235 lines
18 KiB
Markdown
235 lines
18 KiB
Markdown
---
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title: Discoverable Partitions Specification
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category: Concepts
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layout: default
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---
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# The Discoverable Partitions Specification
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_TL;DR: Let's automatically discover, mount and enable the root partition,
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`/home/`, `/srv/`, `/var/` and `/var/tmp/` and the swap partitions based on
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GUID Partition Tables (GPT)!_
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This specification describes the use of GUID Partition Table (GPT) UUIDs to
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enable automatic discovery of partitions and their intended mountpoints.
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Traditionally Linux has made little use of partition types, mostly just
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defining one UUID for file system/data partitions and another one for swap
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partitions. With this specification, we introduce additional partition types
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for specific uses. This has many benefits:
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* OS installers can automatically discover and make sense of partitions of
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existing Linux installations.
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* The OS can discover and mount the necessary file systems with a non-existent
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or incomplete `/etc/fstab` file and without the `root=` kernel command line
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option.
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* Container managers (such as nspawn and libvirt-lxc) can introspect and set up
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file systems contained in GPT disk images automatically and mount them to the
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right places, thus allowing booting the same, identical images on bare metal
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and in Linux containers. This enables true, natural portability of disk
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images between physical machines and Linux containers.
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* As a help to administrators and users partition manager tools can show more
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descriptive information about partitions tables.
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Note that the OS side of this specification is currently implemented in
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[systemd](http://systemd.io/) 211 and newer in the
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[systemd-gpt-auto-generator(8)](http://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html)
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generator tool. Note that automatic discovery of the root only works if the
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boot loader communicates this information to the OS, by implementing the [Boot
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Loader
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Interface](https://systemd.io/BOOT_LOADER_INTERFACE).
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## Defined Partition Type UUIDs
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| Partition Type UUID | Name | Allowed File Systems | Explanation |
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|---------------------|------|----------------------|-------------|
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| `44479540-f297-41b2-9af7-d131d5f0458a` | _Root Partition (x86)_ | Any native, optionally in LUKS | On systems with matching architecture, the first partition with this type UUID on the disk containing the active EFI ESP is automatically mounted to the root directory <tt>/</tt>. If the partition is encrypted with LUKS or has dm-verity integrity data (see below), the device mapper file will be named `/dev/mapper/root`. |
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| `4f68bce3-e8cd-4db1-96e7-fbcaf984b709` | _Root Partition (x86-64)_ | ditto | ditto |
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| `69dad710-2ce4-4e3c-b16c-21a1d49abed3` | _Root Partition (32-bit ARM)_ | ditto | ditto |
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| `b921b045-1df0-41c3-af44-4c6f280d3fae` | _Root Partition (64-bit ARM/AArch64)_ | ditto | ditto |
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| `993d8d3d-f80e-4225-855a-9daf8ed7ea97` | _Root Partition (Itanium/IA-64)_ | ditto | ditto |
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| `60d5a7fe-8e7d-435c-b714-3dd8162144e1` | _Root Partition (RISC-V 32-bit)_ | ditto | ditto |
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| `72ec70a6-cf74-40e6-bd49-4bda08e8f224` | _Root Partition (RISC-V 64-bit)_ | ditto | ditto |
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| `d13c5d3b-b5d1-422a-b29f-9454fdc89d76` | _Root Verity Partition (x86)_ | A dm-verity superblock followed by hash data | On systems with matching architecture, contains dm-verity integrity hash data for the matching root partition. If this feature is used the partition UUID of the root partition should be the first 128bit of the root hash of the dm-verity hash data, and the partition UUID of this dm-verity partition should be the final 128bit of it, so that the root partition and its verity partition can be discovered easily, simply by specifying the root hash. |
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| `2c7357ed-ebd2-46d9-aec1-23d437ec2bf5` | _Root Verity Partition (x86-64)_ | ditto | ditto |
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| `7386cdf2-203c-47a9-a498-f2ecce45a2d6` | _Root Verity Partition (32-bit ARM)_ | ditto | ditto |
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| `df3300ce-d69f-4c92-978c-9bfb0f38d820` | _Root Verity Partition (64-bit ARM/AArch64)_ | ditto | ditto |
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| `86ed10d5-b607-45bb-8957-d350f23d0571` | _Root Verity Partition (Itanium/IA-64)_ | ditto | ditto |
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| `ae0253be-1167-4007-ac68-43926c14c5de` | _Root Verity Partition (RISC-V 32-bit)_ | ditto | ditto |
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| `b6ed5582-440b-4209-b8da-5ff7c419ea3d` | _Root Verity Partition (RISC-V 64-bit)_ | ditto | ditto |
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| `75250d76-8cc6-458e-bd66-bd47cc81a812` | _`/usr/` Partition (x86)_ | Any native, optionally in LUKS | Similar semantics to root partition, but just the `/usr/` partition. |
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| `8484680c-9521-48c6-9c11-b0720656f69e` | _`/usr/` Partition (x86-64)_ | ditto | ditto |
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| `7d0359a3-02b3-4f0a-865c-654403e70625` | _`/usr/` Partition (32-bit ARM)_ | ditto | ditto |
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| `b0e01050-ee5f-4390-949a-9101b17104e9` | _`/usr/` Partition (64-bit ARM/AArch64)_ | ditto | ditto |
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| `4301d2a6-4e3b-4b2a-bb94-9e0b2c4225ea` | _`/usr/` Partition (Itanium/IA-64)_ | ditto | ditto |
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| `b933fb22-5c3f-4f91-af90-e2bb0fa50702` | _`/usr/` Partition (RISC-V 32-bit)_ | ditto | ditto |
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| `beaec34b-8442-439b-a40b-984381ed097d` | _`/usr/` Partition (RISC-V 64-bit)_ | ditto | ditto |
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| `8f461b0d-14ee-4e81-9aa9-049b6fb97abd` | _`/usr/` Verity Partition (x86)_ | Any native, optionally in LUKS | Similar semantics to root Verity partition, but just for the `/usr/` partition. |
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| `77ff5f63-e7b6-4633-acf4-1565b864c0e6` | _`/usr/` Verity Partition (x86-64)_ | ditto | ditto |
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| `c215d751-7bcd-4649-be90-6627490a4c05` | _`/usr/` Verity Partition (32-bit ARM)_ | ditto | ditto |
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| `6e11a4e7-fbca-4ded-b9e9-e1a512bb664e` | _`/usr/` Verity Partition (64-bit ARM/AArch64)_ | ditto | ditto |
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| `6a491e03-3be7-4545-8e38-83320e0ea880` | _`/usr/` Verity Partition (Itanium/IA-64)_ | ditto | ditto |
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| `cb1ee4e3-8cd0-4136-a0a4-aa61a32e8730` | _`/usr/` Verity Partition (RISC-V 32-bit)_ | ditto | ditto |
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| `8f1056be-9b05-47c4-81d6-be53128e5b54` | _`/usr/` Verity Partition (RISC-V 64-bit)_ | ditto | ditto |
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| `933ac7e1-2eb4-4f13-b844-0e14e2aef915` | _Home Partition_ | Any native, optionally in LUKS | The first partition with this type UUID on the disk containing the root partition is automatically mounted to `/home/`. If the partition is encrypted with LUKS, the device mapper file will be named `/dev/mapper/home`. |
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| `3b8f8425-20e0-4f3b-907f-1a25a76f98e8` | _Server Data Partition_ | Any native, optionally in LUKS | The first partition with this type UUID on the disk containing the root partition is automatically mounted to `/srv/`. If the partition is encrypted with LUKS, the device mapper file will be named `/dev/mapper/srv`. |
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| `4d21b016-b534-45c2-a9fb-5c16e091fd2d` | _Variable Data Partition_ | Any native, optionally in LUKS | The first partition with this type UUID on the disk containing the root partition is automatically mounted to `/var/` — under the condition that its partition UUID matches the first 128 bit of `HMAC-SHA256(machine-id, 0x4d21b016b53445c2a9fb5c16e091fd2d)` (i.e. the SHA256 HMAC hash of the binary type UUID keyed by the machine ID as read from [`/etc/machine-id`](https://www.freedesktop.org/software/systemd/man/machine-id.html). This special requirement is made because `/var/` (unlike the other partition types listed here) is inherently private to a specific installation and cannot possibly be shared between multiple OS installations on the same disk, and thus should be bound to a specific instance of the OS, identified by its machine ID. If the partition is encrypted with LUKS, the device mapper file will be named `/dev/mapper/var`. |
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| `7ec6f557-3bc5-4aca-b293-16ef5df639d1` | _Temporary Data Partition_ | Any native, optionally in LUKS | The first partition with this type UUID on the disk containing the root partition is automatically mounted to `/var/tmp/`. If the partition is encrypted with LUKS, the device mapper file will be named `/dev/mapper/tmp`. Note that the intended mount point is indeed `/var/tmp/`, not `/tmp/`. The latter is typically maintained in memory via <tt>tmpfs</tt> and does not require a partition on disk. In some cases it might be desirable to make `/tmp/` persistent too, in which case it is recommended to make it a symlink or bind mount to `/var/tmp/`, thus not requiring its own partition type UUID. |
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| `0657fd6d-a4ab-43c4-84e5-0933c84b4f4f` | _Swap_ | Swap | All swap partitions on the disk containing the root partition are automatically enabled. |
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| `c12a7328-f81f-11d2-ba4b-00a0c93ec93b` | _EFI System Partition_ | VFAT | The ESP used for the current boot is automatically mounted to `/efi/` (or `/boot/` as fallback), unless a different partition is mounted there (possibly via `/etc/fstab`, or because the Extended Boot Loader Partition — see below — exists) or the directory is non-empty on the root disk. This partition type is defined by the [UEFI Specification](http://www.uefi.org/specifications). |
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| `bc13c2ff-59e6-4262-a352-b275fd6f7172` | _Extended Boot Loader Partition_ | Typically VFAT | The Extended Boot Loader Partition (XBOOTLDR) used for the current boot is automatically mounted to <tt>/boot/</tt>, unless a different partition is mounted there (possibly via <tt>/etc/fstab</tt>) or the directory is non-empty on the root disk. This partition type is defined by the [Boot Loader Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION). |
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| `0fc63daf-8483-4772-8e79-3d69d8477de4` | _Other Data Partitions_ | Any native, optionally in LUKS | No automatic mounting takes place for other Linux data partitions. This partition type should be used for all partitions that carry Linux file systems. The installer needs to mount them explicitly via entries in <tt>/etc/fstab</tt>. Optionally, these partitions may be encrypted with LUKS. |
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Other GPT type IDs might be used on Linux, for example to mark software RAID or
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LVM partitions. The definitions of those GPT types is outside of the scope of
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this specification.
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[systemd-id128(1)](http://www.freedesktop.org/software/systemd/man/systemd-id128.html)
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may be used to list those UUIDs.
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## Partition Names
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For partitions of the types listed above it is recommended to use
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human-friendly, descriptive partition names in the GPT partition table, for
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example "*Home*", "*Server* *Data*", "*Fedora* *Root*" and similar, possibly
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localized.
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## Partition Flags
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For the root, `/usr/`, server data, home, variable data, temporary data and swap
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partitions, the partition flag bit 63 ("*no-auto*") may be used to turn off
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auto-discovery for the specific partition. If set, the partition will not be
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automatically mounted or enabled.
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For the root, `/usr/`, server data, home, variable data and temporary data
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partitions, the partition flag bit 60 ("*read-only*") may be used to mark a
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partition for read-only mounts only. If set, the partition will be mounted
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read-only instead of read-write. Note that the variable data partition and the
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temporary data partition will generally not be able to serve their purpose if
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marked read-only, since by their very definition they are supposed to be
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mutable. (The home and server data partitions are generally assumed to be
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mutable as well, but the requirement for them is not equally strong.) Because
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of that, while the read-only flag is defined and supported, it's almost never a
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good idea to actually use it for these partitions.
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Note that these two flag definitions happen to map nicely to the ones used by
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Microsoft Basic Data Partitions.
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## Suggested Mode of Operation
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An *installer* that repartitions the hard disk _should_ use the above UUID
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partition types for appropriate partitions it creates.
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An *installer* which supports a "manual partitioning" interface _may_ choose to
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pre-populate the interface with swap, `/home/`, `/srv/`, `/var/tmp/` partitions
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of pre-existing Linux installations, identified with the GPT type UUIDs
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above. The installer should not pre-populate such an interface with any
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identified root, `/usr` or `/var/` partition unless the intention is to
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overwrite an existing operating system that might be installed.
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An *installer* _may_ omit creating entries in `/etc/fstab` for root, `/home/`,
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`/srv/`, `/var/`, `/var/tmp` and for the swap partitions if they use these UUID
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partition types, and are the first partitions on the disk of each type. If the
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ESP shall be mounted to `/efi/` (or `/boot/`), it may additionally omit
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creating the entry for it in `/etc/fstab`. If an extended boot partition is
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used, or if the EFI partition shall not be mounted to `/efi/` or `/boot/`, it
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_must_ create `/etc/fstab` entries for them. If other partitions are used (for
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example for `/usr/` or `/var/lib/mysql/`), the installer _must_ register these
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in `/etc/fstab`. The `root=` parameter passed to the kernel by the boot loader
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may be omitted if the root partition is the first one on the disk of its type.
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If the root partition is not the first one on the disk, the `root=` parameter
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_must_ be passed to the kernel by the boot loader. An installer that mounts a
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root, `/usr/`, `/home/`, `/srv/`, `/var/`, or `/var/tmp/` file system with the
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partition types defined as above which contains a LUKS header _must_ call the
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device mapper device "root", "usr", "home", "srv", "var" or "tmp",
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respectively. This is necessary to ensure that the automatic discovery will
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never result in different device mapper names than any static configuration by
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the installer, thus eliminating possible naming conflicts and ambiguities.
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An *operating* *system* _should_ automatically discover and mount the first
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root partition that does not have the no-auto flag set (as described above) by
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scanning the disk containing the currently used EFI ESP. It _should_
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automatically discover and mount the first `/usr/`, `/home/`, `/srv/`, `/var/`,
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`/var/tmp/` and swap partitions that do not have the no-auto flag set by
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scanning the disk containing the discovered root partition. It should
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automatically discover and mount the partition containing the currently used
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EFI ESP to `/efi/` (or `/boot/` as fallback). It should automatically discover
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and mount the partition containing the currently used Extended Boot Loader
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Partition to `/boot/`. It _should not_ discover or automatically mount
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partitions with other UUID partition types, or partitions located on other
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disks, or partitions with the no-auto flag set. User configuration shall
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always override automatic discovery and mounting. If a root, `/usr/`,
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`/home/`, `/srv/`, `/boot/`, `/var/`, `/var/tmp/`, `/efi/`, `/boot/` or swap
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partition is listed in `/etc/fstab` or with `root=` on the kernel command line,
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it _must_ take precedence over automatically discovered partitions. If a
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`/home/`, `/usr/`, `/srv/`, `/boot/`, `/var/`, `/var/tmp/`, `/efi/` or `/boot/`
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directory is found to be populated already in the root partition, the automatic
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discovery _must not_ mount any discovered file system over it.
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A *container* *manager* should automatically discover and mount the root,
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`/usr/`, `/home/`, `/srv/`, `/var/`, `/var/tmp/` partitions inside a container
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disk image. It may choose to mount any discovered ESP and/or XBOOOTLDR
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partition to `/efi/` or `/boot/`. It should ignore any swap should they be
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included in a container disk image.
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If a btrfs file system is automatically discovered and mounted by the operating
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system/container manager it will be mounted with its *default* subvolume. The
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installer should make sure to set the default subvolume correctly using "btrfs
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subvolume set-default".
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## Sharing of File Systems between Installations
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If two Linux-based operating systems are installed on the same disk, the scheme
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above suggests that they may share the swap, `/home/`, `/srv/`, `/var/tmp/`,
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ESP, XBOOTLDR. However, they should each have their own root, `/usr/` and
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`/var/` partition.
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## Frequently Asked Questions
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### Why are you taking my `/etc/fstab` away?
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We are not. `/etc/fstab` always overrides automatic discovery and is indeed
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mentioned in the specifications. We are simply trying to make the boot and
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installation processes of Linux a bit more robust and self-descriptive.
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### Why did you only define the root partition for x86, x86-64, ARM, ARM64, ia64?
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The automatic discovery of the root partition is defined to operate on the disk
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containing the current EFI System Partition (ESP). Since EFI only exists on
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x86, x86-64, ia64, and ARM so far, we only defined root partition UUIDs for
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these architectures. Should EFI become more common on other architectures, we
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can define additional UUIDs for them.
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### Why define distinct root partition UUIDs for the various architectures?
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This allows disk images that may be booted on multiple architectures to use
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discovery of the appropriate root partition on each architecture.
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### Doesn't this break multi-boot scenarios?
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No, it doesn't. The specification says that installers may not stop creating
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`/etc/fstab` or stop including `root=` on the kernel command line, unless the used
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partitions are the first ones of their type on the disk. Additionally,
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`/etc/fstab` and `root=` both override automatic discovery. Multi-boot is hence
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well supported, since it doesn't change anything for anything but the first
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installation.
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That all said, it's not expected that generic installers generally stop setting
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`root=` and creating `/etc/fstab` anyway. The option to drop these configuration
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bits is primarily something for appliance-like devices. However, generic
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installers should *still* set the right GPT partition types for the partitions
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they create so that container managers, partition tools and administrators can
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benefit. Phrased differently, this specification introduces A) the
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*recommendation* to use the newly defined partition types to tag things
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properly and B) the *option* to then drop `root=` and `/etc/fstab`. While we
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advertise A) to *all* installers, we only propose B) for simpler,
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appliance-like installations.
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### What partitioning tools will create a DPS-compliant partition table?
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As of util-linux 2.25.2, the fdisk tool provides type codes to create the root,
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home, and swap partitions that the DPS expects, but the gdisk tool (version
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0.8.10) and its variants do not support creation of a root file system with a
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matching type code. By default, fdisk will create an old-style MBR, not a GPT,
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so typing 'l' to list partition types will not show the choices that the root
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partition with the correct UUID. You must first create an empty GPT and then
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type 'l' in order for the DPS-compliant type codes to be available.
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