Use as a lower store with nix

This document describes how to configure snix as the lower layer in your Local Overlay  nix store.

Run the snix daemon#

snix daemon is the component exposing castore and store data. By default, these live inside /var/lib/snix-{castore,store}, so make sure they are writable for the user you’re executing it with. See snix store daemon --help for customization options.

You can run the daemon with:

snix store daemon

Mount the store#

To expose the store paths and their contents as a file system, if can be FUSE-mounted with the following command:

snix store mount /path/to/mount

This mount will talk to the previously invoked daemon.

Note that by default, this mount won’t allow listing files and directories at the root of the store, if you want to enable it, use the --list-root flag, but be careful with it if your store is really large.

Run snix nix-daemon#

snix nix-daemon -l /tmp/snix-daemon.sock \
    --unix-listen-unlink

This will launch the snix nix-daemon listening on a unix domain socket.

Nix will communicate with it to get metadata about store paths.

Create an overlayfs mount#

Bind mount your real /nix store on the side, so that nix has direct access to it, this is optional but allows you to have access to your real nix store without unmounting:

mount --bind /nix /opt/nix

mount -t overlay overlay \
  -o lowerdir=/path/to/mount \
  -o upperdir=/opt/nix \
  /nix

Configure nix to use the daemon#

With all of the above out of the way, we are ready to configure nix. In the proposed setup we will configure nix-daemon with an overlay store but for the Nix CLI you can just configure nix with the overlay store.

nix-daemon#

The daemon can be configured in the following way:

Add the following line to your /etc/nix.conf

store = local-overlay://?state=/opt/nix/var/nix&upper-layer=/opt/nix/store&check-mount=false&lower-store=unix%3A%2F%2F%2Ftmp%2Fsnix-daemon.sock

Personal nix config#

With the above configuration in your /etc/nix.conf, we need to tell nix not to use it but instead use store = daemon so that only the daemon is aware of the Local Overlay Store.

This can be achieved by either setting the env variable NIX_CONFIG='store = daemon or by adding store = daemon to your $XDG_CONFIG_HOME/nix.conf file.

Profit#

With the above setup you should now be able to have nix use Snix castore as its lower store.