Metamagical

Suppose, like, me, you are a NixOS girlthing ¹. Suppose then, (tragedy of tragedies) that you are forced by grave and terrible extenuating circumstances to use a non-deterministic, impure, imperative (gasp) operating system (i.e. not NixOS) on your trusty ThinkPad.

"Never fear!" you declare ², "I'll just run Nix on something else". Hmm, but what something else? Perhaps Fedora, you think to yourself, since that has an actually-up-to-date kernel ³. So, you go to install Fedora, find yourself face-to-face with a delightful installer partitioning tool, and have some fun with partitions, putting the Nix store on a shiny separate LVM volume ⁴ so you can resize it later if you ~~install 17 versions of Haskell~~ need more space.

You boot Fedora, install Nix, set up home-manager, and are off to the races. "Wonderful! Superb!", you declare, and, your work for the day done, you shut down your laptop. Unbeknownst to you, however, you've just made a grave mistake.

See, when nix installs itself on non-NixOS systems, it sets up a pair of systemd unit files (nix-daemon.service, and nix-daemon.socket) that manage the daemon. Unfortunately, init systems ⁵. Particularly, systemd is in charge of managing both unit files and mounts. To do this, it reads its unit files ⁶, including mounts, and then starts stuff. This means we have an ordering issue: it'll try to read our nix-daemon unit file before /nix is mounted, and find yourself with a case of missing services.

The fix, then, is to mount /nix before systemd starts, but since systemd is normally in charge of mounts, we've gotta do a little trickery. In particular, we'll use the initrd. The initrd is normally in charge of mounting the root partition so we can actually boot off of it, plus some other tidbits of early setup.

Thankfully, Fedora's initrd system, Dracut⁷⁸ is pretty flexible, so we can ask it to mount extra volumes by adding relevant entries to /etc/fstab.sys, which is almost exactly like normal /etc/fstab but gets processed sooner ⁹. You may need to create the file, but you can just copy the relevant line from /etc/fstab, since they have the same format.

We'll also need to tell the kernel it's allowed to process the volume we want to mount -- by default, Fedora is set up using rd.lvm.lv which restricts the initrd to working with only the lvs listed ¹⁰. Add an extra kernel flag in /etc/defaults/grub (grub2) or /etc/kernel/cmdline (systemd-boot): rd.lvm.lv=<vg>/<lv-holding-nix> ¹¹

Once you've done those, regenerate dracut ¹² as well as grub or systemd-boot ¹³, then reboot. You should fine that systemd ~~summons the Nix demon~~ starts nix-daemon automatically and you can run nix run nixpkgs#hello and nix develop to your heart's content.

or any other NixOS-loving entity, of any genders or none at all

it's important to speak like you're in a 4-color comic at all times

the kernel, of course, you need because you have made the Grave Mistake ™ of using a laptop with a discrete graphics card ¹⁴, which is made by the the folks over at nvidia ¹⁵, and, having the frankly absurd requirement of needing to use external monitors, so you'll need working nouveau for your system ¹⁶.

⁴

with luks2 underneath your lvm pv, of course

⁵

a traditional cause of much woe and teeth gnashing amongst linux folks

⁶

and /etc/fstab, which turns into a set of virutal units, so that systemd has a single source of what it's supposed to do when

⁷

yes, that's the Gentoo wiki. Still one of the best sources

⁸

also, yes, half the stuff for Fedora is named after towns in MA. It's a thing

⁹

specifically, partway-through-Dracut sooner.

¹⁰

normally, this is just the root and swap volumes, like rd.lvm.lv=primary/root rd.lvm.lv=primary/swap

¹¹

if your vg is named primary, and the lv is nix, you should end up with a piece of kernel command line that looks something like rd.lvm.lv=primary/root rd.lvm.lv=primary/swap rd.lvm.lv=primary/nix

¹²

dracut --regenerate-all --force

¹³

kernel-install is a way do this, not sure if there's a less cumbersome way

¹⁴

first mistake

¹⁵

second mistake

¹⁶

"why,", you might ask yourself, "do I need working nouveau for external monitors?". Well, self, laptop manufacturers hate you. More specifically, they frequently wire things up so that the integrated graphics drive the laptop monitor, and the discrete graphics drive any and all external connections (HDMI, displayport, thunderbolt).

"That's not so bad...", you might think. "Anyway, I can just use the shiny new (kinda) open-source nvidia drivers" ¹⁷ But! wait just a darned minute! there's more! You see, self, nvidia often hates standards. Specifically, the standards that a lot of the smaller wlroots-compositors ¹⁸ use to draw things to the screen via wayland ¹⁹.

So, unless we want weird graphical artifacts, disappearing cursors, and passive- agressive command line flags, we'll need nouveau, which actually works here, but only recent kernels and such support a bunch of the newer-ish graphics cards from the past couple years.

¹⁷

hey Fedora folks, did you finally have that "nvidia open source driver release party"?

¹⁸

well, it's not entirely nvidia's fault, the wlroots folks are a bit curmudgeonly, too

¹⁹

oh yeah, I forgot to mention, we're using wayland