docs/DISCOVERABLE_PARTITIONS.md

   1 ---
   2 title: Discoverable Partitions Specification
   3 category: Concepts
   4 layout: default
   5 ---
   6 # The Discoverable Partitions Specification
   7
   8 _TL;DR: Let's automatically discover, mount and enable the root partition,
   9 `/home/`, `/srv/`, `/var/` and `/var/tmp/` and the swap partitions based on
  10 GUID Partition Tables (GPT)!_
  11
  12 This specification describes the use of GUID Partition Table (GPT) UUIDs to
  13 enable automatic discovery of partitions and their intended mountpoints.
  14 Traditionally Linux has made little use of partition types, mostly just
  15 defining one UUID for file system/data partitions and another one for swap
  16 partitions. With this specification, we introduce additional partition types
  17 for specific uses. This has many benefits:
  18
  19 * OS installers can automatically discover and make sense of partitions of
  20   existing Linux installations.
  21 * The OS can discover and mount the necessary file systems with a non-existent
  22   or incomplete `/etc/fstab` file and without the `root=` kernel command line
  23   option.
  24 * Container managers (such as nspawn and libvirt-lxc) can introspect and set up
  25   file systems contained in GPT disk images automatically and mount them to the
  26   right places, thus allowing booting the same, identical images on bare metal
  27   and in Linux containers. This enables true, natural portability of disk
  28   images between physical machines and Linux containers.
  29 * As a help to administrators and users partition manager tools can show more
  30   descriptive information about partitions tables.
  31
  32 Note that the OS side of this specification is currently implemented in
  33 [systemd](http://systemd.io/) 211 and newer in the
  34 [systemd-gpt-auto-generator(8)](http://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html)
  35 generator tool. Note that automatic discovery of the root only works if the
  36 boot loader communicates this information to the OS, by implementing the [Boot
  37 Loader
  38 Interface](https://systemd.io/BOOT_LOADER_INTERFACE).
  39
  40 ## Defined Partition Type UUIDs
  41
  42 | Partition Type UUID | Name | Allowed File Systems | Explanation |
  43 |---------------------|------|----------------------|-------------|
  44 | `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`. |
  45 | `4f68bce3-e8cd-4db1-96e7-fbcaf984b709` | _Root Partition (x86-64)_ | ditto | ditto |
  46 | `69dad710-2ce4-4e3c-b16c-21a1d49abed3` | _Root Partition (32-bit ARM)_ | ditto | ditto |
  47 | `b921b045-1df0-41c3-af44-4c6f280d3fae` | _Root Partition (64-bit ARM/AArch64)_ | ditto | ditto |
  48 | `993d8d3d-f80e-4225-855a-9daf8ed7ea97` | _Root Partition (Itanium/IA-64)_ | ditto | ditto |
  49 | `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. |
  50 | `2c7357ed-ebd2-46d9-aec1-23d437ec2bf5` | _Root Verity Partition (x86-64)_ | ditto | ditto |
  51 | `7386cdf2-203c-47a9-a498-f2ecce45a2d6` | _Root Verity Partition (32-bit ARM)_ | ditto | ditto |
  52 | `df3300ce-d69f-4c92-978c-9bfb0f38d820` | _Root Verity Partition (64-bit ARM/AArch64)_ | ditto | ditto |
  53 | `86ed10d5-b607-45bb-8957-d350f23d0571` | _Root Verity Partition (Itanium/IA-64)_  | ditto | ditto |
  54 | `75250d76-8cc6-458e-bd66-bd47cc81a812` | _`/usr/` Partition (x86)_ | Any native, optionally in LUKS | Similar semantics to root partition, but just the `/usr/` partition. |
  55 | `8484680c-9521-48c6-9c11-b0720656f69e` | _`/usr/` Partition (x86-64)_ | ditto | ditto |
  56 | `7d0359a3-02b3-4f0a-865c-654403e70625` | _`/usr/` Partition (32-bit ARM)_ | ditto | ditto |
  57 | `b0e01050-ee5f-4390-949a-9101b17104e9` | _`/usr/` Partition (64-bit ARM/AArch64)_ | ditto | ditto |
  58 | `4301d2a6-4e3b-4b2a-bb94-9e0b2c4225ea` | _`/usr/` Partition (Itanium/IA-64)_ | ditto | ditto |
  59 | `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. |
  60 | `77ff5f63-e7b6-4633-acf4-1565b864c0e6` | _`/usr/` Verity Partition (x86-64)_ | ditto | ditto |
  61 | `c215d751-7bcd-4649-be90-6627490a4c05` | _`/usr/` Verity Partition (32-bit ARM)_ | ditto | ditto |
  62 | `6e11a4e7-fbca-4ded-b9e9-e1a512bb664e` | _`/usr/` Verity Partition (64-bit ARM/AArch64)_ | ditto | ditto |
  63 | `6a491e03-3be7-4545-8e38-83320e0ea880` | _`/usr/` Verity Partition (Itanium/IA-64)_ | ditto | ditto |
  64 | `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`. |
  65 | `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`. |
  66 | `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`. |
  67 | `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. |
  68 | `0657fd6d-a4ab-43c4-84e5-0933c84b4f4f` | _Swap_ | Swap | All swap partitions on the disk containing the root partition are automatically enabled. |
  69 | `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). |
  70 | `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). |
  71 | `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. |
  72
  73 Other GPT type IDs might be used on Linux, for example to mark software RAID or
  74 LVM partitions. The definitions of those GPT types is outside of the scope of
  75 this specification.
  76
  77 [systemd-id128(1)](http://www.freedesktop.org/software/systemd/man/systemd-id128.html)
  78 may be used to list those UUIDs.
  79
  80 ## Partition Names
  81
  82 For partitions of the types listed above it is recommended to use
  83 human-friendly, descriptive partition names in the GPT partition table, for
  84 example "*Home*", "*Server* *Data*", "*Fedora* *Root*" and similar, possibly
  85 localized.
  86
  87 ## Partition Flags
  88
  89 For the root, `/usr/`, server data, home, variable data, temporary data and swap
  90 partitions, the partition flag bit 63 ("*no-auto*") may be used to turn off
  91 auto-discovery for the specific partition.  If set, the partition will not be
  92 automatically mounted or enabled.
  93
  94 For the root, `/usr/` server data, home, variable data and temporary data
  95 partitions, the partition flag bit 60 ("*read-only*") may be used to mark a
  96 partition for read-only mounts only.  If set, the partition will be mounted
  97 read-only instead of read-write. Note that the variable data partition and the
  98 temporary data partition will generally not be able to serve their purpose if
  99 marked read-only, since by their very definition they are supposed to be
 100 mutable. (The home and server data partitions are generally assumed to be
 101 mutable as well, but the requirement for them is not equally strong.) Because
 102 of that, while the read-only flag is defined and supported, it's almost never a
 103 good idea to actually use it for these partitions.
 104
 105 Note that these two flag definitions happen to map nicely to the ones used by
 106 Microsoft Basic Data Partitions.
 107
 108 ## Suggested Mode of Operation
 109
 110 An *installer* that repartitions the hard disk _should_ use the above UUID
 111 partition types for appropriate partitions it creates.
 112
 113 An *installer* which supports a "manual partitioning" interface _may_ choose to
 114 pre-populate the interface with swap, `/home/`, `/srv/`, `/var/tmp/` partitions
 115 of pre-existing Linux installations, identified with the GPT type UUIDs
 116 above. The installer should not pre-populate such an interface with any
 117 identified root, `/usr` or `/var/` partition unless the intention is to
 118 overwrite an existing operating system that might be installed.
 119
 120 An *installer* _may_ omit creating entries in `/etc/fstab` for root, `/home/`,
 121 `/srv/`, `/var/`, `/var/tmp` and for the swap partitions if they use these UUID
 122 partition types, and are the first partitions on the disk of each type. If the
 123 ESP shall be mounted to `/efi/` (or `/boot/`), it may additionally omit
 124 creating the entry for it in `/etc/fstab`.  If an extended boot partition is
 125 used, or if the EFI partition shall not be mounted to `/efi/` or `/boot/`, it
 126 _must_ create `/etc/fstab` entries for them.  If other partitions are used (for
 127 example for `/usr/` or `/var/lib/mysql/`), the installer _must_ register these
 128 in `/etc/fstab`.  The `root=` parameter passed to the kernel by the boot loader
 129 may be omitted if the root partition is the first one on the disk of its type.
 130 If the root partition is not the first one on the disk, the `root=` parameter
 131 _must_ be passed to the kernel by the boot loader.  An installer that mounts a
 132 root, `/usr/`, `/home/`, `/srv/`, `/var/`, or `/var/tmp/` file system with the
 133 partition types defined as above which contains a LUKS header _must_ call the
 134 device mapper device "root", "usr", "home", "srv", "var" or "tmp",
 135 respectively.  This is necessary to ensure that the automatic discovery will
 136 never result in different device mapper names than any static configuration by
 137 the installer, thus eliminating possible naming conflicts and ambiguities.
 138
 139 An *operating* *system* _should_ automatically discover and mount the first
 140 root partition that does not have the no-auto flag set (as described above) by
 141 scanning the disk containing the currently used EFI ESP.  It _should_
 142 automatically discover and mount the first `/usr/`, `/home/`, `/srv/`, `/var/`,
 143 `/var/tmp/` and swap partitions that do not have the no-auto flag set by
 144 scanning the disk containing the discovered root partition.  It should
 145 automatically discover and mount the partition containing the currently used
 146 EFI ESP to `/efi/` (or `/boot/` as fallback).  It should automatically discover
 147 and mount the partition containing the currently used Extended Boot Loader
 148 Partition to `/boot/`. It _should not_ discover or automatically mount
 149 partitions with other UUID partition types, or partitions located on other
 150 disks, or partitions with the no-auto flag set.  User configuration shall
 151 always override automatic discovery and mounting.  If a root, `/usr/`,
 152 `/home/`, `/srv/`, `/boot/`, `/var/`, `/var/tmp/`, `/efi/`, `/boot/` or swap
 153 partition is listed in `/etc/fstab` or with `root=` on the kernel command line,
 154 it _must_ take precedence over automatically discovered partitions.  If a
 155 `/home/`, `/usr/`, `/srv/`, `/boot/`, `/var/`, `/var/tmp/`, `/efi/` or `/boot/`
 156 directory is found to be populated already in the root partition, the automatic
 157 discovery _must not_ mount any discovered file system over it.
 158
 159 A *container* *manager* should automatically discover and mount the root,
 160 `/usr/`, `/home/`, `/srv/`, `/var/`, `/var/tmp/` partitions inside a container
 161 disk image.  It may choose to mount any discovered ESP and/or XBOOOTLDR
 162 partition to `/efi/` or `/boot/`. It should ignore any swap should they be
 163 included in a container disk image.
 164
 165 If a btrfs file system is automatically discovered and mounted by the operating
 166 system/container manager it will be mounted with its *default* subvolume.  The
 167 installer should make sure to set the default subvolume correctly using "btrfs
 168 subvolume set-default".
 169
 170 ## Sharing of File Systems between Installations
 171
 172 If two Linux-based operating systems are installed on the same disk, the scheme
 173 above suggests that they may share the swap, `/home/`, `/srv/`, `/var/tmp/`,
 174 ESP, XBOOTLDR. However, they should each have their own root, `/usr/` and
 175 `/var/` partition.
 176
 177 ## Frequently Asked Questions
 178
 179 ### Why are you taking my `/etc/fstab` away?
 180
 181 We are not. `/etc/fstab` always overrides automatic discovery and is indeed
 182 mentioned in the specifications.  We are simply trying to make the boot and
 183 installation processes of Linux a bit more robust and self-descriptive.
 184
 185 ### Why did you only define the root partition for x86, x86-64, ARM, ARM64, ia64?
 186
 187 The automatic discovery of the root partition is defined to operate on the disk
 188 containing the current EFI System Partition (ESP). Since EFI only exists on
 189 x86, x86-64, ia64, and ARM so far, we only defined root partition UUIDs for
 190 these architectures.  Should EFI become more common on other architectures, we
 191 can define additional UUIDs for them.
 192
 193 ### Why define distinct root partition UUIDs for the various architectures?
 194
 195 This allows disk images that may be booted on multiple architectures to use
 196 discovery of the appropriate root partition on each architecture.
 197
 198 ### Doesn't this break multi-boot scenarios?
 199
 200 No, it doesn't.  The specification says that installers may not stop creating
 201 `/etc/fstab` or stop including `root=` on the kernel command line, unless the used
 202 partitions are the first ones of their type on the disk. Additionally,
 203 `/etc/fstab` and `root=` both override automatic discovery.  Multi-boot is hence
 204 well supported, since it doesn't change anything for anything but the first
 205 installation.
 206
 207 That all said, it's not expected that generic installers generally stop setting
 208 `root=` and creating `/etc/fstab` anyway. The option to drop these configuration
 209 bits is primarily something for appliance-like devices.  However, generic
 210 installers should *still* set the right GPT partition types for the partitions
 211 they create so that container managers, partition tools and administrators can
 212 benefit.  Phrased differently, this specification introduces A) the
 213 *recommendation* to use the newly defined partition types to tag things
 214 properly and B) the *option* to then drop `root=` and `/etc/fstab`.  While we
 215 advertise A) to *all* installers, we only propose B) for simpler,
 216 appliance-like installations.
 217
 218 ### What partitioning tools will create a DPS-compliant partition table?
 219
 220 As of util-linux 2.25.2, the fdisk tool provides type codes to create the root,
 221 home, and swap partitions that the DPS expects, but the gdisk tool (version
 222 0.8.10) and its variants do not support creation of a root file system with a
 223 matching type code.  By default, fdisk will create an old-style MBR, not a GPT,
 224 so typing 'l' to list partition types will not show the choices that the root
 225 partition with the correct UUID.  You must first create an empty GPT and then
 226 type 'l' in order for the DPS-compliant type codes to be available.