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167 lines
8.3 KiB
XML
167 lines
8.3 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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SPDX-License-Identifier: LGPL-2.1+
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-->
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<refentry id="systemd.offline-updates">
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<refentryinfo>
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<title>systemd.offline-updates</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd.offline-updates</refentrytitle>
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<manvolnum>7</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd.offline-updates</refname>
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<refpurpose>Implementation of offline updates in systemd</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Implementing Offline System Updates</title>
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<para>This man page describes how to implement "offline" system updates with systemd. By "offline"
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OS updates we mean package installations and updates that are run with the system booted into a
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special system update mode, in order to avoid problems related to conflicts of libraries and
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services that are currently running with those on disk. This document is inspired by this
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<ulink url="https://wiki.gnome.org/Design/OS/SoftwareUpdates">GNOME design whiteboard</ulink>.
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</para>
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<para>The logic:</para>
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<orderedlist>
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<listitem>
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<para>The package manager prepares system updates by downloading all (RPM or DEB or
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whatever) packages to update off-line in a special directory
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<filename noindex="true">/var/lib/system-update</filename> (or
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another directory of the package/upgrade manager's choice).</para>
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</listitem>
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<listitem>
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<para>When the user OK'ed the update, the symlink <filename>/system-update</filename> is
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created that points to <filename noindex="true">/var/lib/system-update</filename> (or
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wherever the directory with the upgrade files is located) and the system is rebooted. This
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symlink is in the root directory, since we need to check for it very early at boot, at a
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time where <filename>/var</filename> is not available yet.</para>
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</listitem>
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<listitem>
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<para>Very early in the new boot
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<citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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checks whether <filename>/system-update</filename> exists. If so, it (temporarily and for
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this boot only) redirects (i.e. symlinks) <filename>default.target</filename> to
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<filename>system-update.target</filename>, a special target that pulls in the base system
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(i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but little
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else) and the system update units.</para>
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</listitem>
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<listitem>
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<para>The system now continues to boot into <filename>default.target</filename>, and
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thus into <filename>system-update.target</filename>. This target pulls in all system
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update units. Only one service should perform an update (see the next point), and all
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the other ones should exit cleanly with a "success" return code and without doing
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anything. Update services should be ordered after <filename>sysinit.target</filename>
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so that the update starts after all file systems have been mounted.</para>
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</listitem>
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<listitem>
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<para>As the first step, an update service should check if the
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<filename>/system-update</filename> symlink points to the location used by that update
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service. In case it does not exist or points to a different location, the service must exit
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without error. It is possible for multiple update services to be installed, and for multiple
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update services to be launched in parallel, and only the one that corresponds to the tool
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that <emphasis>created</emphasis> the symlink before reboot should perform any actions. It
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is unsafe to run multiple updates in parallel.</para>
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</listitem>
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<listitem>
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<para>The update service should now do its job. If applicable and possible, it should
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create a file system snapshot, then install all packages. After completion (regardless
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whether the update succeeded or failed) the machine must be rebooted, for example by
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calling <command>systemctl reboot</command>. In addition, on failure the script should
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revert to the old file system snapshot (without the symlink).</para>
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</listitem>
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<listitem>
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<para>The upgrade scripts should exit only after the update is finished. It is expected
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that the service which performs the upgrade will cause the machine to reboot after it
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is done. If the <filename>system-update.target</filename> is successfully reached, i.e.
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all update services have run, and the <filename>/system-update</filename> symlink still
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exists, it will be removed and the machine rebooted as a safety measure.</para>
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</listitem>
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<listitem>
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<para>After a reboot, now that the <filename>/system-update</filename> symlink is gone,
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the generator won't redirect <filename>default.target</filename> anymore and the system
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now boots into the default target again.</para>
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</listitem>
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</orderedlist>
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</refsect1>
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<refsect1>
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<title>Recommendations</title>
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<orderedlist>
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<listitem>
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<para>To make things a bit more robust we recommend hooking the update script into
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<filename>system-update.target</filename> via a <filename noindex='true'>.wants/</filename>
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symlink in the distribution package, rather than depending on <command>systemctl
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enable</command> in the postinst scriptlets of your package. More specifically, for your
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update script create a .service file, without [Install] section, and then add a symlink like
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<filename noindex='true'>/usr/lib/systemd/system-update.target.wants/foobar.service</filename>
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→ <filename noindex='true'>../foobar.service</filename> to your package.</para>
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</listitem>
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<listitem>
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<para>Make sure to remove the <filename>/system-update</filename> symlink as early as
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possible in the update script to avoid reboot loops in case the update fails.</para>
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</listitem>
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<listitem>
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<para>Use <varname>FailureAction=reboot</varname> in the service file for your update script
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to ensure that a reboot is automatically triggered if the update fails.
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<varname>FailureAction=</varname> makes sure that the specified unit is activated if your
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script exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds
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you should trigger the reboot in your own code, for example by invoking logind's
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<command>Reboot()</command> call or calling <command>systemctl reboot</command>. See
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<ulink url="https://www.freedesktop.org/wiki/Software/systemd/logind">logind dbus API</ulink>
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for details.</para>
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</listitem>
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<listitem>
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<para>The update service should declare <varname>DefaultDependencies=false</varname>,
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<varname>Requires=sysinit.target</varname>, <varname>After=sysinit.target</varname>,
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<varname>After=system-update-pre.target</varname>
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and explicitly pull in any other services it requires.</para>
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</listitem>
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<listitem>
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<para>It may be desirable to always run an auxiliary unit when booting
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into offline-updates mode, which itself does not install updates. To
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do this create a .service file with
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<varname>Wants=system-update-pre.target</varname> and
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<varname>Before=system-update-pre.target</varname> and add a symlink
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to that file under
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<filename noindex='true'>/usr/lib/systemd/system-update.target.wants</filename>
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.</para>
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</listitem>
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</orderedlist>
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</refsect1>
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<refsect1>
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<title>See also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry project='mankier'><refentrytitle>dnf.plugin.system-upgrade</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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