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<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>
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</style><a href="index.html">Index </a>·
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<span style="float:right">systemd 221</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="idm47365628124416"></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="http://linux.die.net/man/8/dracut"><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="http://man7.org/linux/man-pages/man7/boot.7.html"><span class="citerefentry"><span class="refentrytitle">boot</span>(7)</span></a>.</p></div><div class="refsect1"><a name="idm47365623214304"></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 \_________________ | ___________________/
\|/
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><p><code class="filename">timers.target</code> is pulled-in by
<code class="filename">basic.target</code> asynchronously. This allows
timers units to depend on services which become only available
later in boot.</p></div><div class="refsect1"><a name="idm47365623197104"></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="idm47365623179456"></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="idm47365623173664"></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="http://man7.org/linux/man-pages/man7/boot.7.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="http://linux.die.net/man/8/dracut"><span class="citerefentry"><span class="refentrytitle">dracut</span>(8)</span></a>
</p></div></div></body></html>
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