man/systemd-soft-reboot.service.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   5
   6 <refentry id="systemd-soft-reboot.service">
   7
   8   <refentryinfo>
   9     <title>systemd-soft-reboot.service</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>systemd-soft-reboot.service</refentrytitle>
  15     <manvolnum>8</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>systemd-soft-reboot.service</refname>
  20     <refpurpose>Userspace reboot operation</refpurpose>
  21   </refnamediv>
  22
  23   <refsynopsisdiv>
  24     <para><filename>systemd-soft-reboot.service</filename></para>
  25   </refsynopsisdiv>
  26
  27   <refsect1>
  28     <title>Description</title>
  29
  30     <para><filename>systemd-soft-reboot.service</filename> is a system service that is pulled in by
  31     <filename>soft-reboot.target</filename> and is responsible for performing a userspace-only reboot
  32     operation. When invoked, it will send the <constant>SIGTERM</constant> signal to any processes left
  33     running (but does not wait for the processes to exit), and follow up with <constant>SIGKILL</constant>.
  34     If the <filename>/run/nextroot/</filename> directory exists (which may be a regular directory, a
  35     directory mount point or a symlink to either) then it will switch the file system root to it. It then
  36     reexecutes the service manager off the (possibly now new) root file system, which will enqueue a new boot
  37     transaction as in a normal reboot.</para>
  38
  39     <para>Such a userspace-only reboot operation permits updating or resetting the entirety of userspace with
  40     minimal downtime, as the reboot operation does <emphasis>not</emphasis> transition through:</para>
  41
  42     <itemizedlist>
  43       <listitem><para>The second phase of regular shutdown, as implemented by
  44       <citerefentry><refentrytitle>systemd-shutdown</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para></listitem>
  45
  46       <listitem><para>The third phase of regular shutdown, i.e. the return to the initrd context.
  47       </para></listitem>
  48
  49       <listitem><para>The hardware reboot operation.</para></listitem>
  50
  51       <listitem><para>The firmware initialization.</para></listitem>
  52
  53       <listitem><para>The boot loader initialization.</para></listitem>
  54
  55       <listitem><para>The kernel initialization.</para></listitem>
  56
  57       <listitem><para>The initrd initialization.</para></listitem>
  58     </itemizedlist>
  59
  60     <para>However this form of reboot comes with drawbacks as well:</para>
  61
  62     <itemizedlist>
  63       <listitem><para>The OS update remains incomplete, as the kernel is not reset and continues
  64       running.</para></listitem>
  65
  66       <listitem><para>Kernel settings (such as <filename>/proc/sys/</filename> settings, a.k.a. "sysctl", or
  67       <filename>/sys/</filename> settings) are not reset.</para></listitem>
  68     </itemizedlist>
  69
  70     <para>These limitations may be addressed by various means, which are outside of the scope of this
  71     documentation, such as kernel live-patching and sufficiently comprehensive
  72     <filename>/etc/sysctl.d/</filename> files.</para>
  73   </refsect1>
  74
  75   <refsect1>
  76     <title>Resource Pass-Through</title>
  77
  78     <para>Various runtime OS resources can passed from a system runtime to the next, through the userspace
  79     reboot operation. Specifically:</para>
  80
  81     <itemizedlist>
  82       <listitem><para>File descriptors placed in the file descriptor store of services that remain active
  83       until the very end are passed to the next boot, where they are placed in the file descriptor store of
  84       the same unit. For this to work, units must declare <varname>DefaultDependencies=no</varname> (and
  85       avoid a manual <varname>Conflicts=shutdown.target</varname> or similar) to ensure they are not
  86       terminated as usual during the system shutdown operation. Alternatively, use
  87       <varname>FileDescriptorStorePreserve=</varname> to allow the file descriptor store to remain pinned
  88       even when the unit is down. See
  89       <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
  90       details about the file descriptor store.</para></listitem>
  91
  92       <listitem><para>Similar to this, file descriptors associated with <filename>.socket</filename> units
  93       remain open (and connectible) if the units are not stopped during the transition. (Achieved by
  94       <varname>DefaultDependencies=no</varname>.)</para></listitem>
  95
  96       <listitem><para>The <filename>/run/</filename> file system remains mounted and populated and may be
  97       used to pass state information between such userspace reboot cycles.</para></listitem>
  98
  99       <listitem><para>Service processes may continue to run over the transition, past soft-reboot and into
 100       the next session, if they are placed in services that remain active until the very end of shutdown
 101       (which again is achieved via <varname>DefaultDependencies=no</varname>). They must also be set up to
 102       avoid being killed by the aforementioned <constant>SIGTERM</constant> and <constant>SIGKILL</constant>
 103       via <varname>SurviveFinalKillSignal=yes</varname>, and also be configured to avoid being stopped on
 104       isolate via <varname>IgnoreOnIsolate=yes</varname>. They also have to be configured to be stopped on
 105       normal shutdown, reboot and maintenance mode. Finally, they have to be ordered after
 106       <constant>basic.target</constant> to ensure correct ordeering on boot. Note that in case any new or
 107       custom units are used to isolate to, or that implement an equivalent shutdown functionality, they will
 108       also have to be configured manually for correct ordering and conflicting. For example:</para>
 109
 110       <programlisting>[Unit]
 111 Description=My surviving service
 112 SurviveFinalKillSignal=yes
 113 IgnoreOnIsolate=yes
 114 DefaultDependencies=no
 115 After=basic.target
 116 Conflicts=reboot.target
 117 Before=reboot.target
 118 Conflicts=kexec.target
 119 Before=kexec.target
 120 Conflicts=poweroff.target
 121 Before=poweroff.target
 122 Conflicts=halt.target
 123 Before=halt.target
 124 Conflicts=rescue.target
 125 Before=rescue.target
 126 Conflicts=emergency.target
 127 Before=emergency.target
 128
 129 [Service]
 130 Type=oneshot
 131 ExecStart=sleep infinity
 132       </programlisting>
 133       </listitem>
 134
 135       <listitem><para>File system mounts may remain mounted during the transition, and complex storage
 136       attached, if configured to remain until the very end of the shutdown process. (Also achieved via
 137       <varname>DefaultDependencies=no</varname>, and by avoiding
 138       <varname>Conflicts=umount.target</varname>)</para></listitem>
 139     </itemizedlist>
 140
 141     <para>Even though passing resources from one soft reboot cycle to the next is possible this way, we
 142     strongly suggest to use this functionality sparingly only, as it creates a more fragile system as
 143     resources from different versions of the OS and applications might be mixed with unforeseen
 144     consequences. In particular it's recommended to <emphasis>avoid</emphasis> allowing processes to survive
 145     the soft reboot operation, as this means code updates will necessarily be incomplete, and processes
 146     typically pin various other resources (such as the file system they are backed by), thus increasing
 147     memory usage (as two versions of the OS/application/file system might be kept in memory). Leaving
 148     processes running during a soft-reboot operation requires disconnecting the service comprehensively from
 149     the rest of the OS, i.e. minimizing IPC and reducing sharing of resources with the rest of the OS. A
 150     possible mechanism to achieve this is the concept of <ulink
 151     url="https://systemd.io/PORTABLE_SERVICES">Portable Services</ulink>, but make sure no resource from the
 152     host's OS filesystems is pinned via <varname>BindPaths=</varname> or similar unit settings, otherwise the
 153     old, originating filesystem will remain mounted as long as the unit is running.</para>
 154   </refsect1>
 155
 156   <refsect1>
 157     <title>Notes</title>
 158
 159     <para>Note that because
 160     <citerefentry><refentrytitle>systemd-shutdown</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
 161     not executed, the executables in <filename>/usr/lib/systemd/system-shutdown/</filename> are not executed
 162     either.</para>
 163
 164     <para>Note that <filename>systemd-soft-reboot.service</filename> (and related units) should never be
 165     executed directly. Instead, trigger system shutdown with a command such as <literal>systemctl
 166     soft-reboot</literal>.</para>
 167
 168     <para>Note that if a new root file system has been set up on <literal>/run/nextroot/</literal>, a
 169     <command>soft-reboot</command> will be performed when the <command>reboot</command> command is
 170     invoked.</para>
 171   </refsect1>
 172
 173   <refsect1>
 174     <title>See Also</title>
 175     <para><simplelist type="inline">
 176       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 177       <member><citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 178       <member><citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
 179       <member><citerefentry><refentrytitle>systemd-poweroff.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 180       <member><citerefentry><refentrytitle>systemd-suspend.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 181       <member><citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
 182     </simplelist></para>
 183   </refsect1>
 184
 185 </refentry>