microvm.nix is a Flake to run lightweight NixOS virtual machines on NixOS. Starting with the reasons why for the remainder of this chapter, this handbook guides you through the provisioning of MicroVMs on your NixOS machine.
NixOS makes running services a breeze. Being able to quickly rollback configuration is a life-saver. Not so much however on systems that are shared by multiple services where maintenance of one affects others.
Increase stability by partitioning services into virtual NixOS systems that can be updated individually.
microvm.nix can isolate your /nix/store into exactly what is required for the guest's NixOS: the root filesystem is a read-only erofs/squashfs file-systems that include only the binaries of your configuration. Of course, that holds only true until you mount the host's /nix/store as a share for faster build times, or mount the store with a writable overlay for Nix builds inside the VM.
Linux containers are not a single technology but a plethora of kernel features that serve to isolate various system resources so that the running system appears as one. It is still one shared Linux kernel with a huge attack surface.
Virtual machines on the other hand run their own OS kernel, reducing the attack surface to the hypervisor and its device drivers. The resource usage however incurs some overhead when compared with containers, with memory allocation being especially inflexible.
microvm.nix is a tool that helps you building the guest's OS and
running it in ways that are easier than writing a Dockerfile, once
you know how to put a NixOS config into a flake.nix file.
Full virtualization has been available for a long time with QEMU and VirtualBox. The MicroVM machine type highlights that virtualization overhead has been reduced a lot by replacing emulated devices with virtio interfaces that have been optimized for this environment.
This Flake offers you to run your MicroVMs not only on QEMU but with other Hypervisors that have been explicitly authored for virtio. Some of them are written in Rust, a programming language that is renowned for being safer than C.