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<?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-or-later -->
<refentry id="systemd-soft-reboot.service">
<refentryinfo>
<title>systemd-soft-reboot.service</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>systemd-soft-reboot.service</refentrytitle>
<manvolnum>8</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd-soft-reboot.service</refname>
<refpurpose>Userspace reboot operation</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>systemd-soft-reboot.service</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><filename>systemd-soft-reboot.service</filename> is a system service that is pulled in by
<filename>soft-reboot.target</filename> and is responsible for performing a userspace-only reboot
operation. When invoked, it will send the <constant>SIGTERM</constant> signal to any processes left
running (but does not follow up with <constant>SIGKILL</constant>, and does not wait for the processes to
exit). If the <filename>/run/nextroot/</filename> directory exists (which may be a regular directory, a
directory mount point or a symlink to either) then it will switch the file system root to it. It then
reexecutes the service manager off the (possibly now new) root file system, which will enqueue a new boot
transaction as in a normal reboot.</para>
<para>Such a userspace-only reboot operation permits updating or resetting the entirety of userspace with
minimal downtime, as the reboot operation does <emphasis>not</emphasis> transition through:</para>
<itemizedlist>
<listitem><para>The second phase of regular shutdown, as implemented by
<citerefentry><refentrytitle>systemd-shutdown</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para></listitem>
<listitem><para>The third phase of regular shutdown, i.e. the return to the initrd
context</para></listitem>
<listitem><para>The hardware reboot operation</para></listitem>
<listitem><para>The firmware initialization</para></listitem>
<listitem><para>The boot loader initialization</para></listitem>
<listitem><para>The kernel initialization</para></listitem>
<listitem><para>The initrd initialization</para></listitem>
</itemizedlist>
<para>However this form of reboot comes with drawbacks as well:</para>
<itemizedlist>
<listitem><para>The OS update remains incomplete, as the kernel is not reset and continues
running.</para></listitem>
<listitem><para>Kernel settings (such as <filename>/proc/sys/</filename> settings, a.k.a. "sysctl", or
<filename>/sys/</filename> settings) are not reset.</para></listitem>
</itemizedlist>
<para>These limitations may be addressed by various means, which are outside of the scope of this
documentation, such as kernel live-patching and sufficiently comprehensive
<filename>/etc/sysctl.d/</filename> files.</para>
</refsect1>
<refsect1>
<title>Resource Pass-Through</title>
<para>Various runtime OS resources can passed from a system runtime to the next, through the userspace
reboot operation. Specifically:</para>
<itemizedlist>
<listitem><para>File descriptors placed in the file descriptor store of services that remain active
until the very end are passed to the next boot, where they are placed in the file descriptor store of
the same unit. For this to work, units must declare <varname>DefaultDependencies=no</varname> (and
avoid a manual <varname>Conflicts=shutdown.target</varname> or similar) to ensure they are not
terminated as usual during the system shutdown operation. Alternatively, use
<varname>FileDescriptorStorePreserve=</varname> to allow the file descriptor store to remain pinned
even when the unit is down. See
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
details about the file descriptor store.</para></listitem>
<listitem><para>Similar to this, file descriptors associated with <filename>.socket</filename> units
remain open (and connectible) if the units are not stopped during the transition. (Achieved by
<varname>DefaultDependencies=no</varname>.)</para></listitem>
<listitem><para>The <filename>/run/</filename> file system remains mounted and populated and may be
used to pass state information between such userspace reboot cycles.</para></listitem>
<listitem><para>Service processes may continue to run over the transition, if they are placed in
services that remain active until the very end of shutdown (which again is achieved via
<varname>DefaultDependencies=no</varname>). They must also be set up to avoid being killed by the
aforementioned <constant>SIGTERM</constant> spree (as per <ulink
url="https://systemd.io/ROOT_STORAGE_DAEMONS">systemd and Storage Daemons for the Root File
System</ulink>).</para></listitem>
<listitem><para>File system mounts may remain mounted during the transition, and complex storage
attached, if configured to remain until the very end of the shutdown process. (Also achieved via
<varname>DefaultDependencies=no</varname>, and by avoiding
<varname>Conflicts=umount.target</varname>)</para></listitem>
</itemizedlist>
<para>Even though passing resources from one soft reboot cycle to the next is possible this way, we
strongly suggest to use this functionality sparingly only, as it creates a more fragile system as
resources from different versions of the OS and applications might be mixed with unforeseen
consequences. In particular it's recommended to <emphasis>avoid</emphasis> allowing processes to survive
the soft reboot operation, as this means code updates will necessarily be incomplete, and processes
typically pin various other resources (such as the file system they are backed by), thus increasing
memory usage (as two versions of the OS/application/file system might be kept in memory). Leaving
processes running during a soft-reboot operation requires disconnecting the service comprehensively from
the rest of the OS, i.e. minimizing IPC and reducing sharing of resources with the rest of the OS. A
possible mechanism to achieve this is the concept of <ulink
url="https://systemd.io/PORTABLE_SERVICES">Portable Services</ulink>, but make sure no resource from the
host's OS filesystems is pinned via <varname>BindPaths=</varname> or similar unit settings, otherwise the
old, originating filesystem will remain mounted as long as the unit is running.</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>Note that because
<citerefentry><refentrytitle>systemd-shutdown</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
not executed, the executables in <filename>/usr/lib/systemd/system-shutdown/</filename> are not executed
either.</para>
<para>Note that <filename>systemd-soft-reboot.service</filename> (and related units) should never be
executed directly. Instead, trigger system shutdown with a command such as <literal>systemctl
soft-reboot</literal>.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-poweroff.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-suspend.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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