Thomas Koch: Minimal overhead VMs with Nix and MicroVM
Posted on March 17, 2024
Joachim Breitner wrote about a Convenient sandboxed development environment and thus reminded me to blog about MicroVM. I ve toyed around with it a little but not yet seriously used it as I m currently not coding.
MicroVM is a nix based project to configure and run minimal VMs. It can mount and thus reuse the hosts nix store inside the VM and thus has a very small disk footprint. I use MicroVM on a debian system using the nix package manager.
The MicroVM author uses the project to host production services. Otherwise I consider it also a nice way to learn about NixOS after having started with the nix package manager and before making the big step to NixOS as my main system.
The guests root filesystem is a tmpdir, so one must explicitly define folders that should be mounted from the host and thus be persistent across VM reboots.
I defined the VM as a nix flake since this is how I started from the MicroVM projects example:
description = "Haskell dev MicroVM";
inputs.impermanence.url = "github:nix-community/impermanence";
inputs.microvm.url = "github:astro/microvm.nix";
inputs.microvm.inputs.nixpkgs.follows = "nixpkgs";
outputs = self, impermanence, microvm, nixpkgs :
let
persistencePath = "/persistent";
system = "x86_64-linux";
user = "thk";
vmname = "haskell";
nixosConfiguration = nixpkgs.lib.nixosSystem
inherit system;
modules = [
microvm.nixosModules.microvm
impermanence.nixosModules.impermanence
( pkgs, ... :
environment.persistence.$ persistencePath =
hideMounts = true;
users.$ user =
directories = [
"git" ".stack"
];
;
;
environment.sessionVariables =
TERM = "screen-256color";
;
environment.systemPackages = with pkgs; [
ghc
git
(haskell-language-server.override supportedGhcVersions = [ "94" ]; )
htop
stack
tmux
tree
vcsh
zsh
];
fileSystems.$ persistencePath .neededForBoot = nixpkgs.lib.mkForce true;
microvm =
forwardPorts = [
from = "host"; host.port = 2222; guest.port = 22;
from = "guest"; host.port = 5432; guest.port = 5432; # postgresql
];
hypervisor = "qemu";
interfaces = [
type = "user"; id = "usernet"; mac = "00:00:00:00:00:02";
];
mem = 4096;
shares = [
# use "virtiofs" for MicroVMs that are started by systemd
proto = "9p";
tag = "ro-store";
# a host's /nix/store will be picked up so that no
# squashfs/erofs will be built for it.
source = "/nix/store";
mountPoint = "/nix/.ro-store";
proto = "virtiofs";
tag = "persistent";
source = "~/.local/share/microvm/vms/$ vmname /persistent";
mountPoint = persistencePath;
socket = "/run/user/1000/microvm-$ vmname -persistent";
];
socket = "/run/user/1000/microvm-control.socket";
vcpu = 3;
volumes = [];
writableStoreOverlay = "/nix/.rwstore";
;
networking.hostName = vmname;
nix.enable = true;
nix.nixPath = ["nixpkgs=$ builtins.storePath <nixpkgs> "];
nix.settings =
extra-experimental-features = ["nix-command" "flakes"];
trusted-users = [user];
;
security.sudo =
enable = true;
wheelNeedsPassword = false;
;
services.getty.autologinUser = user;
services.openssh =
enable = true;
;
system.stateVersion = "24.11";
systemd.services.loadnixdb =
description = "import hosts nix database";
path = [pkgs.nix];
wantedBy = ["multi-user.target"];
requires = ["nix-daemon.service"];
script = "cat $ persistencePath /nix-store-db-dump nix-store --load-db";
;
time.timeZone = nixpkgs.lib.mkDefault "Europe/Berlin";
users.users.$ user =
extraGroups = [ "wheel" "video" ];
group = "user";
isNormalUser = true;
openssh.authorizedKeys.keys = [
"ssh-rsa REDACTED"
];
password = "";
;
users.users.root.password = "";
users.groups.user = ;
)
];
;
in
packages.$ system .default = nixosConfiguration.config.microvm.declaredRunner;
;
[Unit]
Description=MicroVM for Haskell development
Requires=microvm-virtiofsd-persistent@.service
After=microvm-virtiofsd-persistent@.service
AssertFileNotEmpty=%h/.local/share/microvm/vms/%i/flake/flake.nix
[Service]
Type=forking
ExecStartPre=/usr/bin/sh -c "[ /nix/var/nix/db/db.sqlite -ot %h/.local/share/microvm/nix-store-db-dump ] nix-store --dump-db >%h/.local/share/microvm/nix-store-db-dump"
ExecStartPre=ln -f -t %h/.local/share/microvm/vms/%i/persistent/ %h/.local/share/microvm/nix-store-db-dump
ExecStartPre=-%h/.local/state/nix/profile/bin/tmux new -s microvm -d
ExecStart=%h/.local/state/nix/profile/bin/tmux new-window -t microvm: -n "%i" "exec %h/.local/state/nix/profile/bin/nix run --impure %h/.local/share/microvm/vms/%i/flake"
[Unit]
Description=serve host persistent folder for dev VM
AssertPathIsDirectory=%h/.local/share/microvm/vms/%i/persistent
[Service]
ExecStart=%h/.local/state/nix/profile/bin/virtiofsd \
--socket-path=$ XDG_RUNTIME_DIR /microvm-%i-persistent \
--shared-dir=%h/.local/share/microvm/vms/%i/persistent \
--gid-map :995:%G:1: \
--uid-map :1000:%U:1:
After more than one a year, a new minor FAI version is available, but
it includes some interesting new features.
Here a the items from the NEWS file:
fai (6.2) unstable; urgency=low
I can t actually think of a meme with this GIF, that the internal 
KDE Plasma 6
The new 7945HX CPU from AMD is currently the most powerful. I d love to have one of them, to replace the now aging 6 core Xeon that I ve been using for more than 5 years. So, I ve been searching for a laptop with that CPU.
Absolutely all of the laptops I found with this CPU also embed a very powerful RTX 40 0 series GPU, that I have no use: I don t play games, and I don t do AI. I just want something that builds Debian packages fast (like Ceph, that takes more than 1h to build for me ). The more cores I get, the faster all OpenStack unit tests are running too (stestr does a moderately good job at spreading the tests to all cores). That d be ok if I had to pay more for a GPU that I don t need, and I would have deal with the annoyance of the NVidia driver, if only I could find something with a correct size. But I can only find 16 or bigger laptops, that wont fit in my scooter back case (most of the time, these laptops have an 17 inch screen: that s a way too big).
Currently, I found:
The more I say, the more depressed I will become so will leave it for now. In many ways the destruction seems similar to the destruction that