Transient Nix Builders on Packet.com

I use Packet.com's spot market to run transient, powerful Nix builders. The files here are custom to my builders and my use case. However, they could easily serve as a nice template for you to use.

I would accept PRs parameterizing the code.

Principles of Operation

This repository creates a bootable iPXE image. We assume each boot starts with an empty set of disks. In case this is not true, it erases all of the disks and creates one large ZFS stripe across all disks. If you have any disks attached, it WILL erase it on every boot.

Each machine is stateless. As soon as it boots it is ready to build. When the machine shuts down, all data is lost.

To customize

1. Edit ./user.nix to have your user and your user's key. If you have a key which is only used by Nix's remote builder protocol, then they might belong in in the sshKeys list at the top.

2. Edit ./instances/m2.xlarge.x86.nix to match the hardware you'll be deploying to. These machines are all Packet.com's m2.xlarge.x86 type, so if you're also using those, it is ready to go.

Building

m2.xlarge.x86

You can simply nix-build ./instances/m2.xlarge.x86.nix in this directory and create a bootable image. On the other hand, I use ./build-x86_64-linux.sh, which instantiates locally and builds on my netboot server. The remote server builds much faster and saves my battery life.

c2.large.arm

In principle this one is just as easy. If you're on a machine which can build aarch64 binaries, then you can just run nix-build ./instances/c2.large.arm.nix. However, I am not and this is a bit annoying.

Therefore, I've written ./build-aarch64-linux.sh which requires a configuration file of this format:

buildHost       root@my.aarch64.builder
pxeHost         netboot@my.netboot.server
pxeDir          /var/lib/nginx/netboot/
opensslServer   my.netboot.server
opensslPort     61616

This will copy the derivations to buildHost for building, and then set up an openssl-wrapped netcat tunnel from buildHost to opensslServer:opensslPort for transfering the build products.

My laptop will SSH to the pxeHost and launch openssl and netcat, then SSH to the buildHost and initiate a connection to opensslServer:opensslPort. If this doesn't work, make sure that port is open.

Deploying

After building, copy the resulting directory's files to a web accessible directory and instruct the server to boot from the netboot.ipxe file in the result.

On Packet, edit ./create-spot-request.sh to include the Packet API information, and the URL of the netboot.ipxe. This might be expensive! Make sure you understand what it will cost.

I always use their spot market, but you could deploy this to a regular or reserved server just the same.

If you use their spot market, this repository leaves it up as an exercise to the reader to implement server discovery. Although, if you're using Hydra, an importer exists at https://github.com/NixOS/nixos-org-configurations/tree/master/hydra-packet-importer already.

Implementation Notes

• A naive implementation of a remote Nix builder might stick with the default unionfs. However, this approach uses a lot of extra CPU and causes unstable and broken builds for more complex builds. Because of this, we switched to making a full, proper filesystem across all the disks present. See: NixOS/nixpkgs#64126

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btw: I don't work for Packet. Just a fan.