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238 lines
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</style><a href="index.html">Index </a>·
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<a href="systemd.directives.html">Directives </a>·
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<a href="../python-systemd/index.html">Python </a>·
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<a href="../libudev/index.html">libudev </a>·
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<a href="../libudev/index.html">gudev </a><span style="float:right">systemd 204</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="idm259799556448"></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
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the system boot. Immediately after power-up, the
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system BIOS will do minimal hardware initialization,
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and hand control over to a boot loader stored on a
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persistent storage device. This boot loader will then
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invoke an OS kernel from disk (or the network). In the
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Linux case this kernel (optionally) extracts and
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executes an initial RAM disk image (initrd) such as
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<a href="dracut.html"><span class="citerefentry"><span class="refentrytitle">dracut</span>(8)</span></a>
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which looks for the root file system (possibly using
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<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>
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for this). After the root file system is found and
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mounted the initrd hands over control to the host's
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system manager (such as
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<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>)
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stored on the OS image which is then responsible for
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probing all remaining hardware, mounting all necessary
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file systems and spawning all configured
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services.</p><p>On shutdown the system manager stops all
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services, unmounts all file systems (detaching the
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storage technologies backing them), and then
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(optionally) jumps back into the initrd code which
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unmounts/detaches the root file system and the storage
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it resides on. As last step the system is powered down.</p><p>Additional information about the system boot
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process may be found in
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<a href="boot.html"><span class="citerefentry"><span class="refentrytitle">boot</span>(7)</span></a>.</p></div><div class="refsect1"><a name="idm259795769488"></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
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responsible for initializing the required file
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systems, services and drivers that are necessary for
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operation of the system. On
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<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>
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systems this process is split up in various discrete
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steps which are exposed as target units. (See
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<a href="systemd.target.html"><span class="citerefentry"><span class="refentrytitle">systemd.target</span>(5)</span></a>
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for detailed information about target units.) The
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boot-up process is highly parallelized so that the
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order in which specific target units are reached is not
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deterministic, but still adheres to a limited amount
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of ordering structure.</p><p>When systemd starts up the system it will
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activate all units that are dependencies of
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<code class="filename">default.target</code> (as well as
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recursively all dependencies of these
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dependencies). Usually
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<code class="filename">default.target</code> is simply an alias
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of <code class="filename">graphical.target</code> or
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<code class="filename">multi-user.target</code> depending on
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whether the system is configured for a graphical UI or
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only for a text console. To enforce minimal ordering
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between the units pulled in a number of well-known
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target units are available, as listed on
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<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
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these well-known units and their position in the
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boot-up logic. The arrows describe which units are
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pulled in and ordered before which other units. Units
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near the top are started before units nearer to the
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bottom of the chart.</p><pre class="programlisting">local-fs-pre.target
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v
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(various mounts and (various swap (various cryptsetup
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fsck services...) devices...) devices...) (various low-level (various low-level
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| | | services: udevd, API VFS mounts:
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v v v tmpfiles, random mqueue, configfs,
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local-fs.target swap.target cryptsetup.target seed, sysctl, ...) debugfs, ...)
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| | | | |
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\__________________|_________________ | ___________________|____________________/
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\|/
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v
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sysinit.target
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____________________________________/|\________________________________________
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/ | | | \
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| | | | |
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v v | v v
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(various (various | (various rescue.service
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timers...) paths...) | sockets...) |
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| | | | v
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v v | v <span class="emphasis"><em>rescue.target</em></span>
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timers.target paths.target | sockets.target
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| | | |
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\__________________|_________________ | ___________________/
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\|/
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v
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basic.target
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____________________________________/| emergency.service
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/ | | |
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| | | v
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v v v <span class="emphasis"><em>emergency.target</em></span>
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display- (various system (various system
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manager.service services services)
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| required for |
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| graphical UIs) v
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| | <span class="emphasis"><em>multi-user.target</em></span>
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| | |
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\_________________ | _________________/
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\|/
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v
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<span class="emphasis"><em>graphical.target</em></span></pre><p>Target units that are commonly used as boot
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targets are <span class="emphasis"><em>emphasized</em></span>. These
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units are good choices as goal targets, for
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example by passing them to the
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<code class="varname">systemd.unit=</code> kernel command line
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option (see
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<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>)
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or by symlinking <code class="filename">default.target</code>
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to them.</p></div><div class="refsect1"><a name="idm259799649264"></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
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be set up using systemd as well. In this case boot up
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inside the initrd follows the following
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structure.</p><p>The default target in the initrd is
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<code class="filename">initrd.target</code>. The bootup process
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begins identical to the system manager bootup (see
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above) until it reaches
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<code class="filename">basic.target</code>. From there, systemd
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approaches the special target
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<code class="filename">initrd.target</code>. If the root device
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can be mounted at <code class="filename">/sysroot</code>, the
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<code class="filename">sysroot.mount</code> unit becomes active
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and <code class="filename">initrd-root-fs.target</code> is
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reached. The service
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<code class="filename">initrd-parse-etc.service</code> scans
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<code class="filename">/sysroot/etc/fstab</code> for a possible
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<code class="filename">/usr</code> mount point and additional
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entries marked with the
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<span class="emphasis"><em>x-initrd.mount</em></span> option. All
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entries found are mounted below
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<code class="filename">/sysroot</code>, and
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<code class="filename">initrd-fs.target</code> is reached. The
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service <code class="filename">initrd-cleanup.service</code>
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isolates to the
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<code class="filename">initrd-switch-root.target</code>, where
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cleanup services can run. As the very last step, the
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<code class="filename">initrd-switch-root.service</code> is
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activated, which will cause the system to switch its
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root to <code class="filename">/sysroot</code>.
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</p><pre class="programlisting"> : (beginning identical to above)
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:
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v
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basic.target
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| emergency.service
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______________________/| |
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/ | v
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| sysroot.mount <span class="emphasis"><em>emergency.target</em></span>
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| |
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| v
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| initrd-root-fs.target
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| |
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| v
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v initrd-parse-etc.service
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(custom initrd |
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services...) v
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| (sysroot-usr.mount and
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| various mounts marked
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| with fstab option
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| x-initrd.mount...)
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| |
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| v
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| initrd-fs.target
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\______________________ |
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\|
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v
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initrd.target
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v
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initrd-cleanup.service
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isolates to
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initrd-switch-root.target
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v
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______________________/|
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/ v
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| initrd-udevadm-cleanup-db.service
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v |
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(custom initrd |
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services...) |
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\______________________ |
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\|
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v
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initrd-switch-root.target
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v
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initrd-switch-root.service
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v
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Transition to Host OS</pre></div><div class="refsect1"><a name="idm259799624480"></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
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various target units with some minimal ordering
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structure applied:</p><pre class="programlisting"> (conflicts with (conflicts with
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all system all file system
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services) mounts, swaps,
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| cryptsetup
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| devices, ...)
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v v
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shutdown.target umount.target
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\_______ ______/
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\ /
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v
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(various low-level
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services)
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v
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final.target
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_____________________________________/ \_________________________________
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/ | | \
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v v v v
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systemd-reboot.service systemd-poweroff.service systemd-halt.service systemd-kexec.service
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v v v v
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<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="idm259799616688"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
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<a href="systemd.html"><span class="citerefentry"><span class="refentrytitle">systemd</span>(1)</span></a>,
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<a href="boot.html"><span class="citerefentry"><span class="refentrytitle">boot</span>(7)</span></a>,
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<a href="systemd.special.html"><span class="citerefentry"><span class="refentrytitle">systemd.special</span>(7)</span></a>,
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<a href="systemd.target.html"><span class="citerefentry"><span class="refentrytitle">systemd.target</span>(5)</span></a>,
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<a href="dracut.html"><span class="citerefentry"><span class="refentrytitle">dracut</span>(8)</span></a>
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</p></div></div></body></html>
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