How to create a USI (Unified System Images which)?

[septatrix]

The recent blog post "A re-introduction to mkosi" talked about USIs (Unified System Images which are full OS images packed in a UKI) though they are not a first-class output format (i.e. one cannot say mkosi --format usi. The systemd or UAPI projects also do not seem to document what exactly constitutes a USI.

Is it "just" a normal system (i.e. os-release instead of initrd-release) bundled with the kernel similar to a traditional initrd? If this is the case how is this handled by systemd? Is there still an initrd stage for early-boot units anymore etc...?

[Winterhuman]

I have a blog post about building USIs with mkosi: https://overhead.neocities.org/blog/build-usi-mkosi

But TLDR: Use Format=uki plus MakeInitrd=no to create a USI.

[septatrix]

Thanks, that was an interesting read. Did you run into some problems due to services not being run which only run during the initrd phase? I was thinking about stuff like systemd-pcrphase-initrd.service etc.

PS: Maybe consider adding syntax highlighting to your blog for the systemd files (INI syntax should be sufficient)

[Winterhuman]

systemd-pcrphase-initrd.service is inactive for me, but generally speaking, not creating /etc/initrd-release is all systemd needs to understand that the UKI isn't an initramfs

[edorgeville]

Very interesting read, thanks @Winterhuman