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164 lines
10 KiB
XML
164 lines
10 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="systemd.v">
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<refentryinfo>
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<title>systemd.v</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd.v</refentrytitle>
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<manvolnum>7</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd.v</refname>
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<refpurpose>Directory with Versioned Resources</refpurpose>
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</refnamediv>
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<refsect1>
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<title>Description</title>
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<para>In various places systemd components accept paths whose trailing components have the
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<literal>.v/</literal> suffix, pointing to a directory. These components will then automatically look for
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suitable files inside the directory, do a version comparison and open the newest file found (by
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version). Specifically, two expressions are supported:</para>
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<itemizedlist>
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<listitem><para>When looking for files with a suffix <replaceable>.SUFFIX</replaceable>, and a path
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<filename>…<replaceable>PATH</replaceable>/<replaceable>NAME</replaceable><replaceable>.SUFFIX</replaceable>.v/</filename>
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is specified, then all files
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<filename>…<replaceable>PATH</replaceable>/<replaceable>NAME</replaceable><replaceable>.SUFFIX</replaceable>.v/<replaceable>NAME</replaceable>_*<replaceable>.SUFFIX</replaceable></filename>
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are enumerated, filtered, sorted and the newest file used. The primary sorting key is the
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<emphasis>variable part</emphasis>, here indicated by the wildcard
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<literal>*</literal>.</para></listitem>
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<listitem><para>When a path
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<filename>…<replaceable>PATH</replaceable>.v/<replaceable>NAME</replaceable>___<replaceable>.SUFFIX</replaceable></filename>
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is specified (i.e. the penultimate component of the path ends in <literal>.v</literal> and the final
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component contains a triple underscore), then all files
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<filename>…<replaceable>PATH</replaceable>.v/<replaceable>NAME</replaceable>_*<replaceable>.SUFFIX</replaceable></filename>
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are enumerated, filtered, sorted and the newest file used (again, by the <emphasis>variable
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part</emphasis>, here indicated by the wildcard <literal>*</literal>).</para></listitem>
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</itemizedlist>
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<para>To illustrate this in an example, consider a directory <filename>/var/lib/machines/mymachine.raw.v/</filename>, which is populated with three files:</para>
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<itemizedlist>
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<listitem><para><filename>mymachine_7.5.13.raw</filename></para></listitem>
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<listitem><para><filename>mymachine_7.5.14.raw</filename></para></listitem>
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<listitem><para><filename>mymachine_7.6.0.raw</filename></para></listitem>
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</itemizedlist>
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<para>Invoke a tool such as <citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry> with a command line like the following:</para>
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<programlisting># systemd-nspawn --image=/var/lib/machines/mymachine.raw.v --boot</programlisting>
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<para>Then this would automatically be resolved to the equivalent of:</para>
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<programlisting># systemd-nspawn --image=/var/lib/machines/mymachine.raw.v/mymachine_7.6.0.raw --boot</programlisting>
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<para>Much of systemd's functionality that expects a path to a disk image or OS directory hierarchy
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support the <literal>.v/</literal> versioned directory mechanism, for example
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<citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-dissect</refentrytitle><manvolnum>1</manvolnum></citerefentry> or
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the <varname>RootDirectory=</varname>/<varname>RootImage=</varname> settings of service files (see
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>).</para>
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<para>Use the
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<citerefentry><refentrytitle>systemd-vpick</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool to
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resolve <literal>.v/</literal> paths from the command line, for example for usage in shell
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scripts.</para>
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</refsect1>
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<refsect1>
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<title>Filtering and Sorting</title>
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<para>The variable part of the filenames in the <literal>.v/</literal> directories are filtered and
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compared primarily with a version comparison, implementing <ulink
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url="https://uapi-group.org/specifications/specs/version_format_specification/">Version Format
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Specification</ulink>. However, additional rules apply:</para>
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<itemizedlist>
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<listitem><para>If the variable part is suffixed by one or two integer values ("tries left" and "tries
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done") in the formats <filename>+<replaceable>LEFT</replaceable></filename> or
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<filename>+<replaceable>LEFT</replaceable>-<replaceable>DONE</replaceable></filename>, then these
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indicate usage attempt counters. The idea is that each time before a file is attempted to be used, its
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"tries left" counter is decreased, and the "tries done" counter increased (simply by renaming the
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file). When the file is successfully used (which for example could mean for an OS image: successfully
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booted) the counters are removed from the file name, indicating that the file has been validated to
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work correctly. This mechanism mirrors the boot assessment counters defined by <ulink
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url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT/">Automatic Boot Assessment</ulink>. Any filenames
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with no boot counters or with a non-zero "tries left" counter are sorted before filenames with a zero
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"tries left" counter.</para></listitem>
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<listitem><para>Preceding the use counters (if they are specified), an optional CPU architecture
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identifier may be specified in the filename (separated from the version with an underscore), as defined
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in the architecture vocabulary of the <varname>ConditionArchitecture=</varname> unit file setting, as
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documented in
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. Files
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whose name indicates an architecture not supported locally are filtered and not considered for the
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version comparison.</para></listitem>
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<listitem><para>The rest of the variable part is the version string.</para></listitem>
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</itemizedlist>
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<para>Or in other words, the files in the <literal>.v/</literal> directories should follow one of these
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naming structures:</para>
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<itemizedlist>
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<listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
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<listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable>_<replaceable>ARCHITECTURE</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
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<listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable>+<replaceable>LEFT</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
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<listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable>+<replaceable>LEFT</replaceable>-<replaceable>DONE</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
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<listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable>_<replaceable>ARCHITECTURE</replaceable>+<replaceable>LEFT</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
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<listitem><para><filename><replaceable>NAME</replaceable>_<replaceable>VERSION</replaceable>_<replaceable>ARCHITECTURE</replaceable>+<replaceable>LEFT</replaceable>-<replaceable>DONE</replaceable><replaceable>.SUFFIX</replaceable></filename></para></listitem>
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</itemizedlist>
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</refsect1>
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<refsect1>
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<title>Example</title>
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<para>Here's a more comprehensive example, further extending the one described above. Consider a
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directory <filename>/var/lib/machines/mymachine.raw.v/</filename>, which is populated with the following
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files:</para>
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<itemizedlist>
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<listitem><para><filename>mymachine_7.5.13.raw</filename></para></listitem>
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<listitem><para><filename>mymachine_7.5.14_x86-64.raw</filename></para></listitem>
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<listitem><para><filename>mymachine_7.6.0_arm64.raw</filename></para></listitem>
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<listitem><para><filename>mymachine_7.7.0_x86-64+0-5.raw</filename></para></listitem>
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</itemizedlist>
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<para>Now invoke the following command on an x86-64 machine:</para>
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<programlisting>$ systemd-vpick --suffix=.raw /var/lib/machines/mymachine.raw.v/</programlisting>
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<para>This would resolve the specified path to
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<filename>/var/lib/machines/mymachine.raw.v/mymachine_7.5.14_x86-64.raw</filename>. Explanation: even
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though <filename>mymachine_7.7.0_x86-64+0-5.raw</filename> has the newest version, it is not preferred
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because its tries left counter is zero. And even though <filename>mymachine_7.6.0_arm64.raw</filename>
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has the second newest version it is also not considered, in this case because we operate on an x86_64
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system and the image is intended for arm64 CPUs. Finally, the <filename>mymachine_7.5.13.raw</filename>
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image is not considered because it is older than <filename>mymachine_7.5.14_x86-64.raw</filename>.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para><simplelist type="inline">
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<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd-vpick</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd-dissect</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>systemd-sysupdate</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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</simplelist></para>
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</refsect1>
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</refentry>
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