docs: import initrd interface documentation from fdo wiki

Imported from https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface/

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+---
+title: Initrd Interface
+category: Interfaces
+layout: default
+---
+
+
+# The initrd Interface of systemd
+
+The Linux initrd mechanism (short for "initial RAM disk") refers to a small
+file system archive that is unpacked by the kernel and contains the first
+userspace code that runs. It typically finds and transitions into the actual
+root file system to use. systemd supports both initrd and initrd-less boots. If
+an initrd is used it is a good idea to pass a few bits of runtime information
+from the initrd to systemd in order to avoid duplicate work and to provide
+performance data to the administrator. In this page we attempt to roughly
+describe the interfaces that exist between the initrd and systemd. These
+interfaces are currently used by dracut and the ArchLinux initrds.
+
+* The initrd should mount `/run/` as a tmpfs and pass it pre-mounted when
+  jumping into the main system when executing systemd. The mount options should
+  be `mode=755,nodev,nosuid,strictatime`
+
+* It's highly recommended that the initrd also mounts `/usr/` (if split off) as
+  appropriate and passes it pre-mounted to the main system, to avoid the
+  problems described in [Booting without /usr is
+  Broken](http://freedesktop.org/wiki/Software/systemd/separate-usr-is-broken).
+
+* If the executable `/run/initramfs/shutdown` exists systemd will use it to
+  jump back into the initrd on shutdown. `/run/initramfs/` should be a usable
+  initrd environment to which systemd will pivot back and the `shutdown`
+  executable in it should be able to detach all complex storage that for
+  example was needed to mount the root file system. It's the job of the initrd
+  to set up this directory and executable in the right way so that this works
+  correctly. The shutdown binary is invoked with the shutdown verb as `argv[1]`,
+  optionally followed (in `argv[2]`, `argv[3]`, … systemd's original command
+  line options, for example `--log-level=` and similar.
+
+* Storage daemons run from the initrd should follow the the guide on [systemd
+  and Storage Daemons for the Root File
+  System](https://systemd.io/ROOT_STORAGE_DAEMONS) to survive properly from the
+  boot initrd all the way to the point where systemd jumps back into the initrd
+  for shutdown.
+
+One last clarification: we use the term _initrd_ very generically here
+describing any kind of early boot file system, regardless whether that might be
+implemented as an actual ramdisk, ramfs or tmpfs. We recommend using _initrd_
+in this sense as a term that is unrelated to the actual backing technologies
+used.
+
+Oh, and one last question before closing: instead of implementing these
+features in your own distro's initrd, may I suggest just using Dracut instead?
+It's all already implemented there!
+
+## Using systemd inside an initrd
+
+It is also possible and recommended to implement the initrd itself based on
+systemd. Here are a few terse notes:
+
+* Provide `/etc/initrd-release` in the initrd image. The idea is that it follows
+  the same format as the usual `/etc/os-release` but describes the initial RAM
+  disk implementation rather than the OS. systemd uses the existence of this
+  file as a flag whether to run in initial RAM disk mode, or not.
+
+* When run in initrd mode, systemd and its components will read a couple of
+  additional command line arguments, which are generally prefixed with `rd.`
+
+* To transition into the main system image invoke `systemctl switch-root`.
+
+* The switch-root operation will result in a killing spree of all running
+  processes. Some processes might need to be excluded from that, see the guide
+  on [systemd and Storage Daemons for the Root File
+  System](https://systemd.io/ROOT_STORAGE_DAEMONS).