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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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<a href="../python-systemd/index.html">Python </a>·
<a href="../libudev/index.html">libudev </a>·
<a href="../libudev/index.html">gudev </a><span style="float:right">systemd 215</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="idm214175133552"></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="idm214172705392"></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="idm214174200976"></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="idm214171319504"></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="idm214172155856"></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>
|
