Preview changes

nixarchy preview (or Omarchy menu → Install → Preview changes) builds what your /etc/nixos evaluates to right now and boots it in a VM window — so you can look at a configuration change before switching the machine to it. Apply changes offers the same thing as a question, at the moment you are about to switch.

Underneath it is nixosConfigurations.<host>.config.system.build.vm: your own configuration re-evaluated with nixpkgs’ qemu-vm.nix layered on — the same thing nixos-rebuild build-vm builds. The guest shares your machine’s /nix/store read-only, so the first preview builds the delta of your change, not a second copy of the desktop, and the disk image carries state rather than a system.

What a green preview proves, and what it does not

A preview is strong evidence, not proof. The VM re-evaluates your configuration with VM filesystems and a VM bootloader, so its toplevel is not the one nixos-rebuild switch would activate. What it verifies: your options evaluate, your services start, your session comes up. What it does not verify: your bootloader, your real GPU, your real disks. A change that touches any of those still meets its first real test at the switch — which is one nixarchy rollback away from undone.

The disk, and why the command sometimes refuses

Each preview gets a disk image in ~/.local/share/nixarchy/preview/. Left to its own devices, qemu’s runner would reuse that disk forever — and a reused disk replays the previous preview’s state (logins, notification history, anything written in the guest) into the one you are looking at, which quietly turns “what does my change do?” into “what did my last preview do?”.

So when a previous preview’s disk is there, the command refuses to guess:

• nixarchy preview --fresh — delete it and preview on a clean disk
• nixarchy preview --keep — boot on it, previous state and all

--keep is for looking at something that needs state to exist — a second boot, a service that migrates data. Everything else wants --fresh.

Speed

Without /dev/kvm (no hardware virtualisation, or you are not in the kvm group) the preview runs under software emulation, several times slower — near-unusable for a graphical desktop. The command warns loudly; the lag is the emulation, not your configuration. The guest gets 8192 MB by default; --memory 4096 runs a smaller one, and 4096 is the floor.

Not Sandboxes

Sandboxes (nixarchy vm) are disposable generic NixOS MicroVMs — a clean room for software you do not trust. A preview is the opposite: your own configuration, booted to be looked at. The sandbox page says “Not nixarchy’s own test VM” about itself; this is that VM.