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c5915c636a
This imports most of http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates/ to turn it into a man page. Similarly for the man page about generators, this will make it easier to keep up-to-date, keep a history of changes, and make it more discoverable for end-users.
152 lines
6.6 KiB
XML
152 lines
6.6 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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This file is part of systemd.
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Copyright 2013 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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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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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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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 is implemented starting with systemd 183.</para>
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<para>Here are some guidelines how to implement "offline" OS 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>
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(or wherever the upgrade package directory is called) 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 a systemd generator checks whether
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<filename>/system-update</filename> exists. If so, it (temporarily and for this boot only)
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redirects (i.e. symlinks) <filename>default.target</filename> to
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<filename>system-update.target</filename>, a new target that is intended to pull in the base
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system (i.e. <filename>sysinit.target</filename>, so that all file systems are mounted but
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little 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 thus
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into <filename>system-update.target</filename>. This target pulls in the OS update script,
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which is executed after all file systems are mounted.</para>
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</listitem>
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<listitem>
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<para>The system update script now creates a btrfs snapshot (if possible), then installs all
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RPMs. After completion (regardless whether the update succeeded or failed) the
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/system-update symlink is removed. In addition, on failure it reverts to the old btrfs state
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(modulo the aforementioned symlink), on success it leaves the newly made changes in
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place.</para>
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</listitem>
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<listitem>
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<para>The system is rebooted. Since the <filename>/system-update</filename> symlink is gone,
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the generator won't redirect <filename>default.target</filename> after reboot and the
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system 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 early in the
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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>OnFailure=reboot.target</varname> in the service file for your update
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script to ensure that a reboot is automatically triggered if the update fails.
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<varname>OnFailure=</varname> makes sure that the specified unit is activated if your script
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exits uncleanly (by non-zero error code, or signal/coredump). If your script succeeds you
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should trigger the reboot in your own code, for example by invoking logind's
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<command>Reboot()</command> call. See
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<ulink url="http://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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</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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<ulink url="http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates/">Implementing Offline System Updates</ulink>,
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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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</para>
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</refsect1>
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</refentry>
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