Add COMPARISON.md

COMPARISON.md

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+# Comparison with other tools
+
+## `nix build .\#nixosConfigurations.$SERVER.config.system.build.vm && ./result/bin/run-ai-vm`
+
+- Has a long list of ui quirks and gotchas:
+  - Opens a graphical window to show textual tty output.
+  - You can switch by setting `virtualisation.vmVariant.virtualisation.graphics = false;`, but then it still messes up your terminal.
+  - Creates a `.qcow2` file on the first run that will be re-used on subsequent runs.
+    Meaning you can change your NixOS config declaration and silently just not get any of your changes.
+  - You have to configure a way to log in, either with `virtualisation.vmVariant.services.getty.autologinUser = "root";` or some other manual way.
+  - Networking (e.g. forwarding ports to ssh in, setting up networking between VMs) is all up to manual configuration.
+- Is not very discoverable.
+- Seems more like a low-level building block than a thought out UI.
+
+## microvm.nix
+
+- Solves some of the problems with running the vm scripts. (E.g. doesn't mess up your terminal.)
+- Requires more manual configuration and work for e.g. ssh access and networking
+  between VMs. Both in terms of nix config and running commands on the host.
+- Seems to be mostly targeted at running production systems in VMs on NixOS machines.
+
+## nixos-container
+
+- Allows to spin up containers (not full VMs) from NixOS configs.
+- Seems to only work on NixOS hosts.
+- Shares the network with the host (could lead to port clashes), or requires manual configuration.
+- Logging into containers requires manually setting up ssh.
+
+## `nixos-rebuild build-vm`
+
+- Provides a slightly nicer way to run `nix build .\#nixosConfigurations.$SERVER.config.system.build.vm`.
+- Is meant to manage non-VM NixOS systems, i.e. `nixos-rebuild switch`.
+- Building and running VMs for testing is an additional feature.
+
+## `nixos-build-vms`
+
+- Doesn't work with flake files.
+- Seems a bit bitrotten? Couldn't figure out how to make it work.
+- It's a ~50 line bash script around `nix-build`.