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:
The twentyfirst release of 
A short status update what happened last month. Work in progress is marked as WiP:
GNOME Calls
I think while developing Wayland-as-an-ecosystem we are now entrenched into narrow concepts of how a desktop should work. While discussing Wayland protocol additions, a lot of concepts clash, people from different desktops with different design philosophies debate the merits of those over and over again never reaching any conclusion (just as you will never get an answer out of humans whether sushi or pizza is the clearly superior food, or whether CSD or SSD is better). Some people want to use Wayland as a vehicle to force applications to submit to their desktop s design philosophies, others prefer the smallest and leanest protocol possible, other developers want the most elegant behavior possible. To be clear, I think those are all very valid approaches.
But this also creates problems: By switching to Wayland compositors, we are already forcing a lot of porting work onto toolkit developers and application developers. This is annoying, but just work that has to be done. It becomes frustrating though if Wayland provides toolkits with absolutely no way to reach their goal in any reasonable way. For Nate s Photoshop analogy: Of course Linux does not break Photoshop, it is Adobe s responsibility to port it. But what if Linux was missing a crucial syscall that Photoshop needed for proper functionality and Adobe couldn t port it without that? In that case it becomes much less clear on who is to blame for Photoshop not being available.
A lot of Wayland protocol work is focused on the environment and design, while applications and work to port them often is considered less. I think this happens because the overlap between application developers and developers of the desktop environments is not necessarily large, and the overlap with people willing to engage with Wayland upstream is even smaller. The combination of Windows developers porting apps to Linux and having involvement with toolkits or Wayland is pretty much nonexistent. So they have less of a voice.
I will also bring my two protocol MRs to their conclusion for sure, because as application developers we need clarity on what the platform (either all desktops or even just a few) supports and will or will not support in future. And the only way to get something good done is by contribution and friendly discussion.
This post should have marked the beginning of my yearly roundups of the favourite books and movies I read and watched in 2023.
However, due to coming down with a nasty bout of flu recently and other sundry commitments, I wasn't able to undertake writing the necessary four or five blog posts In lieu of this, however, I will simply present my (unordered and unadorned) highlights for now. Do get in touch if this (or any of my previous posts) have spurred you into picking something up yourself
The Emacs part is superceded by 
It was pointed out to me that I have not blogged about this, so better now than never:
Since 2021 I am together with four other hosts producing a regular podcast about Haskell, the 
Now, I think it's safe to assume this program is dead and buried, and
anyways I'm running 
For multi-monitor setups,