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725ad3b062
It's useful to be able to combine a regular /usr/ file system with a tmpfs as root, for an OS that boots up in volatile mode on every single boot. Let's add explicit support for this via root=tmpfs. Note the relationship to the existing systemd.volatile= option: 1. The kernel command line "root=/dev/… systemd.volatile=yes" will mount the specified root fs, and then hide everything at the top by overmounting it with a tmpfs, except for the /usr subtree. 2. The kernel command line "root=tmpfs mount.usr=/dev/…" otoh will mount a toot fs at the top (just like the case above), but will then mount the top-level dir of the fs specified in mount.usr= directly below it. Or to say this differently: in the first case /usr/ from the physical storage fs is going to become /usr/ of the hierarchy ultimately booted, while in the second case / from the physical storage fs is going to become /usr of the hierarchy booted. Philosophically I figure systemd.volatile= is more an option for "one-off" boots, while root=tmpfs is something to have as default mode of operation for suitable images. This is currently hard to test reasonably, since Dracut refuses to accept root=tmpfs. This needs to be addressed separately though.
247 lines
12 KiB
XML
247 lines
12 KiB
XML
<?xml version="1.0"?>
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<!--*-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-fstab-generator">
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<refentryinfo>
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<title>systemd-fstab-generator</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-fstab-generator</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-fstab-generator</refname>
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<refpurpose>Unit generator for /etc/fstab</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>/usr/lib/systemd/system-generators/systemd-fstab-generator</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><filename>systemd-fstab-generator</filename> is a generator
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that translates <filename>/etc/fstab</filename> (see
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<citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for details) into native systemd units early at boot and when
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configuration of the system manager is reloaded. This will
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instantiate mount and swap units as necessary.</para>
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<para>The <varname>passno</varname> field is treated like a simple
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boolean, and the ordering information is discarded. However, if
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the root file system is checked, it is checked before all the
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other file systems.</para>
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<para>See
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<citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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and
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<citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for more information about special <filename>/etc/fstab</filename>
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mount options this generator understands.</para>
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<para>One special topic is handling of symbolic links. Historical init
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implementations supported symlinks in <filename>/etc/fstab</filename>.
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Because mount units will refuse mounts where the target is a symbolic link,
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this generator will resolve any symlinks as far as possible when processing
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<filename>/etc/fstab</filename> in order to enhance backwards compatibility.
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If a symlink target does not exist at the time that this generator runs, it
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is assumed that the symlink target is the final target of the mount.</para>
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<para><filename>systemd-fstab-generator</filename> implements
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<citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
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</refsect1>
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<refsect1>
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<title>Kernel Command Line</title>
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<para><filename>systemd-fstab-generator</filename> understands the
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following kernel command line parameters:</para>
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<variablelist class='kernel-commandline-options'>
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<varlistentry>
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<term><varname>fstab=</varname></term>
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<term><varname>rd.fstab=</varname></term>
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<listitem><para>Takes a boolean argument. Defaults to
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<literal>yes</literal>. If <literal>no</literal>, causes the
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generator to ignore any mounts or swap devices configured in
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<filename>/etc/fstab</filename>. <varname>rd.fstab=</varname>
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is honored only by the initial RAM disk (initrd) while
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<varname>fstab=</varname> is honored by both the main system
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and the initrd.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>root=</varname></term>
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<listitem><para>Configures the operating system's root filesystem to mount when running in the
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initrd. This accepts a device node path (usually <filename>/dev/disk/by-uuid/…</filename> or
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<filename>/dev/disk/by-label/…</filename> or similar), or the special values <literal>gpt-auto</literal>
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and <literal>tmpfs</literal>.</para>
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<para>Use <literal>gpt-auto</literal> to explicitly request automatic root file system discovery via
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<citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
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<para>Use <literal>tmpfs</literal> in order to mount a <citerefentry
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project='man-pages'><refentrytitle>tmpfs</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
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system as root file system of the OS. This is useful in combination with
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<varname>mount.usr=</varname> (see below) in order to combine a volatile root file system with a
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separate, immutable <filename>/usr/</filename> file system. Also see
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<varname>systemd.volatile=</varname> below.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>rootfstype=</varname></term>
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<listitem><para>Takes the root filesystem type that will be
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passed to the mount command. <varname>rootfstype=</varname> is
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honored by the initrd.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>rootflags=</varname></term>
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<listitem><para>Takes the root filesystem mount options to use. <varname>rootflags=</varname> is
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honored by the initrd.</para>
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<para>Note that unlike most kernel command line options this setting does not override settings made
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in configuration files (specifically: the mount option string in
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<filename>/etc/fstab</filename>). See
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<citerefentry><refentrytitle>systemd-remount-fs.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>mount.usr=</varname></term>
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<listitem><para>Takes the <filename>/usr/</filename> filesystem
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to be mounted by the initrd. If
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<varname>mount.usrfstype=</varname> or
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<varname>mount.usrflags=</varname> is set, then
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<varname>mount.usr=</varname> will default to the value set in
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<varname>root=</varname>.</para>
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<para>Otherwise, this parameter defaults to the
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<filename>/usr/</filename> entry found in
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<filename>/etc/fstab</filename> on the root filesystem.</para>
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<para><varname>mount.usr=</varname> is honored by the initrd.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>mount.usrfstype=</varname></term>
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<listitem><para>Takes the <filename>/usr/</filename> filesystem
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type that will be passed to the mount command. If
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<varname>mount.usr=</varname> or
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<varname>mount.usrflags=</varname> is set, then
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<varname>mount.usrfstype=</varname> will default to the value
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set in <varname>rootfstype=</varname>.</para>
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<para>Otherwise, this value will be read from the
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<filename>/usr/</filename> entry in
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<filename>/etc/fstab</filename> on the root filesystem.</para>
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<para><varname>mount.usrfstype=</varname> is honored by the
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initrd.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>mount.usrflags=</varname></term>
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<listitem><para>Takes the <filename>/usr/</filename> filesystem
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mount options to use. If <varname>mount.usr=</varname> or
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<varname>mount.usrfstype=</varname> is set, then
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<varname>mount.usrflags=</varname> will default to the value
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set in <varname>rootflags=</varname>.</para>
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<para>Otherwise, this value will be read from the
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<filename>/usr/</filename> entry in
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<filename>/etc/fstab</filename> on the root filesystem.</para>
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<para><varname>mount.usrflags=</varname> is honored by the
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initrd.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>systemd.volatile=</varname></term>
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<listitem><para>Controls whether the system shall boot up in volatile mode. Takes a boolean argument or the
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special value <option>state</option>.</para>
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<para>If false (the default), this generator makes no changes to the mount tree and the system is booted up in
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normal mode.</para>
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<para>If true the generator ensures
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<citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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is run as part of the initial RAM disk ("initrd"). This service changes the mount table before transitioning to
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the host system, so that a volatile memory file system (<literal>tmpfs</literal>) is used as root directory,
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with only <filename>/usr/</filename> mounted into it from the configured root file system, in read-only
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mode. This way the system operates in fully stateless mode, with all configuration and state reset at boot and
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lost at shutdown, as <filename>/etc/</filename> and <filename>/var/</filename> will be served from the (initially
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unpopulated) volatile memory file system.</para>
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<para>If set to <option>state</option> the generator will leave the root directory mount point unaltered,
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however will mount a <literal>tmpfs</literal> file system to <filename>/var/</filename>. In this mode the normal
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system configuration (i.e. the contents of <literal>/etc/</literal>) is in effect (and may be modified during
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system runtime), however the system state (i.e. the contents of <literal>/var/</literal>) is reset at boot and
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lost at shutdown.</para>
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<para>If this setting is set to <literal>overlay</literal> the root file system is set up as
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<literal>overlayfs</literal> mount combining the read-only root directory with a writable
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<literal>tmpfs</literal>, so that no modifications are made to disk, but the file system may be modified
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nonetheless with all changes being lost at reboot.</para>
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<para>Note that in none of these modes the root directory, <filename>/etc/</filename>, <filename>/var/</filename>
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or any other resources stored in the root file system are physically removed. It's thus safe to boot a system
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that is normally operated in non-volatile mode temporarily into volatile mode, without losing data.</para>
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<para>Note that with the exception of <literal>overlay</literal> mode, enabling this setting will
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only work correctly on operating systems that can boot up with only <filename>/usr/</filename>
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mounted, and are able to automatically populate <filename>/etc/</filename>, and also
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<filename>/var/</filename> in case of <literal>systemd.volatile=yes</literal>.</para>
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<para>Also see <varname>root=tmpfs</varname> above, for a method to combine a
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<literal>tmpfs</literal> file system with a regular <filename>/usr/</filename> file system (as
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configured via <varname>mount.usr=</varname>). The main distinction between
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<varname>systemd.volatile=yes</varname>, and <varname>root=tmpfs</varname> in combination
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<varname>mount.usr=</varname> is that the former operates on top of a regular root file system and
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temporarily obstructs the files and directories above its <filename>/usr/</filename> subdirectory,
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while the latter does not hide any files, but simply mounts a unpopulated tmpfs as root file system
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and combines it with a user picked <filename>/usr/</filename> file system.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>systemd.swap</varname></term>
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<listitem><para>Takes a boolean argument or enables the option if specified
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without an argument. If disabled, causes the generator to ignore
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any swap devices configured in <filename>/etc/fstab</filename>.
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Defaults to enabled.</para></listitem>
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</varlistentry>
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</variablelist>
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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 project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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