SYSTEMD-GPT-AUTO-GENERATOR(8)

NAME
systemd-gpt-auto-generator - Generator for automatically discovering and mounting root, /home/, /srv/, /var/ and /var/tmp/ partitions, as well as discovering and enabling swap partitions, based on GPT partition type GUIDs

SYNOPSIS

/usr/lib/systemd/system-generators/systemd-gpt-auto-generator

DESCRIPTION

systemd-gpt-auto-generator is a unit generator that automatically discovers root, /home/, /srv/, /var/, /var/tmp/, 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 UEFI Specification[1], chapter 5. It implements the Discoverable Partitions Specification[2]. 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 fstab(5)), the units this generator creates are overridden, but additional implicit dependencies might be created.

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 LoaderDevicePartUUID of the 4a67b082-0a4c-41cf-b6c7-440b29bb8c4f 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 Boot Loader Interface[3] for details.

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.

systemd-gpt-auto-generator is useful for centralizing file system configuration in the partition table and making configuration in /etc/fstab or on the kernel command line unnecessary.

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

Table 1. Partition Type GUIDs
Partition Type GUID | Name | Mount Point | Explanation
SD_GPT_ROOT_X86_64 4f68bce3-e8cd-4db1-96e7-fbcaf984b709 | Root Partition (x86-64) | / | The first partition with this type UUID, located on the same disk as the ESP, is used as the root file system / on AMD64 / 64-bit x86 systems.
SD_GPT_ROOT_ARM64 b921b045-1df0-41c3-af44-4c6f280d3fae | Root Partition (64-bit ARM) | / | The first partition with this type UUID, located on the same disk as the ESP, is used as the root file system / on AArch64 / 64-bit ARM systems.
SD_GPT_ROOT_ALPHA SD_GPT_ROOT_ARC SD_GPT_ROOT_ARM SD_GPT_ROOT_ARM64 SD_GPT_ROOT_IA64 SD_GPT_ROOT_LOONGARCH64 SD_GPT_ROOT_MIPS_LE SD_GPT_ROOT_MIPS64_LE SD_GPT_ROOT_PARISC SD_GPT_ROOT_PPC SD_GPT_ROOT_PPC64 SD_GPT_ROOT_PPC64_LE SD_GPT_ROOT_RISCV32 SD_GPT_ROOT_RISCV64 SD_GPT_ROOT_S390 SD_GPT_ROOT_S390X SD_GPT_ROOT_TILEGX SD_GPT_ROOT_X86 SD_GPT_ROOT_X86_64 SD_GPT_USR_ALPHA SD_GPT_USR_ARC SD_GPT_USR_ARM SD_GPT_USR_IA64 SD_GPT_USR_LOONGARCH64 SD_GPT_USR_MIPS_LE SD_GPT_USR_MIPS64_LE SD_GPT_USR_PARISC SD_GPT_USR_PPC SD_GPT_USR_PPC64 SD_GPT_USR_PPC64_LE SD_GPT_USR_RISCV32 SD_GPT_USR_RISCV64 SD_GPT_USR_S390 SD_GPT_USR_S390X SD_GPT_USR_TILEGX SD_GPT_USR_X86 | root partitions for other architectures | / | 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 /. For the full list and constant values, see Discoverable Partitions Specification[2].
SD_GPT_HOME 933ac7e1-2eb4-4f13-b844-0e14e2aef915 | Home Partition | /home/ | The first partition with this type UUID on the same disk as the ESP is mounted to /home/.
SD_GPT_SRV 3b8f8425-20e0-4f3b-907f-1a25a76f98e8 | Server Data Partition | /srv/ | The first partition with this type UUID on the same disk as the ESP is mounted to /srv/.
SD_GPT_VAR 4d21b016-b534-45c2-a9fb-5c16e091fd2d | Variable Data Partition | /var/ | The first partition with this type UUID on the same disk as the ESP is mounted to /var/ — 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 machine-id(5).
SD_GPT_TMP 7ec6f557-3bc5-4aca-b293-16ef5df639d1 | Temporary Data Partition | /var/tmp/ | The first partition with this type UUID on the same disk as the ESP is mounted to /var/tmp/.
SD_GPT_SWAP 0657fd6d-a4ab-43c4-84e5-0933c84b4f4f | Swap | n/a | All partitions with this type UUID on the same disk as the ESP are used as swap.
SD_GPT_ESP c12a7328-f81f-11d2-ba4b-00a0c93ec93b | EFI System Partition (ESP) | /efi/ or /boot/ | The first partition with this type UUID located on the same disk as the root partition is mounted to /boot/ or /efi/, see below.
SD_GPT_XBOOTLDR bc13c2ff-59e6-4262-a352-b275fd6f7172 | Extended Boot Loader Partition | /boot/ | The first partition with this type UUID located on the same disk as the root partition is mounted to /boot/, see below.

This generator understands the following attribute flags for partitions:

Table 2. Partition Attribute Flags
Flag | Applicable to | Explanation
SD_GPT_FLAG_READ_ONLY 0x1000000000000000 | /, /home/, /srv/, /var/, /var/tmp/, Extended Boot Loader Partition | Partition is mounted read-only
SD_GPT_FLAG_NO_AUTO 0x8000000000000000 | /, /home/, /srv/, /var/, /var/tmp/, Extended Boot Loader Partition | Partition is not mounted automatically
SD_GPT_FLAG_NO_BLOCK_IO_PROTOCOL 0x0000000000000002 | EFI System Partition (ESP) | Partition is not mounted automatically

The /home/, /srv/, /var/ and /var/tmp/ partitions may be encrypted in LUKS format. In this case, a device mapper device is set up under the names /dev/mapper/home, /dev/mapper/srv, /dev/mapper/var and /dev/mapper/tmp. Note that this might create conflicts if the same partition is listed in /etc/crypttab with a different device mapper device name.

When systemd is running in the initrd the / partition may be encrypted in LUKS format as well. In this case, a device mapper device is set up under the name /dev/mapper/root, and a sysroot.mount is set up that mounts the device under /sysroot. For more information, see bootup(7).

The root partition can be specified by symlinking /run/systemd/volatile-root to /dev/block/$major:$minor. This is especially useful if the root mount has been replaced by some form of volatile file system (overlayfs).

Mount and automount units for the EFI System Partition (ESP) are generated on EFI systems. The ESP is mounted to /boot/ (except if an Extended Boot Loader partition exists, see below), unless a mount point directory /efi/ 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 /boot/ (or /efi/ if it exists) is an explicitly configured mount (for example, listed in fstab(5)) or where the /boot/ (or /efi/) mount point is non-empty, no mount units are generated.

If the disk contains an Extended Boot Loader partition, as defined in the Boot Loader Specification[4], it is made available at /boot/ (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 /boot/. Make sure to create both /efi/ and /boot/ to ensure both partitions are mounted.

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

systemd-gpt-auto-generator implements systemd.generator(7).

KERNEL COMMAND LINE

systemd-gpt-auto-generator understands the following kernel command line parameters:

systemd.gpt_auto, rd.systemd.gpt_auto
    Those options take an optional boolean argument, and default to yes. The generator is enabled by default, and a false value may be used to disable it (e.g. "systemd.gpt_auto=0").

root=
    When used with the special value "gpt-auto", automatic discovery of the root partition based on the GPT partition type is enabled. Any other value disables this generator.

rw, ro
    Mount the root partition read-write or read-only initially.

    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 systemd-remount-fs.service(8).

SEE ALSO

systemd(1), systemd.mount(5), systemd.swap(5), systemd-fstab-generator(8), systemd-cryptsetup@.service(8), machine-id(5), cryptsetup(8), fstab(5), btrfs(8)

NOTES

1.  UEFI Specification
    https://uefi.org/specifications

2.  Discoverable Partitions Specification
    https://systemd.io/DISCOVERABLE_PARTITIONS

3.  Boot Loader Interface
    https://systemd.io/BOOT_LOADER_INTERFACE

4.  Boot Loader Specification
    https://systemd.io/BOOT_LOADER_SPECIFICATION