systemd-stable/man/systemd-gpt-auto-generator.xml

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<refentry id="systemd-gpt-auto-generator" conditional='HAVE_BLKID'>

  <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>,
    <filename>/srv/</filename>, <filename>/var/</filename> and <filename>/var/tmp/</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>, <filename>/var/</filename>,
    <filename>/var/tmp/</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://uapi-group.org/specifications/specs/discoverable_partitions_specification">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 the root partition on the same physical disk where the EFI System
    Partition (ESP) is located. Note that support from the boot loader is required: the EFI variable
    <varname>LoaderDevicePartUUID</varname> of the <constant>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</constant>
    vendor UUID is used to determine from which partition, and hence the disk from which the system was
    booted. If the boot loader does not set this variable, this generator will not be able to autodetect the
    root partition. See the <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader
    Interface</ulink> for details.</para>

    <para>Similarly, this generator will only look for the other partitions on the same physical disk as the
    root partition. In this case, boot loader support is not required. 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> or on
    the kernel command line 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><constant>SD_GPT_ROOT_X86_64</constant> <constant>4f68bce3-e8cd-4db1-96e7-fbcaf984b709</constant></entry>
            <entry><filename>Root Partition (x86-64)</filename></entry>
            <entry><filename>/</filename></entry>
            <entry>The first partition with this type UUID, located on the same disk as the ESP, is used as the root file system <filename>/</filename> on AMD64 / 64-bit x86 systems.</entry>
          </row>
          <row>
            <entry><constant>SD_GPT_ROOT_ARM64</constant> <constant>b921b045-1df0-41c3-af44-4c6f280d3fae</constant></entry>
            <entry><filename>Root Partition (64-bit ARM)</filename></entry>
            <entry><filename>/</filename></entry>
            <entry>The first partition with this type UUID, located on the same disk as the ESP, is used as the root file system <filename>/</filename> on AArch64 / 64-bit ARM systems.</entry>
          </row>
          <row>
            <entry>
              <constant>SD_GPT_ROOT_ALPHA</constant> <constant>SD_GPT_ROOT_ARC</constant> <constant>SD_GPT_ROOT_ARM</constant> <constant>SD_GPT_ROOT_ARM64</constant> <constant>SD_GPT_ROOT_IA64</constant> <constant>SD_GPT_ROOT_LOONGARCH64</constant> <constant>SD_GPT_ROOT_MIPS_LE</constant> <constant>SD_GPT_ROOT_MIPS64_LE</constant> <constant>SD_GPT_ROOT_PARISC</constant> <constant>SD_GPT_ROOT_PPC</constant> <constant>SD_GPT_ROOT_PPC64</constant> <constant>SD_GPT_ROOT_PPC64_LE</constant> <constant>SD_GPT_ROOT_RISCV32</constant> <constant>SD_GPT_ROOT_RISCV64</constant> <constant>SD_GPT_ROOT_S390</constant> <constant>SD_GPT_ROOT_S390X</constant> <constant>SD_GPT_ROOT_TILEGX</constant> <constant>SD_GPT_ROOT_X86</constant> <constant>SD_GPT_ROOT_X86_64</constant> <constant>SD_GPT_USR_ALPHA</constant> <constant>SD_GPT_USR_ARC</constant> <constant>SD_GPT_USR_ARM</constant> <constant>SD_GPT_USR_IA64</constant> <constant>SD_GPT_USR_LOONGARCH64</constant> <constant>SD_GPT_USR_MIPS_LE</constant> <constant>SD_GPT_USR_MIPS64_LE</constant> <constant>SD_GPT_USR_PARISC</constant> <constant>SD_GPT_USR_PPC</constant> <constant>SD_GPT_USR_PPC64</constant> <constant>SD_GPT_USR_PPC64_LE</constant> <constant>SD_GPT_USR_RISCV32</constant> <constant>SD_GPT_USR_RISCV64</constant> <constant>SD_GPT_USR_S390</constant> <constant>SD_GPT_USR_S390X</constant> <constant>SD_GPT_USR_TILEGX</constant> <constant>SD_GPT_USR_X86</constant>
            </entry>
            <entry>root partitions for other architectures</entry>
            <entry><filename>/</filename></entry>
            <entry>The first partition with the type UUID matching the architecture, located on the same disk as the ESP, is used as the root file system <filename>/</filename>. For the full list and constant values, see <ulink url="https://uapi-group.org/specifications/specs/discoverable_partitions_specification">Discoverable Partitions Specification</ulink>.</entry>
          </row>
          <row>
            <entry><constant>SD_GPT_HOME</constant> <constant>933ac7e1-2eb4-4f13-b844-0e14e2aef915</constant></entry>
            <entry>Home Partition</entry>
            <entry><filename>/home/</filename></entry>
            <entry>The first partition with this type UUID on the same disk as the ESP is mounted to <filename>/home/</filename>.</entry>
          </row>
          <row>
            <entry><constant>SD_GPT_SRV</constant> <constant>3b8f8425-20e0-4f3b-907f-1a25a76f98e8</constant></entry>
            <entry>Server Data Partition</entry>
            <entry><filename>/srv/</filename></entry>
            <entry>The first partition with this type UUID on the same disk as the ESP is mounted to <filename>/srv/</filename>.</entry>
          </row>
          <row>
            <entry><constant>SD_GPT_VAR</constant> <constant>4d21b016-b534-45c2-a9fb-5c16e091fd2d</constant></entry>
            <entry>Variable Data Partition</entry>
            <entry><filename>/var/</filename></entry>
            <entry>The first partition with this type UUID on the same disk as the ESP is mounted to <filename>/var/</filename> — under the condition its partition UUID matches the first 128 bit of the HMAC-SHA256 of the GPT type uuid of this partition keyed by the machine ID of the installation stored in <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</entry>
          </row>
          <row>
            <entry><constant>SD_GPT_TMP</constant> <constant>7ec6f557-3bc5-4aca-b293-16ef5df639d1</constant></entry>
            <entry>Temporary Data Partition</entry>
            <entry><filename>/var/tmp/</filename></entry>
            <entry>The first partition with this type UUID on the same disk as the ESP is mounted to <filename>/var/tmp/</filename>.</entry>
          </row>
          <row>
            <entry><constant>SD_GPT_SWAP</constant> <constant>0657fd6d-a4ab-43c4-84e5-0933c84b4f4f</constant></entry>
            <entry>Swap</entry>
            <entry>n/a</entry>
            <entry>All partitions with this type UUID on the same disk as the ESP are used as swap.</entry>
          </row>
          <row>
            <entry><constant>SD_GPT_ESP</constant> <constant>c12a7328-f81f-11d2-ba4b-00a0c93ec93b</constant></entry>
            <entry>EFI System Partition (ESP)</entry>
            <entry><filename>/efi/</filename> or <filename>/boot/</filename></entry>
            <entry>The first partition with this type UUID located on the same disk as the root partition is mounted to <filename>/boot/</filename> or <filename>/efi/</filename>, see below.</entry>
          </row>
          <row>
            <entry><constant>SD_GPT_XBOOTLDR</constant> <constant>bc13c2ff-59e6-4262-a352-b275fd6f7172</constant></entry>
            <entry>Extended Boot Loader Partition</entry>
            <entry><filename>/boot/</filename></entry>
            <entry>The first partition with this type UUID located on the same disk as the root 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 Attribute Flags</title>
      <tgroup cols='3' align='left' colsep='1' rowsep='1'>
        <colspec colname="flag" />
        <colspec colname="where" />
        <colspec colname="explanation" />
        <thead>
          <row>
            <entry>Flag</entry>
            <entry>Applicable to</entry>
            <entry>Explanation</entry>
          </row>
        </thead>
        <tbody>
          <row>
            <entry><constant>SD_GPT_FLAG_READ_ONLY</constant> <constant>0x1000000000000000</constant></entry>
            <entry><filename>/</filename>, <filename>/home/</filename>, <filename>/srv/</filename>, <filename>/var/</filename>, <filename>/var/tmp/</filename>, Extended Boot Loader Partition</entry>
            <entry>Partition is mounted read-only</entry>
          </row>

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

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

    <para>The <filename>/home/</filename>, <filename>/srv/</filename>, <filename>/var/</filename> and
    <filename>/var/tmp/</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>,
    <filename>/dev/mapper/srv</filename>, <filename>/dev/mapper/var</filename> and
    <filename>/dev/mapper/tmp</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>The root partition can be specified by symlinking <filename>/run/systemd/volatile-root</filename>
    to <filename>/dev/block/$major:$minor</filename>. This is especially useful if the root mount has been
    replaced by some form of volatile file system (overlayfs).
    </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://uapi-group.org/specifications/specs/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><refentrytitle>machine-id</refentrytitle><manvolnum>5</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>