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09e917ea4d
So, i think "erofs" is probably the better, more modern alternative to "squashfs". Many of the benefits don't matter too much to us I guess, but there's one thing that stands out: erofs has a UUID in the superblock, squashfs has not. Having an UUID in the superblock matters if the file systems are used in an overlayfs stack, as overlayfs uses the UUIDs to robustly and persistently reference inodes on layers in case of metadata copy-up. Since we probably want to allow such uses in overlayfs as emplyoed by sysext (and the future syscfg) we probably should ramp up our erofs game early on. Hence let's natively support erofs, test it, and in fact mention it in the docs before squashfs even.
255 lines
15 KiB
XML
255 lines
15 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-sysext"
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>systemd-sysext</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-sysext</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-sysext</refname>
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<refname>systemd-sysext.service</refname>
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<refpurpose>Activates System Extension Images</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<cmdsynopsis>
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<command>systemd-sysext</command>
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<arg choice="opt" rep="repeat">OPTIONS</arg>
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<arg choice="plain">COMMAND</arg>
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</cmdsynopsis>
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<para><literallayout><filename>systemd-sysext.service</filename></literallayout></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><command>systemd-sysext</command> activates/deactivates system extension images. System extension
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images may – dynamically at runtime — extend the <filename>/usr/</filename> and
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<filename>/opt/</filename> directory hierarchies with additional files. This is particularly useful on
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immutable system images where a <filename>/usr/</filename> and/or <filename>/opt/</filename> hierarchy
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residing on a read-only file system shall be extended temporarily at runtime without making any
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persistent modifications.</para>
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<para>System extension images should contain files and directories similar in fashion to regular
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operating system tree. When one or more system extension images are activated, their
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<filename>/usr/</filename> and <filename>/opt/</filename> hierarchies are combined via
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<literal>overlayfs</literal> with the same hierarchies of the host OS, and the host
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<filename>/usr/</filename> and <filename>/opt/</filename> overmounted with it ("merging"). When they are
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deactivated, the mount point is disassembled — again revealing the unmodified original host version of
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the hierarchy ("unmerging"). Merging thus makes the extension's resources suddenly appear below the
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<filename>/usr/</filename> and <filename>/opt/</filename> hierarchies as if they were included in the
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base OS image itself. Unmerging makes them disappear again, leaving in place only the files that were
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shipped with the base OS image itself.</para>
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<para>Files and directories contained in the extension images outside of the <filename>/usr/</filename>
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and <filename>/opt/</filename> hierarchies are <emphasis>not</emphasis> merged, and hence have no effect
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when included in a system extension image. In particular, files in the <filename>/etc/</filename> and
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<filename>/var/</filename> included in a system extension image will <emphasis>not</emphasis> appear in
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the respective hierarchies after activation.</para>
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<para>System extension images are strictly read-only, and the host <filename>/usr/</filename> and
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<filename>/opt/</filename> hierarchies become read-only too while they are activated.</para>
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<para>System extensions are supposed to be purely additive, i.e. they are supposed to include only files
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that do not exist in the underlying basic OS image. However, the underlying mechanism (overlayfs) also
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allows overlaying or removing files, but it is recommended not to make use of this.</para>
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<para>System extension images may be provided in the following formats:</para>
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<orderedlist>
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<listitem><para>Plain directories or btrfs subvolumes containing the OS tree</para></listitem>
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<listitem><para>Disk images with a GPT disk label, following the <ulink
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url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions Specification</ulink></para></listitem>
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<listitem><para>Disk images lacking a partition table, with a naked Linux file system (e.g. erofs,
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squashfs or ext4)</para></listitem>
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</orderedlist>
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<para>These image formats are the same ones that
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<citerefentry><refentrytitle>systemd-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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supports via its <option>--directory=</option>/<option>--image=</option> switches and those that the
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service manager supports via <option>RootDirectory=</option>/<option>RootImage=</option>. Similar to
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them they may optionally carry Verity authentication information.</para>
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<para>System extensions are automatically looked for in the directories
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<filename>/etc/extensions/</filename>, <filename>/run/extensions/</filename>,
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<filename>/var/lib/extensions/</filename>, <filename>/usr/lib/extensions/</filename> and
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<filename>/usr/local/lib/extensions/</filename>. The first two listed directories are not suitable for
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carrying large binary images, however are still useful for carrying symlinks to them. The primary place
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for installing system extensions is <filename>/var/lib/extensions/</filename>. Any directories found in
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these search directories are considered directory based extension images, any files with the
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<filename>.raw</filename> suffix are considered disk image based extension images.</para>
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<para>During boot OS extension images are activated automatically, if the
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<filename>systemd-sysext.service</filename> is enabled. Note that this service runs only after the
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underlying file systems where system extensions may be located have been mounted. This means they are not
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suitable for shipping resources that are processed by subsystems running in earliest boot. Specifically,
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OS extension images are not suitable for shipping system services or
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<citerefentry><refentrytitle>systemd-sysusers</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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definitions. See <ulink url="https://systemd.io/PORTABLE_SERVICES">Portable Services</ulink> for a simple
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mechanism for shipping system services in disk images, in a similar fashion to OS extensions. Note the
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different isolation on these two mechanisms: while system extension directly extend the underlying OS
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image with additional files that appear in a way very similar to as if they were shipped in the OS image
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itself and thus imply no security isolation, portable services imply service level sandboxing in one way
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or another. The <filename>systemd-sysext.service</filename> service is guaranteed to finish start-up
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before <filename>basic.target</filename> is reached; i.e. at the time regular services initialize (those
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which do not use <varname>DefaultDependencies=no</varname>), the files and directories system extensions
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provide are available in <filename>/usr/</filename> and <filename>/opt/</filename> and may be
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accessed.</para>
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<para>Note that there is no concept of enabling/disabling installed system extension images: all
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installed extension images are automatically activated at boot. However, you can place an empty directory
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named like the extension (no <filename>.raw</filename>) in <filename>/etc/extensions/</filename> to "mask"
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an extension with the same name in a system folder with lower precedence.</para>
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<para>A simple mechanism for version compatibility is enforced: a system extension image must carry a
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<filename>/usr/lib/extension-release.d/extension-release.<replaceable>$name</replaceable></filename>
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file, which must match its image name, that is compared with the host <filename>os-release</filename>
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file: the contained <varname>ID=</varname> fields have to match unless <literal>_any</literal> is set
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for the extension. If the extension <varname>ID=</varname> is not <literal>_any</literal>, the
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<varname>SYSEXT_LEVEL=</varname> field (if defined) has to match. If the latter is not defined, the
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<varname>VERSION_ID=</varname> field has to match instead. If the extension defines the
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<varname>ARCHITECTURE=</varname> field and the value is not <literal>_any</literal> it has to match the kernel's
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architecture reported by <citerefentry><refentrytitle>uname</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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but the used architecture identifiers are the same as for <varname>ConditionArchitecture=</varname>
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described in <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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System extensions should not ship a <filename>/usr/lib/os-release</filename> file (as that would be merged
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into the host <filename>/usr/</filename> tree, overriding the host OS version data, which is not desirable).
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The <filename>extension-release</filename> file follows the same format and semantics, and carries the same
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content, as the <filename>os-release</filename> file of the OS, but it describes the resources carried
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in the extension image.</para>
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</refsect1>
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<refsect1>
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<title>Uses</title>
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<para>The primary use case for system images are immutable environments where debugging and development
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tools shall optionally be made available, but not included in the immutable base OS image itself (e.g.
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<citerefentry project='man-pages'><refentrytitle>strace</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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and
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<citerefentry project='man-pages'><refentrytitle>gdb</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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shall be an optionally installable addition in order to make debugging/development easier). System
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extension images should not be misunderstood as a generic software packaging framework, as no dependency
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scheme is available: system extensions should carry all files they need themselves, except for those
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already shipped in the underlying host system image. Typically, system extension images are built at the
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same time as the base OS image — within the same build system.</para>
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<para>Another use case for the system extension concept is temporarily overriding OS supplied resources
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with newer ones, for example to install a locally compiled development version of some low-level
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component over the immutable OS image without doing a full OS rebuild or modifying the nominally
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immutable image. (e.g. "install" a locally built package with <command>DESTDIR=/var/lib/extensions/mytest
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make install && systemd-sysext refresh</command>, making it available in
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<filename>/usr/</filename> as if it was installed in the OS image itself.) This case works regardless if
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the underlying host <filename>/usr/</filename> is managed as immutable disk image or is a traditional
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package manager controlled (i.e. writable) tree.</para>
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</refsect1>
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<refsect1>
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<title>Commands</title>
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<para>The following commands are understood:</para>
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<variablelist>
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<varlistentry>
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<term><option>status</option></term>
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<listitem><para>When invoked without any command verb, or when <option>status</option> is specified
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the current merge status is shown, separately for both <filename>/usr/</filename> and
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<filename>/opt/</filename>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>merge</option></term>
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<listitem><para>Merges all currently installed system extension images into
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<filename>/usr/</filename> and <filename>/opt/</filename>, by overmounting these hierarchies with an
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<literal>overlayfs</literal> file system combining the underlying hierarchies with those included in
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the extension images. This command will fail if the hierarchies are already merged.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>unmerge</option></term>
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<listitem><para>Unmerges all currently installed system extension images from
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<filename>/usr/</filename> and <filename>/opt/</filename>, by unmounting the
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<literal>overlayfs</literal> file systems created by <option>merge</option>
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prior.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>refresh</option></term>
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<listitem><para>A combination of <option>unmerge</option> and <option>merge</option>: if already
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mounted the existing <literal>overlayfs</literal> instance is unmounted temporarily, and then
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replaced by a new version. This command is useful after installing/removing system extension images,
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in order to update the <literal>overlayfs</literal> file system accordingly. If no system extensions
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are installed when this command is executed, the equivalent of <option>unmerge</option> is
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executed, without establishing any new <literal>overlayfs</literal> instance. Note that currently
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there's a brief moment where neither the old nor the new <literal>overlayfs</literal> file system is
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mounted. This implies that all resources supplied by a system extension will briefly disappear — even
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if it exists continuously during the refresh operation.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>list</option></term>
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<listitem><para>A brief list of installed extension images is shown.</para></listitem>
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</varlistentry>
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<xi:include href="standard-options.xml" xpointer="help" />
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<xi:include href="standard-options.xml" xpointer="version" />
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Options</title>
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<variablelist>
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<varlistentry>
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<term><option>--root=</option></term>
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<listitem><para>Operate relative to the specified root directory, i.e. establish the
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<literal>overlayfs</literal> mount not on the top-level host <filename>/usr/</filename> and
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<filename>/opt/</filename> hierarchies, but below some specified root directory.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><option>--force</option></term>
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<listitem><para>When merging system extensions into <filename>/usr/</filename> and
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<filename>/opt/</filename>, ignore version incompatibilities, i.e. force merging regardless of
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whether the version information included in the extension images matches the host or
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not.</para></listitem>
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</varlistentry>
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<xi:include href="standard-options.xml" xpointer="no-pager" />
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<xi:include href="standard-options.xml" xpointer="no-legend" />
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<xi:include href="standard-options.xml" xpointer="json" />
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Exit status</title>
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<para>On success, 0 is returned.</para>
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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-nspawn</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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