find and document automation to dump/load hosts /nix/store db

[thkoch2001]

The hosts /nix/store is mounted inside the microvm. However nix inside the VM does not use objects from /nix/store. It does not know these objects since they are not in the Nix database (/nix/var/nix/db).

Probably the best solution is currently worked on as [RFC 0152] local-overlay store.

A workaround is to use nix-store --dump-db on the host and nix-store --load-db in the guest VM.

Of course this is a hack. But for my use case of local project development VMs it should work fine.

I've not yet figured out a nice automation for this. Ideally, a db-dump would be made on microvm boot, copied in the VM and loaded inside the VM.

[thkoch2001]

Documenting my current implementation here before I forget any details.

I'm running my microvm as a user systemd service, templated by the vm name. There is a dependency on a virtiofsd service. Before starting the VM, the microvm service does:

[Service]
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

The first line dumps the hosts nix database to ~/.local/share/microvm/nix-store-db-dump if the db.sqlite is not older than the dump file.
The second line hard-links the dump into a folder that gets mounted into the VM.

In the microvm flake.nix I got:

              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";
              };

[Dummyc0m]

It looks like local-overlay stores have been merged as an experimental feature. https://nix.dev/manual/nix/2.22/store/types/experimental-local-overlay-store

I'm curious if there's any interest/development in the VM guest use case... If no one has tried it yet I might spend a weekend working on it.