man/systemd-gpt-auto-generator.xml

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<refentry id="systemd-gpt-auto-generator">

  <refentryinfo>
    <title>systemd-gpt-auto-generator</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-gpt-auto-generator</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-gpt-auto-generator</refname>
    <refpurpose>Generator for automatically discovering
    and mounting root, <filename>/home</filename> and
    <filename>/srv</filename> partitions, as well as
    discovering and enabling swap partitions, based on GPT
    partition type GUIDs.</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>/usr/lib/systemd/system-generators/systemd-gpt-auto-generator</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><filename>systemd-gpt-auto-generator</filename> is a unit generator that automatically discovers
    root, <filename>/home/</filename>, <filename>/srv/</filename>, the EFI System Partition, the Extended
    Boot Loader Partition and swap partitions and creates mount and swap units for them, based on the
    partition type GUIDs of GUID partition tables (GPT), see <ulink
    url="https://uefi.org/specifications">UEFI Specification</ulink>, chapter 5.  It implements the <ulink
    url="https://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/">Discoverable Partitions
    Specification</ulink>. Note that this generator has no effect on non-GPT systems, and on specific mount
    points that are directories already containing files. Also, on systems where the units are explicitly
    configured (for example, listed in <citerefentry
    project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>), the
    units this generator creates are overridden, but additional implicit dependencies might be
    created.</para>

    <para>This generator will only look for root partitions on the
    same physical disk the EFI System Partition (ESP) is located on.
    It will only look for the other partitions on the same physical
    disk the root file system is located on. These partitions will not
    be searched for on systems where the root file system is distributed
    on multiple disks, for example via btrfs RAID.</para>

    <para><filename>systemd-gpt-auto-generator</filename> is useful
    for centralizing file system configuration in the partition table
    and making configuration in <filename>/etc/fstab</filename> unnecessary.
    </para>

    <para>This generator looks for the partitions based on their
    partition type GUID. The following partition type GUIDs are
    identified:</para>

    <table>
      <title>Partition Type GUIDs</title>
      <tgroup cols='3' align='left' colsep='1' rowsep='1'>
        <colspec colname="guid" />
        <colspec colname="name" />
        <colspec colname="where" />
        <colspec colname="explanation" />
        <thead>
          <row>
            <entry>Partition Type GUID</entry>
            <entry>Name</entry>
            <entry>Mount Point</entry>
            <entry>Explanation</entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry>44479540-f297-41b2-9af7-d131d5f0458a</entry>
            <entry><filename>Root Partition (x86)</filename></entry>
            <entry><filename>/</filename></entry>
            <entry>On 32-bit x86 systems, the first x86 root partition on the disk the EFI ESP is located on is mounted to the root directory <filename>/</filename>.</entry>
          </row>
          <row>
            <entry>4f68bce3-e8cd-4db1-96e7-fbcaf984b709</entry>
            <entry><filename>Root Partition (x86-64)</filename></entry>
            <entry><filename>/</filename></entry>
            <entry>On 64-bit x86 systems, the first x86-64 root partition on the disk the EFI ESP is located on is mounted to the root directory <filename>/</filename>.</entry>
          </row>
          <row>
            <entry>69dad710-2ce4-4e3c-b16c-21a1d49abed3</entry>
            <entry><filename>Root Partition (32-bit ARM)</filename></entry>
            <entry><filename>/</filename></entry>
            <entry>On 32-bit ARM systems, the first ARM root partition on the disk the EFI ESP is located on is mounted to the root directory <filename>/</filename>.</entry>
          </row>
          <row>
            <entry>b921b045-1df0-41c3-af44-4c6f280d3fae</entry>
            <entry><filename>Root Partition (64-bit ARM)</filename></entry>
            <entry><filename>/</filename></entry>
            <entry>On 64-bit ARM systems, the first ARM root partition on the disk the EFI ESP is located on is mounted to the root directory <filename>/</filename>.</entry>
          </row>
          <row>
            <entry>993d8d3d-f80e-4225-855a-9daf8ed7ea97</entry>
            <entry><filename>Root Partition (Itanium/IA-64)</filename></entry>
            <entry><filename>/</filename></entry>
            <entry>On Itanium systems, the first Itanium root partition on the disk the EFI ESP is located on is mounted to the root directory <filename>/</filename>.</entry>
          </row>
          <row>
            <entry>933ac7e1-2eb4-4f13-b844-0e14e2aef915</entry>
            <entry>Home Partition</entry>
            <entry><filename>/home/</filename></entry>
            <entry>The first home partition on the disk the root partition is located on is mounted to <filename>/home</filename>.</entry>
          </row>
          <row>
            <entry>3b8f8425-20e0-4f3b-907f-1a25a76f98e8</entry>
            <entry>Server Data Partition</entry>
            <entry><filename>/srv/</filename></entry>
            <entry>The first server data partition on the disk the root partition is located on is mounted to <filename>/srv</filename>.</entry>
          </row>
          <row>
            <entry>0657fd6d-a4ab-43c4-84e5-0933c84b4f4f</entry>
            <entry>Swap</entry>
            <entry>n/a</entry>
            <entry>All swap partitions located on the disk the root partition is located on are enabled.</entry>
          </row>
          <row>
            <entry>c12a7328-f81f-11d2-ba4b-00a0c93ec93b</entry>
            <entry>EFI System Partition (ESP)</entry>
            <entry><filename>/efi/</filename> or <filename>/boot/</filename></entry>
            <entry>The first ESP located on the disk the root partition is located on is mounted to <filename>/boot</filename> or <filename>/efi</filename>, see below.</entry>
          </row>
          <row>
            <entry>bc13c2ff-59e6-4262-a352-b275fd6f7172</entry>
            <entry>Extended Boot Loader Partition</entry>
            <entry><filename>/boot/</filename></entry>
            <entry>The first Extended Boot Loader Partition is mounted to <filename>/boot</filename>, see below.</entry>
          </row>
        </tbody>
      </tgroup>
    </table>

    <para>This generator understands the following attribute flags for partitions:</para>

    <table>
      <title>Partition Attributes</title>
      <tgroup cols='4' align='left' colsep='1' rowsep='1'>
        <colspec colname="attribute" />
        <colspec colname="value" />
        <colspec colname="where" />
        <colspec colname="explanation" />
        <thead>
          <row>
            <entry>Name</entry>
            <entry>Value</entry>
            <entry>Applicable to</entry>
            <entry>Explanation</entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry><constant>GPT_FLAG_READ_ONLY</constant></entry>
            <entry>0x1000000000000000</entry>
            <entry><filename>/</filename>, <filename>/home/</filename>, <filename>/srv/</filename>, Extended Boot Loader Partition</entry>
            <entry>Partition is mounted read-only</entry>
          </row>

          <row>
            <entry><constant>GPT_FLAG_NO_AUTO</constant></entry>
            <entry>0x8000000000000000</entry>
            <entry><filename>/</filename>, <filename>/home/</filename>, <filename>/srv/</filename>, Extended Boot Loader Partition</entry>
            <entry>Partition is not mounted automatically</entry>
          </row>

          <row>
            <entry><constant>GPT_FLAG_NO_BLOCK_IO_PROTOCOL</constant></entry>
            <entry>0x0000000000000002</entry>
            <entry>EFI System Partition (ESP)</entry>
            <entry>Partition is not mounted automatically</entry>
          </row>
        </tbody>
      </tgroup>
    </table>

    <para>The <filename>/home/</filename> and <filename>/srv/</filename> partitions may be encrypted in LUKS
    format. In this case, a device mapper device is set up under the names
    <filename>/dev/mapper/home</filename> and <filename>/dev/mapper/srv</filename>. Note that this might
    create conflicts if the same partition is listed in <filename>/etc/crypttab</filename> with a different
    device mapper device name.</para>
    
    <para>When systemd is running in the initrd the <filename>/</filename> partition may be encrypted in LUKS
      format as well. In this case, a device mapper device is set up under the name <filename>/dev/mapper/root</filename>,
      and a <filename>sysroot.mount</filename> is set up that mounts the device under <filename>/sysroot</filename>.
      For more information, see <citerefentry><refentrytitle>bootup</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
    </para>

    <para>Mount and automount units for the EFI System Partition (ESP) are generated on EFI systems. The ESP
    is mounted to <filename>/boot/</filename> (except if an Extended Boot Loader partition exists, see
    below), unless a mount point directory <filename>/efi/</filename> exists, in which case it is mounted
    there. Since this generator creates an automount unit, the mount will only be activated on-demand, when
    accessed. On systems where <filename>/boot/</filename> (or <filename>/efi/</filename> if it exists) is an
    explicitly configured mount (for example, listed in <citerefentry
    project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>) or where
    the <filename>/boot/</filename> (or <filename>/efi/</filename>) mount point is non-empty, no mount units
    are generated.</para>

    <para>If the disk contains an Extended Boot Loader partition, as defined in the <ulink
    url="https://systemd.io/BOOT_LOADER_SPECIFICATION">Boot Loader Specification</ulink>, it is made
    available at <filename>/boot</filename> (by means of an automount point, similar to the ESP, see
    above). If both an EFI System Partition and an Extended Boot Loader partition exist the latter is
    preferably mounted to <filename>/boot/</filename>. Make sure to create both <filename>/efi/</filename>
    and <filename>/boot/</filename> to ensure both partitions are mounted.</para>

    <para>When using this generator in conjunction with btrfs file
    systems, make sure to set the correct default subvolumes on them,
    using <command>btrfs subvolume set-default</command>.</para>

    <para><filename>systemd-gpt-auto-generator</filename> implements
    <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
  </refsect1>

  <refsect1>
    <title>Kernel Command Line</title>

    <para><filename>systemd-gpt-auto-generator</filename> understands the following kernel command line
    parameters:</para>

    <variablelist class='kernel-commandline-options'>

      <varlistentry>
        <term><varname>systemd.gpt_auto</varname></term>
        <term><varname>rd.systemd.gpt_auto</varname></term>

        <listitem><para>Those options take an optional boolean argument, and default to yes.
        The generator is enabled by default, and a negative value may be used to disable it.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>root=</varname></term>

        <listitem><para>When used with the special value <literal>gpt-auto</literal>, automatic discovery of
        the root partition based on the GPT partition type is enabled.  Any other value disables this
        generator.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>rw</varname></term>
        <term><varname>ro</varname></term>

        <listitem><para>Mount the root partition read-write or read-only <emphasis>initially</emphasis>.</para>

        <para>Note that unlike most kernel command line options these settings do not override configuration
        in the file system, and the file system may be remounted later. See
        <citerefentry><refentrytitle>systemd-remount-fs.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
        </para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='die-net'><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>btrfs</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>