1---
2title: Initrd Interface
3category: Interfaces
4layout: default
5SPDX-License-Identifier: LGPL-2.1-or-later
6---
7
8
9# The initrd Interface of systemd
10
11The Linux initrd mechanism (short for "initial RAM disk") refers to a small
12file system archive that is unpacked by the kernel and contains the first
13userspace code that runs. It typically finds and transitions into the actual
14root file system to use. systemd supports both initrd and initrd-less boots. If
15an initrd is used, it is a good idea to pass a few bits of runtime information
16from the initrd to systemd in order to avoid duplicate work and to provide
17performance data to the administrator. In this page we attempt to roughly
18describe the interfaces that exist between the initrd and systemd. These
19interfaces are currently used by dracut and the ArchLinux initrds.
20
21* The initrd should mount `/run/` as a tmpfs and pass it pre-mounted when
22  jumping into the main system when executing systemd. The mount options should
23  be `mode=755,nodev,nosuid,strictatime`.
24
25* It's highly recommended that the initrd also mounts `/usr/` (if split off) as
26  appropriate and passes it pre-mounted to the main system, to avoid the
27  problems described in [Booting without /usr is
28  Broken](http://freedesktop.org/wiki/Software/systemd/separate-usr-is-broken).
29
30* If the executable `/run/initramfs/shutdown` exists systemd will use it to
31  jump back into the initrd on shutdown. `/run/initramfs/` should be a usable
32  initrd environment to which systemd will pivot back and the `shutdown`
33  executable in it should be able to detach all complex storage that for
34  example was needed to mount the root file system. It's the job of the initrd
35  to set up this directory and executable in the right way so that this works
36  correctly. The shutdown binary is invoked with the shutdown verb as `argv[1]`,
37  optionally followed (in `argv[2]`, `argv[3]`, … systemd's original command
38  line options, for example `--log-level=` and similar.
39
40* Storage daemons run from the initrd should follow the guide on
41  [systemd and Storage Daemons for the Root File System](ROOT_STORAGE_DAEMONS.md)
42  to survive properly from the boot initrd all the way to the point where
43  systemd jumps back into the initrd for shutdown.
44
45One last clarification: we use the term _initrd_ very generically here
46describing any kind of early boot file system, regardless whether that might be
47implemented as an actual ramdisk, ramfs or tmpfs. We recommend using _initrd_
48in this sense as a term that is unrelated to the actual backing technologies
49used.
50
51Oh, and one last question before closing: instead of implementing these
52features in your own distro's initrd, may I suggest just using Dracut instead?
53It's all already implemented there!
54
55## Using systemd inside an initrd
56
57It is also possible and recommended to implement the initrd itself based on
58systemd. Here are a few terse notes:
59
60* Provide `/etc/initrd-release` in the initrd image. The idea is that it follows
61  the same format as the usual `/etc/os-release` but describes the initial RAM
62  disk implementation rather than the OS. systemd uses the existence of this
63  file as a flag whether to run in initial RAM disk mode, or not.
64
65* When run in initrd mode, systemd and its components will read a couple of
66  additional command line arguments, which are generally prefixed with `rd.`
67
68* To transition into the main system image invoke `systemctl switch-root`.
69
70* The switch-root operation will result in a killing spree of all running
71  processes. Some processes might need to be excluded from that, see the guide
72  on [systemd and Storage Daemons for the Root File System](ROOT_STORAGE_DAEMONS.md).
73