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Frequently Asked Questions

A few caveats. Contributions to eliminate those are welcome.

Can I support the development and maintenance of this project?

❤ Sponsor

Why build a kernel with a custom configuration?

Hypervisors are not required to be able to load an initrd/initramfs. Therefore we start init from a virtio disk which requires virtio drivers to be built into the kernel statically.

Because we are building our own kernel anyway, we've got the opportunity of adding more custom configuration that is optimized for common MicroVM use-cases.

The microvm.nix project contains instructions for Github to build the microvm-kernel package every night and push the binary to Cachix. In order to use that binary cache, try running nix build github:astro/microvm.nix#microvm-kernel and answer the prompts.

How to centralize logging with journald?

That is possible without even requiring a network transport by just making the journals available to the host as a share. Because journald identifies hosts by their /etc/machine-id, we propose to use static content for that file. Add a NixOS module like the following to your MicroVM configuration:

environment.etc."machine-id" = {
  mode = "0644";
  text =
    # change this to suit your flake's interface
    self.lib.addresses.machineId.${config.networking.hostName} + "\n";
};

microvm.shares = [ {
  # On the host
  source = "/var/lib/microvms/${config.networking.hostName}/journal";
  # In the MicroVM
  mountPoint = "/var/log/journal";
  tag = "journal";
  proto = "virtiofs";
  socket = "journal.sock";
} ];

Last, make the MicroVM journals available to your host. The machine-id must be available.

systemd.tmpfiles.rules = map (vmHost:
  let
    machineId = self.lib.addresses.machineId.${vmHost};
  in
    # creates a symlink of each MicroVM's journal under the host's /var/log/journal
    "L+ /var/log/journal/${machineId} - - - /var/lib/microvms/${vmHost}/journal/${machineId}"
) (builtins.attrNames self.lib.addresses.machineId);

Once your MicroVM's journal data is visible in the /var/log/journal/$machineId/ directories, journalctl can pick it up using the -m/--merge switch.

Can I build with hypervisors from the host's nixpkgs instead of the MicroVM's?

Yes. This scenario is enabled through the flake's lib.buildRunner function. See the nix run microvm#build-microvm script that you will need to customize to fit your deployment scenario.