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634735b56b
This has a long history; see see5261ba9018which originally introduced the behavior. Unfortunately that commit doesn't include any rationale, but IIRC the basic issue is that systemd wants to model the real mount state as units, and symlinks make canonicalization much more difficult. At the same time, on a RHEL6 system (upstart), one can make e.g. `/home` a symlink, and things work as well as they always did; but one doesn't have access to the sophistication of mount units (dependencies, introspection, etc.) Supporting symlinks here will hence make it easier for people to do upgrades to RHEL7 and beyond. The `/home` as symlink case also appears prominently for OSTree; see https://ostree.readthedocs.io/en/latest/manual/adapting-existing/ Further work has landed in the nspawn case for this; see e.g.d944dc9553A basic limitation with doing this in the fstab generator (and that I hit while doing some testing) is that we obviously can't chase symlinks into mounts, since the generator runs early before mounts. Or at least - doing so would require multiple passes over the fstab data (as well as looking at existing mount units), and potentially doing multi-phase generation. I'm not sure it's worth doing that without a real world use case. For now, this will fix at least the OSTree + `/home` <https://bugzilla.redhat.com/show_bug.cgi?id=1382873> case mentioned above, and in general anyone who for whatever reason has symlinks in their `/etc/fstab`.
232 lines
10 KiB
XML
232 lines
10 KiB
XML
<?xml version="1.0"?>
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<!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "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 2012 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-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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<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-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>Takes the root filesystem to mount in the
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initrd. <varname>root=</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>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
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use. <varname>rootflags=</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.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
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directory mount point unaltered, however will mount a <literal>tmpfs</literal> file system to
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<filename>/var</filename>. In this mode the normal system configuration (i.e. the contents of
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<literal>/etc</literal>) is in effect (and may be modified during system runtime), however the system state
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(i.e. the contents of <literal>/var</literal>) is reset at boot and lost at shutdown.</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 enabling this setting will only work correctly on operating systems that can boot up with only
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<filename>/usr</filename> 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></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>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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