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<refentry id="systemd.offline-updates">
<refentryinfo>
<title>systemd.offline-updates</title>
<productname>systemd</productname>
-
- <authorgroup>
- <author>
- <contrib>Developer</contrib>
- <firstname>Lennart</firstname>
- <surname>Poettering</surname>
- <email>lennart@poettering.net</email>
- </author>
- </authorgroup>
</refentryinfo>
<refmeta>
<refsect1>
<title>Implementing Offline System Updates</title>
- <para>This is implemented starting with systemd 183.</para>
-
- <para>Here are some guidelines how to implement "offline" OS updates with systemd. By "offline"
+ <para>This man page describes how to implement "offline" system updates with systemd. By "offline"
OS updates we mean package installations and updates that are run with the system booted into a
special system update mode, in order to avoid problems related to conflicts of libraries and
services that are currently running with those on disk. This document is inspired by this
<listitem>
<para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is
- created that points to <filename noindex="true">/var/lib/system-update</filename>
- (or wherever the upgrade package directory is called) and the system is rebooted. This
+ created that points to <filename noindex="true">/var/lib/system-update</filename> (or
+ wherever the directory with the upgrade files is located) and the system is rebooted. This
symlink is in the root directory, since we need to check for it very early at boot, at a
time where <filename>/var</filename> is not available yet.</para>
</listitem>
<listitem>
- <para>Very early in the new boot a systemd generator checks whether
- <filename>/system-update</filename> exists. If so, it (temporarily and for this boot only)
- redirects (i.e. symlinks) <filename>default.target</filename> to
- <filename>system-update.target</filename>, a new target that is intended to pull in the base
- system (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but
- little else) and the system update units.</para>
+ <para>Very early in the new boot
+ <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ checks whether <filename>/system-update</filename> exists. If so, it (temporarily and for
+ this boot only) redirects (i.e. symlinks) <filename>default.target</filename> to
+ <filename>system-update.target</filename>, a special target that pulls in the base system
+ (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little
+ else) and the system update units.</para>
+ </listitem>
+
+ <listitem>
+ <para>The system now continues to boot into <filename>default.target</filename>, and
+ thus into <filename>system-update.target</filename>. This target pulls in all system
+ update units. Only one service should perform an update (see the next point), and all
+ the other ones should exit cleanly with a "success" return code and without doing
+ anything. Update services should be ordered after <filename>sysinit.target</filename>
+ so that the update starts after all file systems have been mounted.</para>
</listitem>
<listitem>
- <para>The system now continues to boot into <filename>default.target</filename>, and thus
- into <filename>system-update.target</filename>. This target pulls in the OS update script,
- which is executed after all file systems are mounted.</para>
+ <para>As the first step, an update service should check if the
+ <filename>/system-update</filename> symlink points to the location used by that update
+ service. In case it does not exist or points to a different location, the service must exit
+ without error. It is possible for multiple update services to be installed, and for multiple
+ update services to be launched in parallel, and only the one that corresponds to the tool
+ that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It
+ is unsafe to run multiple updates in parallel.</para>
</listitem>
<listitem>
- <para>The system update script now creates a btrfs snapshot (if possible), then installs all
- RPMs. After completion (regardless whether the update succeeded or failed) the
- /system-update symlink is removed. In addition, on failure it reverts to the old btrfs state
- (modulo the aforementioned symlink), on success it leaves the newly made changes in
- place.</para>
+ <para>The update service should now do its job. If applicable and possible, it should
+ create a file system snapshot, then install all packages. After completion (regardless
+ whether the update succeeded or failed) the machine must be rebooted, for example by
+ calling <command>systemctl reboot</command>. In addition, on failure the script should
+ revert to the old file system snapshot (without the symlink).</para>
</listitem>
<listitem>
- <para>The system is rebooted. Since the <filename>/system-update</filename> symlink is gone,
- the generator won't redirect <filename>default.target</filename> after reboot and the
- system now boots into the default target again.</para>
+ <para>The upgrade scripts should exit only after the update is finished. It is expected
+ that the service which performs the upgrade will cause the machine to reboot after it
+ is done. If the <filename>system-update.target</filename> is successfully reached, i.e.
+ all update services have run, and the <filename>/system-update</filename> symlink still
+ exists, it will be removed and the machine rebooted as a safety measure.</para>
+ </listitem>
+
+ <listitem>
+ <para>After a reboot, now that the <filename>/system-update</filename> symlink is gone,
+ the generator won't redirect <filename>default.target</filename> anymore and the system
+ now boots into the default target again.</para>
</listitem>
</orderedlist>
</refsect1>
</listitem>
<listitem>
- <para>Make sure to remove the <filename>/system-update</filename> symlink early in the
- update script to avoid reboot loops in case the update fails.</para>
+ <para>Make sure to remove the <filename>/system-update</filename> symlink as early as
+ possible in the update script to avoid reboot loops in case the update fails.</para>
</listitem>
<listitem>
- <para>Use <varname>OnFailure=reboot.target</varname> in the service file for your update
- script to ensure that a reboot is automatically triggered if the update fails.
- <varname>OnFailure=</varname> makes sure that the specified unit is activated if your script
- exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds you
- should trigger the reboot in your own code, for example by invoking logind's
- <command>Reboot()</command> call. See
- <ulink url="http://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink>
+ <para>Use <varname>FailureAction=reboot</varname> in the service file for your update script
+ to ensure that a reboot is automatically triggered if the update fails.
+ <varname>FailureAction=</varname> makes sure that the specified unit is activated if your
+ script exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds
+ you should trigger the reboot in your own code, for example by invoking logind's
+ <command>Reboot()</command> call or calling <command>systemctl reboot</command>. See
+ <ulink url="https://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink>
for details.</para>
</listitem>
+
+ <listitem>
+ <para>The update service should declare <varname>DefaultDependencies=no</varname>,
+ <varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>,
+ <varname>After=system-update-pre.target</varname>
+ and explicitly pull in any other services it requires.</para>
+ </listitem>
+
+ <listitem>
+ <para>It may be desirable to always run an auxiliary unit when booting
+ into offline-updates mode, which itself does not install updates. To
+ do this create a .service file with
+ <varname>Wants=system-update-pre.target</varname> and
+ <varname>Before=system-update-pre.target</varname> and add a symlink
+ to that file under
+ <filename noindex='true'>/usr/lib/systemd/system-update.target.wants</filename>
+ .</para>
+ </listitem>
</orderedlist>
</refsect1>
<title>See also</title>
<para>
- <ulink url="http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates/">Implementing Offline System Updates</ulink>,
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+ <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+ <citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
projects / thirdparty / systemd.git / blobdiff