autoinit is an experimental minimal initrd for ostree use. It builds a single-executable init process with the only task of mounting the sysroot, running ostree-prepare-root to mount the ostree root and then switchroot into that and boot it.

The advantage of this over a more complicated initrd (such as one from regular dracut) is that it is smaller and does less, so the total time until the full systemd is running in the real rootfs is shorter.

A disadvantage is that the minimal initrd is very inflexible, so it cannot be amended by the user, which means there is no way to run custom code earlier than after the main systemd starts.

The expected usage is to install ostree and autoinit in the build environment, and then run dracut -m autoinit (i.e. dracut with only the autoinit module) to produce an initrd file. This can then be used to boot the system by specifying the required arguments on the kernel command line.

The supported kernel args are:

• autoinit.root=... - The device node to mount the rootfs from (e.g. root=/dev/vda3). This is required.
• autoinit.rootfstype=... - The filesystem type used in the mount, if unspecified rootfs=ext4 is used.
• autoinit.rw - If this is set, /sysroot is mounted read-write.
• autoinit.debug - If this is set, autoinit will print out some debug info
• autoinit.shellat=init - If this is set to init, autoinit will start a bash instead of sysroot init

In addition, ostree-prepare-root needs the standard ostree= argument to be set, so that ostree can tell what deploy to boot.

For fast iteration, the localtest/Makefile file contains some targets that make it easy to test-boot an image.

In the localtest directory, put an ostree-based vm image as image.qcow2, and extract the kernel used as vmlinux and the deployment id used. Then you can test autoinit like so:

$ mkdir -p build
$ cd build
$ meson ..
$ ninja
$ cd ../localtest
$ make run OSTREE_DEPLOYMENT=0b257ba185126d152559cb7395bb9dde2bc4a600687264facd600eebe504eca5