Manual: systemd-gpt-auto-generator
SYSTEMD-GPT-AUTO-GENERATOR(8) systemd-gpt-auto-generator 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 the root partition, /home/, /srv/, /var/,
/var/tmp/, the EFI System Partition (ESP), the Extended Boot Loader Partition (XBOOTLDR), 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 for more details. It implements the UAPI.2 Discoverable Partitions Specification[2].
Note that this generator has no effect on non-GPT systems. It will also not create mount point configuration for
directories which already contain files or if the mount point is explicitly configured in fstab(5). Additionally no
unit will be created for the ESP or the XBOOTLDR partition if mount entries are found in the /boot/ or /efi/
hierarchies in fstab(5).
If the units this generator creates are overridden, for example by units in directories with higher precedence,
drop-ins and additional dependencies created by this generator might still be used.
When run in the initial RAM disk (initrd) this generator can automatically search for the root file system.
Specifically:
• It will 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 for this to work: the EFI variable LoaderDevicePartUUID of the
4a67b082-0a4c-41cf-b6c7-440b29bb8c4f vendor UUID is used to determine from which partition (and hence disk) the
system was booted. If the boot loader does not set this variable, this generator will not be able to detect the
root partition. See the Boot Loader Interface[3] for details.
• Alternatively, it will look for the root file system on a loopback block device whose ".lo_name" field is set to
one of the literal strings "rootdisk" or "rootdisk.raw". This field can be set via losetup's --loop-ref= string.
For images downloaded via systemd-import-generator(8) make sure to set the "blockdev" option and set the local
name string to "rootdisk" to achieve this effect. Note that discovery of the root file system on loopback block
devices like this is only done if "root=gpt-auto" or "root=dissect" is specified explicitly on the kernel command
line, unlike the discovery based on the boot loader reported ESP which is also enabled if no "root=" parameter is
specified at all. (The latter relies on systemd-udevd.service's /dev/gpt-auto-root block device symlink
generation).
When run on the host system (i.e. after successfully transitioning out of the initrd into the root filesystem) this
generator will look for all other partitions on the same physical disk as the root partition. For this discovery,
boot loader support is not required. Moreover, it is not required that the root partition was automatically
discovered by the initrd (as described above) for the discovery of the non-root file partitions to take place. Or in
other words: automatic discovery of the root file system and of the non-root file systems are independent operations,
that do not rely on each other, and are done during two distinct phases of the boot process (one in the initrd, the
other after). These partitions will not be searched for on systems where the root file system is distributed on
multiple disks, for example via btrfs RAID.
The root partition can be configured explicitly 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).
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 │ / │ The first partition │
│ │ │ (x86-64) │ │ with this type UUID, │
│ │ │ │ │ located on the same │
│ │ │ │ │ disk as the ESP used │
│ │ │ │ │ for booting, is used │
│ │ │ │ │ as the root file │
│ │ │ │ │ system / on AMD64 / │
│ │ │ │ │ 64-bit x86 systems. │
├─────────────────────────┼──────────────────────────────────────┼───────────────────────┼─────────────────┼───────────────────────┤
│ SD_GPT_ROOT_ARM64 │ b921b045-1df0-41c3-af44-4c6f280d3fae │ Root Partition │ / │ The first partition │
│ │ │ (64-bit ARM) │ │ with this type UUID, │
│ │ │ │ │ located on the same │
│ │ │ │ │ disk as the ESP used │
│ │ │ │ │ for booting, is used │
│ │ │ │ │ as the root file │
│ │ │ │ │ system / on AArch64 / │
│ │ │ │ │ 64-bit ARM systems. │
├─────────────────────────┼──────────────────────────────────────┼───────────────────────┼─────────────────┼───────────────────────┤
│ SD_GPT_ROOT_ALPHA │ ... │ Root partitions for │ / │ The first partition │
│ SD_GPT_ROOT_ARC │ │ other architectures │ │ with the type UUID │
│ SD_GPT_ROOT_ARM │ │ │ │ matching the │
│ SD_GPT_ROOT_ARM64 │ │ │ │ architecture, located │
│ SD_GPT_ROOT_IA64 │ │ │ │ on the same disk as │
│ SD_GPT_ROOT_LOONGARCH64 │ │ │ │ the ESP used for │
│ SD_GPT_ROOT_MIPS │ │ │ │ booting, is used as │
│ SD_GPT_ROOT_MIPS64 │ │ │ │ the root file system │
│ SD_GPT_ROOT_MIPS_LE │ │ │ │ /. For the full list │
│ SD_GPT_ROOT_MIPS64_LE │ │ │ │ and constant values, │
│ SD_GPT_ROOT_PARISC │ │ │ │ see UAPI.2 │
│ SD_GPT_ROOT_PPC │ │ │ │ Discoverable │
│ SD_GPT_ROOT_PPC64 │ │ │ │ Partitions │
│ SD_GPT_ROOT_PPC64_LE │ │ │ │ Specification[2]. │
│ 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 │ │ │ │ │
├─────────────────────────┼──────────────────────────────────────┼───────────────────────┼─────────────────┼───────────────────────┤
│ SD_GPT_HOME │ 933ac7e1-2eb4-4f13-b844-0e14e2aef915 │ Home Partition │ /home/ │ The first partition │
│ │ │ │ │ with this type UUID │
│ │ │ │ │ on the same disk as │
│ │ │ │ │ the root partition 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 root partition is │
│ │ │ │ │ mounted to /srv/. │
├─────────────────────────┼──────────────────────────────────────┼───────────────────────┼─────────────────┼───────────────────────┤
│ SD_GPT_VAR │ 4d21b016-b534-45c2-a9fb-5c16e091fd2d │ Variable Data │ /var/ │ The first partition │
│ │ │ Partition │ │ with this type UUID │
│ │ │ │ │ on the same disk as │
│ │ │ │ │ the root partition 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). │
│ │ │ │ │ This can be generated │
│ │ │ │ │ using systemd- │
│ │ │ │ │ id128(1). │
├─────────────────────────┼──────────────────────────────────────┼───────────────────────┼─────────────────┼───────────────────────┤
│ SD_GPT_TMP │ 7ec6f557-3bc5-4aca-b293-16ef5df639d1 │ Temporary Data │ /var/tmp/ │ The first partition │
│ │ │ Partition │ │ with this type UUID │
│ │ │ │ │ on the same disk as │
│ │ │ │ │ the root partition 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 root │
│ │ │ │ │ partition are used as │
│ │ │ │ │ swap. │
├─────────────────────────┼──────────────────────────────────────┼───────────────────────┼─────────────────┼───────────────────────┤
│ SD_GPT_ESP │ c12a7328-f81f-11d2-ba4b-00a0c93ec93b │ EFI System Partition │ /efi/ or /boot/ │ The first partition │
│ │ │ (ESP) │ │ 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 │ /boot/ │ The first partition │
│ │ │ 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 │ Value │ Applicable to │ Explanation │
├──────────────────────────────────┼────────────────────┼──────────────────────────┼──────────────────────────┤
│ SD_GPT_FLAG_READ_ONLY │ 0x1000000000000000 │ /, /home/, /srv/, /var/, │ Partition is mounted │
│ │ │ /var/tmp/, Extended Boot │ read-only │
│ │ │ Loader Partition │ │
├──────────────────────────────────┼────────────────────┼──────────────────────────┼──────────────────────────┤
│ SD_GPT_FLAG_NO_AUTO │ 0x8000000000000000 │ /, /home/, /srv/, /var/, │ Partition is not mounted │
│ │ │ /var/tmp/, Extended Boot │ automatically │
│ │ │ Loader Partition │ │
├──────────────────────────────────┼────────────────────┼──────────────────────────┼──────────────────────────┤
│ SD_GPT_FLAG_NO_BLOCK_IO_PROTOCOL │ 0x0000000000000002 │ EFI System Partition │ Partition is not mounted │
│ │ │ (ESP) │ automatically │
└──────────────────────────────────┴────────────────────┴──────────────────────────┴──────────────────────────┘
The /home/, /srv/, /var/, /var/tmp/ and swap 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, /dev/mapper/tmp or
/dev/mapper/swap. 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 with LUKS 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).
Mount and automount units for the EFI System Partition (ESP) and Extended Boot Loader Partition (XBOOTLDR) are
generated on EFI systems. If the disk contains an XBOOTLDR partition, as defined in the UAPI.1 Boot Loader
Specification[4], it is made available at /boot/. This generator creates an automount unit; the mount will only be
activated on-demand when accessed. The mount point will be created if necessary.
The ESP is mounted to /boot/ if that directory exists and is not used for XBOOTLDR, and otherwise to /efi/. Same as
for /boot/, an automount unit is used. The mount point will be created if necessary.
No configuration is created for mount points that are configured in fstab(5) or when the target directory contains
files.
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.
If the system was booted via systemd-stub(7) and the stub reported to userspace that the kernel image was measured to
a TPM2 PCR, then any discovered root and /var/ volume identifiers (and volume encryption key in case it is encrypted)
will be automatically measured into PCR 15 on activation, via systemd-pcrfs@.service(8).
Mount constraint metadata contained in the file systems is validated by pulling in systemd-validatefs@.service(8) for
generated mounts.
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").
Added in version 242.
systemd.image_policy=
Takes an image dissection policy string as argument (as per systemd.image-policy(7)), and allows enforcing a
policy on dissection and use of the automatically discovered GPT partition table entries.
Note that the specified image policy is not taken into account for automatic root or /usr/ file system discovery
unless root=dissect/mount.usr=dissect (or root=dissect-force) are specified. (The policy will always be applied
to the other auto-discoverable partition types.)
Added in version 254.
systemd.image_filter=
Takes an image dissection filter string as argument (as per systemd.image-filter(7)), and allows enforcing a set
of globbing patterns on the partition matching of the automatically discovered GPT partition table entries.
Note that the specified image filter is not taken into account for automatic root or /usr/ file system discovery
unless root=dissect/mount.usr=dissect (or root=dissect-force) are specified. (The filter will always be applied
to the other auto-discoverable partition types.)
Added in version 258.
root=, rootfstype=, rootflags=
When root= is used with the special value "gpt-auto", basic automatic discovery of the root partition based on
the GPT partition type is enabled. Use of the root partition is delayed until factory reset mode is left, in case
it is enabled during the current boot. See Factory Reset[5] for more information on that. If "gpt-auto-force" is
specified automatic discovery of the root partition is enabled, ignoring any factory reset mode.
If root= is set to the special value "dissect" full automatic discovery of the root partition based on GPT
partition information is enabled. This is a superset of root=gpt-auto, as it automatically configures Verity
partitions (including signature-based setup) following the logic defined for that in the UAPI.2 Discoverable
Partitions Specification[6]. Moreover it takes the configured image policy and image filter into account for all
partition types, including the root file system. "root=dissect" will wait for the factory reset phase to be
completed if it is in effect before activating the root file system. Use "root=dissect-force" to ignore the
factory reset phase and activate the root file system immediately.
Any other value (i.e. besides "gpt-auto", "gpt-auto-force", "dissect", "dissect-force") disables automatic root
file system discovery.
If root= is not specified at all on the kernel command line automatic discovery of the root partition via the ESP
reported by the boot loader is also enabled (taking factory reset state into account, i.e. equivalent to
"root=gpt-auto"), however in this case discovery based on the loopback block device ".lo_name" field is not
enabled.
The rootfstype= and rootflags= options are used to select the file system type and options when the root file
system is automatically discovered.
Added in version 242.
mount.usr=, mount.usrfstype=, mount.usrflags=
Similar to root=, rootfstype=, rootflags= (see above), but applies to the /usr/ partition instead. Note that the
"gpt-auto", "gpt-auto-force", "dissect-force" settings that root= understands are not supported by mount.usr=
(however "dissect" is).
Also note that automatic partition discovery for /usr/ must be enabled explicitly, unlike the discovery for the
root file system, which is enabled if no root= parameter is passed at all.
Added in version 258.
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).
Added in version 242.
systemd.swap=
Takes a boolean argument or enables the option if specified without an argument. If disabled, automatic discovery
of swap partition(s) based on GPT partition type is disabled. Defaults to enabled.
Added in version 254.
SEE ALSO
systemd(1), systemd.mount(5), systemd.swap(5), systemd-fstab-generator(8), systemd-cryptsetup@.service(8), systemd-
pcrfs@.service(8), systemd-validatefs@.service(8), machine-id(5), cryptsetup(8), fstab(5), btrfs(8)
NOTES
1. UEFI Specification
https://uefi.org/specifications
2. UAPI.2 Discoverable Partitions Specification
https://uapi-group.org/specifications/specs/discoverable_partitions_specification
3. Boot Loader Interface
https://systemd.io/BOOT_LOADER_INTERFACE
4. UAPI.1 Boot Loader Specification
https://uapi-group.org/specifications/specs/boot_loader_specification
5. Factory Reset
https://systemd.io/FACTORY_RESET
6. UAPI.2 Discoverable Partitions Specification
https://uapi-group.org/specifications/specs/discoverable_partitions_specification/
systemd 259.5 SYSTEMD-GPT-AUTO-GENERATOR(8)