systemd 214</span><hr><div class="refentry"><a name="bootup"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>bootup — System bootup process</p></div><div class="refsect1"><a name="idm214173222816"></a><h2 id="Description">Description<a class="headerlink" title="Permalink to this headline" href="#Description">¶</a></h2><p>A number of different components are involved in
663996b3 MS	23	the system boot. Immediately after power-up, the
	24	system BIOS will do minimal hardware initialization,
	25	and hand control over to a boot loader stored on a
	26	persistent storage device. This boot loader will then
	27	invoke an OS kernel from disk (or the network). In the
14228c0d MB	28	Linux case, this kernel (optionally) extracts and
	29	executes an initial RAM disk image (initrd), such as
	30	generated by
	31	<a href="dracut.html"><span class="citerefentry"><span class="refentrytitle">dracut</span>(8)</span></a>,
663996b3 MS	32	which looks for the root file system (possibly using
	33	<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>
	34	for this). After the root file system is found and
14228c0d	35	mounted, the initrd hands over control to the host's
663996b3 MS	36	system manager (such as
663996b3 MS	37	<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>)
14228c0d	38	stored on the OS image, which is then responsible for
663996b3 MS	39	probing all remaining hardware, mounting all necessary
663996b3 MS	40	file systems and spawning all configured
14228c0d	41	services.</p><p>On shutdown, the system manager stops all
663996b3 MS	42	services, unmounts all file systems (detaching the
	43	storage technologies backing them), and then
	44	(optionally) jumps back into the initrd code which
	45	unmounts/detaches the root file system and the storage
14228c0d	46	it resides on. As a last step, the system is powered down.</p><p>Additional information about the system boot
663996b3	47	process may be found in
60f067b4	48	<a href="boot.html"><span class="citerefentry"><span class="refentrytitle">boot</span>(7)</span></a>.</p></div><div class="refsect1"><a name="idm214169432720"></a><h2 id="System Manager Bootup">System Manager Bootup<a class="headerlink" title="Permalink to this headline" href="#System%20Manager%20Bootup">¶</a></h2><p>At boot, the system manager on the OS image is
663996b3 MS	49	responsible for initializing the required file
	50	systems, services and drivers that are necessary for
	51	operation of the system. On
	52	<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>
14228c0d	53	systems, this process is split up in various discrete
663996b3 MS	54	steps which are exposed as target units. (See
	55	<a href="systemd.target.html"><span class="citerefentry"><span class="refentrytitle">systemd.target</span>(5)</span></a>
	56	for detailed information about target units.) The
	57	boot-up process is highly parallelized so that the
	58	order in which specific target units are reached is not
	59	deterministic, but still adheres to a limited amount
14228c0d	60	of ordering structure.</p><p>When systemd starts up the system, it will
663996b3 MS	61	activate all units that are dependencies of
	62	<code class="filename">default.target</code> (as well as
	63	recursively all dependencies of these
14228c0d	64	dependencies). Usually,
663996b3 MS	65	<code class="filename">default.target</code> is simply an alias
663996b3 MS	66	of <code class="filename">graphical.target</code> or
14228c0d	67	<code class="filename">multi-user.target</code>, depending on
663996b3 MS	68	whether the system is configured for a graphical UI or
663996b3 MS	69	only for a text console. To enforce minimal ordering
14228c0d	70	between the units pulled in, a number of well-known
663996b3 MS	71	target units are available, as listed on
	72	<a href="systemd.special.html"><span class="citerefentry"><span class="refentrytitle">systemd.special</span>(7)</span></a>.</p><p>The following chart is a structural overview of
	73	these well-known units and their position in the
	74	boot-up logic. The arrows describe which units are
	75	pulled in and ordered before which other units. Units
	76	near the top are started before units nearer to the
	77	bottom of the chart.</p><pre class="programlisting">local-fs-pre.target
	78	\|
	79	v
	80	(various mounts and (various swap (various cryptsetup
	81	fsck services...) devices...) devices...) (various low-level (various low-level
	82	\| \| \| services: udevd, API VFS mounts:
	83	v v v tmpfiles, random mqueue, configfs,
	84	local-fs.target swap.target cryptsetup.target seed, sysctl, ...) debugfs, ...)
	85	\| \| \| \| \|
	86	\__________________\|_________________ \| ___________________\|____________________/
	87	\\|/
	88	v
	89	sysinit.target
	90	\|
	91	____________________________________/\|\________________________________________
	92	/ \| \| \| \
	93	\| \| \| \| \|
	94	v v \| v v
	95	(various (various \| (various rescue.service
	96	timers...) paths...) \| sockets...) \|
	97	\| \| \| \| v
	98	v v \| v <span class="emphasis"><em>rescue.target</em></span>
	99	timers.target paths.target \| sockets.target
	100	\| \| \| \|
	101	\__________________\|_________________ \| ___________________/
	102	\\|/
	103	v
	104	basic.target
	105	\|
	106	____________________________________/\| emergency.service
	107	/ \| \| \|
	108	\| \| \| v
	109	v v v <span class="emphasis"><em>emergency.target</em></span>
	110	display- (various system (various system
	111	manager.service services services)
	112	\| required for \|
	113	\| graphical UIs) v
	114	\| \| <span class="emphasis"><em>multi-user.target</em></span>
	115	\| \| \|
	116	\_________________ \| _________________/
	117	\\|/
	118	v
	119	<span class="emphasis"><em>graphical.target</em></span></pre><p>Target units that are commonly used as boot
	120	targets are <span class="emphasis"><em>emphasized</em></span>. These
	121	units are good choices as goal targets, for
	122	example by passing them to the
	123	<code class="varname">systemd.unit=</code> kernel command line
	124	option (see
	125	<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>)
	126	or by symlinking <code class="filename">default.target</code>
60f067b4	127	to them.</p></div><div class="refsect1"><a name="idm214173316512"></a><h2 id="Bootup in the Initial RAM Disk (initrd)">Bootup in the Initial RAM Disk (initrd)<a class="headerlink" title="Permalink to this headline" href="#Bootup%20in%20the%20Initial%20RAM%20Disk%20(initrd)">¶</a></h2><p>The initial RAM disk implementation (initrd) can
14228c0d	128	be set up using systemd as well. In this case, boot up
663996b3 MS	129	inside the initrd follows the following
	130	structure.</p><p>The default target in the initrd is
	131	<code class="filename">initrd.target</code>. The bootup process
	132	begins identical to the system manager bootup (see
	133	above) until it reaches
	134	<code class="filename">basic.target</code>. From there, systemd
	135	approaches the special target
	136	<code class="filename">initrd.target</code>. If the root device
	137	can be mounted at <code class="filename">/sysroot</code>, the
	138	<code class="filename">sysroot.mount</code> unit becomes active
	139	and <code class="filename">initrd-root-fs.target</code> is
	140	reached. The service
	141	<code class="filename">initrd-parse-etc.service</code> scans
	142	<code class="filename">/sysroot/etc/fstab</code> for a possible
	143	<code class="filename">/usr</code> mount point and additional
	144	entries marked with the
	145	<span class="emphasis"><em>x-initrd.mount</em></span> option. All
	146	entries found are mounted below
	147	<code class="filename">/sysroot</code>, and
	148	<code class="filename">initrd-fs.target</code> is reached. The
	149	service <code class="filename">initrd-cleanup.service</code>
	150	isolates to the
	151	<code class="filename">initrd-switch-root.target</code>, where
	152	cleanup services can run. As the very last step, the
	153	<code class="filename">initrd-switch-root.service</code> is
	154	activated, which will cause the system to switch its
	155	root to <code class="filename">/sysroot</code>.
	156	</p><pre class="programlisting"> : (beginning identical to above)
	157	:
	158	v
	159	basic.target
	160	\| emergency.service
	161	______________________/\| \|
	162	/ \| v
	163	\| sysroot.mount <span class="emphasis"><em>emergency.target</em></span>
	164	\| \|
	165	\| v
	166	\| initrd-root-fs.target
	167	\| \|
	168	\| v
	169	v initrd-parse-etc.service
	170	(custom initrd \|
	171	services...) v
	172	\| (sysroot-usr.mount and
	173	\| various mounts marked
	174	\| with fstab option
	175	\| x-initrd.mount...)
	176	\| \|
	177	\| v
	178	\| initrd-fs.target
	179	\______________________ \|
	180	\\|
	181	v
	182	initrd.target
	183	\|
	184	v
	185	initrd-cleanup.service
	186	isolates to
	187	initrd-switch-root.target
	188	\|
	189	v
	190	______________________/\|
	191	/ v
	192	\| initrd-udevadm-cleanup-db.service
193	v \|
194	(custom initrd \|
195	services...) \|
196	\______________________ \|
197	\\|
198	v
199	initrd-switch-root.target
200	\|
201	v
202	initrd-switch-root.service
203	\|
204	v
60f067b4	205	Transition to Host OS</pre></div><div class="refsect1"><a name="idm214173291536"></a><h2 id="System Manager Shutdown">System Manager Shutdown<a class="headerlink" title="Permalink to this headline" href="#System%20Manager%20Shutdown">¶</a></h2><p>System shutdown with systemd also consists of
663996b3 MS	206	various target units with some minimal ordering
	207	structure applied:</p><pre class="programlisting"> (conflicts with (conflicts with
	208	all system all file system
	209	services) mounts, swaps,
	210	\| cryptsetup
	211	\| devices, ...)
	212	\| \|
	213	v v
	214	shutdown.target umount.target
	215	\| \|
	216	\_______ ______/
	217	\ /
	218	v
	219	(various low-level
	220	services)
	221	\|
	222	v
	223	final.target
	224	\|
	225	_____________________________________/ \_________________________________
	226	/ \| \| \
	227	\| \| \| \|
	228	v v v v
	229	systemd-reboot.service systemd-poweroff.service systemd-halt.service systemd-kexec.service
	230	\| \| \| \|
	231	v v v v
60f067b4	232	<span class="emphasis"><em>reboot.target</em></span> <span class="emphasis"><em>poweroff.target</em></span> <span class="emphasis"><em>halt.target</em></span> <span class="emphasis"><em>kexec.target</em></span></pre><p>Commonly used system shutdown targets are <span class="emphasis"><em>emphasized</em></span>.</p></div><div class="refsect1"><a name="idm214173283648"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
663996b3 MS	233	<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
	234	<a href="boot.html"><span class="citerefentry"><span class="refentrytitle">boot</span>(7)</span></a>,
	235	<a href="systemd.special.html"><span class="citerefentry"><span class="refentrytitle">systemd.special</span>(7)</span></a>,
	236	<a href="systemd.target.html"><span class="citerefentry"><span class="refentrytitle">systemd.target</span>(5)</span></a>,
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