man/systemd-fstab-generator.xml

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   6 <refentry id="systemd-fstab-generator">
   7
   8   <refentryinfo>
   9     <title>systemd-fstab-generator</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>systemd-fstab-generator</refentrytitle>
  15     <manvolnum>8</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>systemd-fstab-generator</refname>
  20     <refpurpose>Unit generator for /etc/fstab</refpurpose>
  21   </refnamediv>
  22
  23   <refsynopsisdiv>
  24     <para><filename>/usr/lib/systemd/system-generators/systemd-fstab-generator</filename></para>
  25   </refsynopsisdiv>
  26
  27   <refsect1>
  28     <title>Description</title>
  29
  30     <para><filename>systemd-fstab-generator</filename> is a generator
  31     that translates <filename>/etc/fstab</filename> (see
  32     <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  33     for details) into native systemd units early at boot and when
  34     configuration of the system manager is reloaded. This will
  35     instantiate mount and swap units as necessary.</para>
  36
  37     <para>The <varname>passno</varname> field is treated like a simple
  38     boolean, and the ordering information is discarded. However, if
  39     the root file system is checked, it is checked before all the
  40     other file systems.</para>
  41
  42     <para>See
  43     <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  44     and
  45     <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  46     for more information about special <filename>/etc/fstab</filename>
  47     mount options this generator understands.</para>
  48
  49     <para>One special topic is handling of symbolic links. Historical init
  50     implementations supported symlinks in <filename>/etc/fstab</filename>.
  51     Because mount units will refuse mounts where the target is a symbolic link,
  52     this generator will resolve any symlinks as far as possible when processing
  53     <filename>/etc/fstab</filename> in order to enhance backwards compatibility.
  54     If a symlink target does not exist at the time that this generator runs, it
  55     is assumed that the symlink target is the final target of the mount.</para>
  56
  57     <para><filename>systemd-fstab-generator</filename> implements
  58     <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  59   </refsect1>
  60
  61   <refsect1>
  62     <title>Kernel Command Line</title>
  63
  64     <para><filename>systemd-fstab-generator</filename> understands the
  65     following kernel command line parameters:</para>
  66
  67     <variablelist class='kernel-commandline-options'>
  68
  69       <varlistentry>
  70         <term><varname>fstab=</varname></term>
  71         <term><varname>rd.fstab=</varname></term>
  72
  73         <listitem><para>Takes a boolean argument. Defaults to <literal>yes</literal>. If
  74         <literal>no</literal>, causes the generator to ignore any mounts or swap devices configured in
  75         <filename>/etc/fstab</filename>. <varname>rd.fstab=</varname> is honored only in the initrd, while
  76         <varname>fstab=</varname> is honored by both the main system and the initrd.</para></listitem>
  77       </varlistentry>
  78
  79       <varlistentry>
  80         <term><varname>root=</varname></term>
  81
  82         <listitem><para>Configures the operating system's root filesystem to mount when running in the
  83         initrd. This accepts a device node path (usually <filename>/dev/disk/by-uuid/…</filename> or
  84         <filename>/dev/disk/by-label/…</filename> or similar), or the special values <literal>gpt-auto</literal>,
  85         <literal>fstab</literal>, and <literal>tmpfs</literal>.</para>
  86
  87         <para>Use <literal>gpt-auto</literal> to explicitly request automatic root file system discovery via
  88         <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
  89
  90         <para>Use <literal>fstab</literal> to explicitly request automatic root file system discovery via
  91         the initrd <filename>/etc/fstab</filename> rather than via kernel command line.</para>
  92
  93         <para>Use <literal>tmpfs</literal> in order to mount a <citerefentry
  94         project='man-pages'><refentrytitle>tmpfs</refentrytitle><manvolnum>5</manvolnum></citerefentry> file
  95         system as root file system of the OS. This is useful in combination with
  96         <varname>mount.usr=</varname> (see below) in order to combine a volatile root file system with a
  97         separate, immutable <filename>/usr/</filename> file system. Also see
  98         <varname>systemd.volatile=</varname> below.</para></listitem>
  99       </varlistentry>
 100
 101       <varlistentry>
 102         <term><varname>rootfstype=</varname></term>
 103
 104         <listitem><para>Takes the root filesystem type that will be
 105         passed to the mount command. <varname>rootfstype=</varname> is
 106         honored by the initrd.</para></listitem>
 107       </varlistentry>
 108
 109       <varlistentry>
 110         <term><varname>rootflags=</varname></term>
 111
 112         <listitem><para>Takes the root filesystem mount options to use. <varname>rootflags=</varname> is
 113         honored by the initrd.</para>
 114
 115         <para>Note that unlike most kernel command line options this setting does not override settings made
 116         in configuration files (specifically: the mount option string in
 117         <filename>/etc/fstab</filename>). See
 118         <citerefentry><refentrytitle>systemd-remount-fs.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para></listitem>
 119       </varlistentry>
 120
 121       <varlistentry>
 122         <term><varname>mount.usr=</varname></term>
 123
 124         <listitem><para>Takes the <filename>/usr/</filename> filesystem
 125         to be mounted by the initrd. If
 126         <varname>mount.usrfstype=</varname> or
 127         <varname>mount.usrflags=</varname> is set, then
 128         <varname>mount.usr=</varname> will default to the value set in
 129         <varname>root=</varname>.</para>
 130
 131         <para>Otherwise, this parameter defaults to the
 132         <filename>/usr/</filename> entry found in
 133         <filename>/etc/fstab</filename> on the root filesystem.</para>
 134
 135         <para><varname>mount.usr=</varname> is honored by the initrd.
 136         </para></listitem>
 137       </varlistentry>
 138
 139       <varlistentry>
 140         <term><varname>mount.usrfstype=</varname></term>
 141
 142         <listitem><para>Takes the <filename>/usr/</filename> filesystem
 143         type that will be passed to the mount command. If
 144         <varname>mount.usr=</varname> or
 145         <varname>mount.usrflags=</varname> is set, then
 146         <varname>mount.usrfstype=</varname> will default to the value
 147         set in <varname>rootfstype=</varname>.</para>
 148
 149         <para>Otherwise, this value will be read from the
 150         <filename>/usr/</filename> entry in
 151         <filename>/etc/fstab</filename> on the root filesystem.</para>
 152
 153         <para><varname>mount.usrfstype=</varname> is honored by the
 154         initrd.</para></listitem>
 155       </varlistentry>
 156
 157       <varlistentry>
 158         <term><varname>mount.usrflags=</varname></term>
 159
 160         <listitem><para>Takes the <filename>/usr/</filename> filesystem
 161         mount options to use. If <varname>mount.usr=</varname> or
 162         <varname>mount.usrfstype=</varname> is set, then
 163         <varname>mount.usrflags=</varname> will default to the value
 164         set in <varname>rootflags=</varname>.</para>
 165
 166         <para>Otherwise, this value will be read from the
 167         <filename>/usr/</filename> entry in
 168         <filename>/etc/fstab</filename> on the root filesystem.</para>
 169
 170         <para><varname>mount.usrflags=</varname> is honored by the
 171         initrd.</para></listitem>
 172       </varlistentry>
 173
 174       <varlistentry>
 175         <term><varname>roothash=</varname></term>
 176         <term><varname>usrhash=</varname></term>
 177
 178         <listitem><para>These options are primarily read by
 179         <citerefentry><refentrytitle>systemd-veritysetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>. When
 180         set this indicates that the root file system (or <filename>/usr/</filename>) shall be mounted from
 181         Verity volumes with the specified hashes. If these kernel command line options are set the root (or
 182         <filename>/usr/</filename>) file system is thus mounted from a device mapper volume
 183         <filename>/dev/mapper/root</filename> (or <filename>/dev/mapper/usr</filename>).</para></listitem>
 184       </varlistentry>
 185
 186       <varlistentry>
 187         <term><varname>systemd.volatile=</varname></term>
 188
 189         <listitem><para>Controls whether the system shall boot up in volatile mode. Takes a boolean argument or the
 190         special value <option>state</option>.</para>
 191
 192         <para>If false (the default), this generator makes no changes to the mount tree and the system is booted up in
 193         normal mode.</para>
 194
 195         <para>If true the generator ensures
 196         <citerefentry><refentrytitle>systemd-volatile-root.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 197         is run in the initrd. This service changes the mount table before transitioning to the host system,
 198         so that a volatile memory file system (<literal>tmpfs</literal>) is used as root directory, with only
 199         <filename>/usr/</filename> mounted into it from the configured root file system, in read-only mode.
 200         This way the system operates in fully stateless mode, with all configuration and state reset at boot
 201         and lost at shutdown, as <filename>/etc/</filename> and <filename>/var/</filename> will be served
 202         from the (initially unpopulated) volatile memory file system.</para>
 203
 204         <para>If set to <option>state</option> the generator will leave the root directory mount point unaltered,
 205         however will mount a <literal>tmpfs</literal> file system to <filename>/var/</filename>. In this mode the normal
 206         system configuration (i.e. the contents of <literal>/etc/</literal>) is in effect (and may be modified during
 207         system runtime), however the system state (i.e. the contents of <literal>/var/</literal>) is reset at boot and
 208         lost at shutdown.</para>
 209
 210         <para>If this setting is set to <literal>overlay</literal> the root file system is set up as
 211         <literal>overlayfs</literal> mount combining the read-only root directory with a writable
 212         <literal>tmpfs</literal>, so that no modifications are made to disk, but the file system may be modified
 213         nonetheless with all changes being lost at reboot.</para>
 214
 215         <para>Note that in none of these modes the root directory, <filename>/etc/</filename>, <filename>/var/</filename>
 216         or any other resources stored in the root file system are physically removed. It's thus safe to boot a system
 217         that is normally operated in non-volatile mode temporarily into volatile mode, without losing data.</para>
 218
 219         <para>Note that with the exception of <literal>overlay</literal> mode, enabling this setting will
 220         only work correctly on operating systems that can boot up with only <filename>/usr/</filename>
 221         mounted, and are able to automatically populate <filename>/etc/</filename>, and also
 222         <filename>/var/</filename> in case of <literal>systemd.volatile=yes</literal>.</para>
 223
 224         <para>Also see <varname>root=tmpfs</varname> above, for a method to combine a
 225         <literal>tmpfs</literal> file system with a regular <filename>/usr/</filename> file system (as
 226         configured via <varname>mount.usr=</varname>). The main distinction between
 227         <varname>systemd.volatile=yes</varname>, and <varname>root=tmpfs</varname> in combination
 228         <varname>mount.usr=</varname> is that the former operates on top of a regular root file system and
 229         temporarily obstructs the files and directories above its <filename>/usr/</filename> subdirectory,
 230         while the latter does not hide any files, but simply mounts a unpopulated tmpfs as root file system
 231         and combines it with a user picked <filename>/usr/</filename> file system.</para></listitem>
 232       </varlistentry>
 233
 234       <varlistentry>
 235         <term><varname>systemd.swap</varname></term>
 236
 237         <listitem><para>Takes a boolean argument or enables the option if specified
 238         without an argument. If disabled, causes the generator to ignore
 239         any swap devices configured in <filename>/etc/fstab</filename>.
 240         Defaults to enabled.</para></listitem>
 241       </varlistentry>
 242     </variablelist>
 243   </refsect1>
 244
 245   <refsect1>
 246     <title>See Also</title>
 247     <para>
 248       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 249       <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 250       <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 251       <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 252       <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 253       <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 254       <citerefentry><refentrytitle>kernel-command-line</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 255       <ulink url="https://systemd.io/ENVIRONMENT/">Known Environment Variables</ulink>
 256     </para>
 257   </refsect1>
 258 </refentry>