Building a custom initrd and using it in a mkosi image

Building an image with a mkosi-built initrd is a two step process, because you will build two images - the initrd and your distribution image.

1. Build an initrd image using the cpio output format with the same target distributions as you want to use for your distribution image. mkosi compresses the cpio output format by default.

[Output]
Format=cpio

[Content]
Packages=systemd
         udev
         kmod

1. Invoke mkosi passing the initrd image via the --initrd option or add the Initrd= option to your mkosi config when building your distribution image.

mkosi --initrd=<path-to-initrd-image> ...

This will build an image using the provided initrd image. mkosi will add the kernel modules found in the distribution image to this initrd.