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 The GUID Partition Table (GPT) is mandatory on EFI systems. It allows
  13 identification of partition types with UUIDs. So far Linux has made little use
  14 of this, and mostly just defined one UUID for file system/data partitions and
  15 another one for swap partitions.  With this specification, we introduce
  16 additional partition types to enable automatic discovery of partitions and
  17 their intended mountpoint.  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-existing
  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 decode 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-auto-gpt-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 | `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`. |
  55 | `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`. |
  56 | `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`. |
  57 | `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. |
  58 | `0657fd6d-a4ab-43c4-84e5-0933c84b4f4f` | _Swap_ | Swap | All swap partitions on the disk containing the root partition are automatically enabled. |
  59 | `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). |
  60 | `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). |
  61 | `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. |
  62
  63 Other GPT type IDs might be used on Linux, for example to mark software RAID or
  64 LVM partitions. The definitions of those GPT types is outside of the scope of
  65 this specification.
  66
  67 ## Partition Names
  68
  69 For partitions of the types listed above it is recommended to use
  70 human-friendly, descriptive partition names in the GPT partition table, for
  71 example "*Home*", "*Server* *Data*", "*Fedora* *Root*" and similar, possibly
  72 localized.
  73
  74 ## Partition Flags
  75
  76 For the root, server data, home, variable data, temporary data and swap
  77 partitions, the partition flag bit 63 ("*no-auto*") may be used to turn off
  78 auto-discovery for the specific partition.  If set, the partition will not be
  79 automatically mounted or enabled.
  80
  81 For the root, server data, home, variable data and temporary data partitions,
  82 the partition flag bit 60 ("*read-only*") may be used to mark a partition for
  83 read-only mounts only.  If set, the partition will be mounted read-only instead
  84 of read-write. Note that the variable data partition and the temporary data
  85 partition will generally not be able to serve their purpose if marked
  86 read-only, since by their very definition they are supposed to be mutable. (The
  87 home and server data partitions are generally assumed to be mutable as well,
  88 but the requirement for them is not equally strong.) Because of that, while the
  89 read-only flag is defined and supported, it's almost never a good idea to
  90 actually use it for these partitions.
  91
  92 Note that these two flag definitions happen to map nicely to the ones used by
  93 Microsoft Basic Data Partitions.
  94
  95 ## Suggested Mode of Operation
  96
  97 An *installer* that repartitions the hard disk _should_ use the above UUID
  98 partition types for appropriate partitions it creates.
  99
 100 An *installer* which supports a "manual partitioning" interface _may_ choose to
 101 pre-populate the interface with swap, `/home/`, `/srv/`, `/var/tmp/` partitions
 102 of pre-existing Linux installations, identified with the GPT type UUIDs
 103 above. The installer should not pre-populate such an interface with any
 104 identified root or `/var/` partition unless the intention is to overwrite an
 105 existing operating system that might be installed.
 106
 107 An *installer* _may_ omit creating entries in `/etc/fstab` for root, `/home/`,
 108 `/srv/`, `/var/`, `/var/tmp` and for the swap partitions if they use these UUID
 109 partition types, and are the first partitions on the disk of each type. If the
 110 ESP shall be mounted to `/efi/` (or `/boot/`), it may additionally omit
 111 creating the entry for it in `/etc/fstab`.  If an extended boot partition is
 112 used, or if the EFI partition shall not be mounted to `/efi/` or `/boot/`, it
 113 _must_ create `/etc/fstab` entries for them.  If other partitions are used (for
 114 example for `/usr/` or `/var/lib/mysql/`), the installer _must_ register these
 115 in `/etc/fstab`.  The `root=` parameter passed to the kernel by the boot loader
 116 may be omitted if the root partition is the first one on the disk of its type.
 117 If the root partition is not the first one on the disk, the `root=` parameter
 118 _must_ be passed to the kernel by the boot loader.  An installer that mounts a
 119 root, `/home/`, `/srv/`, `/var/`, or `/var/tmp/` file system with the partition
 120 types defined as above which contains a LUKS header _must_ call the device
 121 mapper device "root", "home", "srv", "var" or "tmp", respectively.  This is
 122 necessary to ensure that the automatic discovery will never result in different
 123 device mapper names than any static configuration by the installer, thus
 124 eliminating possible naming conflicts and ambiguities.
 125
 126 An *operating* *system* _should_ automatically discover and mount the first
 127 root partition that does not have the no-auto flag set (as described above) by
 128 scanning the disk containing the currently used EFI ESP.  It _should_
 129 automatically discover and mount the first `/home/`, `/srv/`, `/var/`,
 130 `/var/tmp/` and swap partitions that do not have the no-auto flag set by
 131 scanning the disk containing the discovered root partition.  It should
 132 automatically discover and mount the partition containing the currently used
 133 EFI ESP to `/efi/` (or `/boot/` as fallback).  It should automatically discover
 134 and mount the partition containing the currently used Extended Boot Loader
 135 Partition to `/boot/`. It _should not_ discover or automatically mount
 136 partitions with other UUID partition types, or partitions located on other
 137 disks, or partitions with the no-auto flag set.  User configuration shall
 138 always override automatic discovery and mounting.  If a root, `/home/`,
 139 `/srv/`, `/boot/`, `/var/`, `/var/tmp/`, `/efi/`, `/boot/` or swap partition is
 140 listed in `/etc/fstab` or with `root=` on the kernel command line, it _must_
 141 take precedence over automatically discovered partitions.  If a `/home/`,
 142 `/srv/`, `/boot/`, `/var/`, `/var/tmp/`, `/efi/` or `/boot/` directory is found
 143 to be populated already in the root partition, the automatic discovery _must
 144 not_ mount any discovered file system over it.
 145
 146 A *container* *manager* should automatically discover and mount the root,
 147 `/home/`, `/srv/`, `/var/`, `/var/tmp/` partitions inside a container disk
 148 image.  It may choose to mount any discovered ESP and/or XBOOOTLDR partition to
 149 `/efi/` or `/boot/`. It should ignore any swap should they be included in a
 150 container disk image.
 151
 152 If a btrfs file system is automatically discovered and mounted by the operating
 153 system/container manager it will be mounted with its *default* subvolume.  The
 154 installer should make sure to set the default subvolume correctly using "btrfs
 155 subvolume set-default".
 156
 157 ## Sharing of File Systems between Installations
 158
 159 If two Linux-based operating systems are installed on the same disk, the scheme
 160 above suggests that they may share the swap, `/home/`, `/srv/`, `/var/tmp/`,
 161 ESP, XBOOTLDR. However, they should each have their own root and `/var/`
 162 partition.
 163
 164 ## Frequently Asked Questions
 165
 166 ### Why are you taking my `/etc/fstab` away?
 167
 168 We are not. `/etc/fstab` always overrides automatic discovery and is indeed
 169 mentioned in the specifications.  We are simply trying to make the boot and
 170 installation processes of Linux a bit more robust and self-descriptive.
 171
 172 ### Why did you only define the root partition for x86, x86-64, ARM, ARM64, ia64?
 173
 174 The automatic discovery of the root partition is defined to operate on the disk
 175 containing the current EFI System Partition (ESP). Since EFI only exists on
 176 x86, x86-64, ia64, and ARM so far, we only defined root partition UUIDs for
 177 these architectures.  Should EFI become more common on other architectures, we
 178 can define additional UUIDs for them.
 179
 180 ### Why define distinct root partition UUIDs for the various architectures?
 181
 182 This allows disk images that may be booted on multiple architectures to use
 183 discovery of the appropriate root partition on each architecture.
 184
 185 ### Doesn't this break multi-boot scenarios?
 186
 187 No, it doesn't.  The specification says that installers may not stop creating
 188 `/etc/fstab` or stop including `root=` on the kernel command line, unless the used
 189 partitions are the first ones of their type on the disk. Additionally,
 190 `/etc/fstab` and `root=` both override automatic discovery.  Multi-boot is hence
 191 well supported, since it doesn't change anything for anything but the first
 192 installation.
 193
 194 That all said, it's not expected that generic installers generally stop setting
 195 `root=` and creating `/etc/fstab` anyway. The option to drop these configuration
 196 bits is primarily something for appliance-like devices.  However, generic
 197 installers should *still* set the right GPT partition types for the partitions
 198 they create so that container managers, partition tools and administrators can
 199 benefit.  Phrased differently, this specification introduces A) the
 200 *recommendation* to use the newly defined partition types to tag things
 201 properly and B) the *option* to then drop `root=` and `/etc/fstab`.  While we
 202 advertise A) to *all* installers, we only propose B) for simpler,
 203 appliance-like installations.
 204
 205 ### What partitioning tools will create a DPS-compliant partition table?
 206
 207 As of util-linux 2.25.2, the fdisk tool provides type codes to create the root,
 208 home, and swap partitions that the DPS expects, but the gdisk tool (version
 209 0.8.10) and its variants do not support creation of a root file system with a
 210 matching type code.  By default, fdisk will create an old-style MBR, not a GPT,
 211 so typing 'l' to list partition types will not show the choices that the root
 212 partition with the correct UUID.  You must first create an empty GPT and then
 213 type 'l' in order for the DPS-compliant type codes to be available.