What causes /run/booted-system to be updated (and when)?

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1

I have a system which I manage using flakes (i.e. nixos-rebuild switch --flake .#box). I noticed that my /nix is pretty big, so today I went and looked at why that could be. I noticed in /nix/store I have a lot of duplicate entries, such as:

$ for j in /nix/store/*ghc-9.4.6/; do echo --- $j; nix-store --query --roots $j|rg -v censored; done
--- /nix/store/3aq8bd4j4vcqcp1qc8aw3wija9bzb1cb-ghc-9.4.6/
/run/booted-system -> /nix/store/yxy7zvv86mci0fi2bpw6v1xn77ls873g-nixos-system-box-23.11.20230915.ace5093
--- /nix/store/s7gic33gdmisapji4v2fml3k5ky1bx84-ghc-9.4.6/
/run/current-system -> /nix/store/g3bfwyf3yzfyym0p1k0ih82k04l4z80f-nixos-system-box-23.11.20231019.7c9cc5a
/nix/var/nix/profiles/system-199-link -> /nix/store/g3bfwyf3yzfyym0p1k0ih82k04l4z80f-nixos-system-box-23.11.20231019.7c9cc5a

I only have a single generation:

$ nixos-rebuild list-generations --flake .#box|cat
Generation   Build-date           NixOS version           Kernel  Configuration Revision  Specialisation
199 current  2023-10-20 20:11:03  23.11.20231019.7c9cc5a  6.1.58

and also a single boot entry:

$ ll /boot/efi/loader/entries
total 4
-rwxr-xr-x 1 root root 422 Oct 29 15:00 nixos-generation-199.conf

I rebooted the system, ran nix-collect-garbage -d, ran nixos-rebuild switch --flake .#box, but only /run/current-system changed, /run/booted-system stayed the same.

Any idea how to update /run/booted-system and have it updated going forward when I switch between generations?

2

/run/booted-system is a symlink created by stage-2 init, that points to the booted generation. It prevents GC of things that are likely in use. To state another way, it’s automatically kept-up-to-date by rebooting after running nixos-rebuild switch ....

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