The host configuration can be built before its cache signing secret exists. Harmonia's socket remains stopped until SOPS installs the signing key at /run/secrets/harmonia/signing-key.

Proxmox storage layout

The host configuration expects three filesystems. Keep the build scratch space separate from the store so a large build cannot fill the root filesystem.

Mount point	Suggested size	Contents
`/`	48 GiB	NixOS and mutable system state
`/var/lib/nix-build`	192 GiB	Disposable build scratch space
`/nix/store`	1 TiB	Fleet closures and binary cache

The build-server policy starts emergency store GC below 64 GiB free and aims for 128 GiB free. Persistent roots under /var/lib/nix-fleet/roots protect the latest fleet builds from that GC. It also limits Nix to two concurrent derivations while allowing each derivation to use every vCPU assigned to the VM.

For the two dedicated ext4 data filesystems, remove the default root-reserved blocks once after formatting; keep the root filesystem's reserve intact:

sudo tune2fs -m 0 /dev/disk/by-label/nix-build
sudo tune2fs -m 0 /dev/disk/by-label/nix-store

Initial deployment

Once the VM is reachable as moons@nix-builder, deploy it from the repository:

nix run .#deploy -- .#nix-builder

deploy-rs uses the target's ssh-ng store, so the system closure is built on the builder rather than copied from the laptop. Automatic and magic rollback remain enabled.

Add the host SOPS recipient

After the VM has a stable SSH host key, derive its age recipient:

ssh-keyscan -t ed25519 nix-builder 2>/dev/null | ssh-to-age

Add the recipient to .sops.yaml and add a creation rule for secrets/hosts/nix-builder/*.yaml. The admin YubiKey recipient should remain in the same key group for recovery.

Generate the cache signing key

Run this on a trusted Nix machine, preferably with the temporary files on a tmpfs:

nix-store --generate-binary-cache-key \
  cache.app.homelabs.run-1 \
  harmonia.private \
  harmonia.public

Create secrets/hosts/nix-builder/system.yaml with SOPS and store the complete contents of harmonia.private at harmonia.signing-key:

harmonia:
  signing-key: cache.app.homelabs.run-1:REDACTED

Copy the complete contents of harmonia.public to modules/systems/nix/homelab-cache/public-key. The private plaintext file must not be committed or retained.

After committing both encrypted/public files, select networking.homelab-cache-client on each client host.

Redeploy the builder and verify the cache after installing the secret:

nix run .#deploy -- .#nix-builder
curl --fail http://nix-builder:5000/nix-cache-info

Normal operation

Run fleet-build on the builder to build and root every NixOS host, or pass a list of host names to build only those hosts. Run fleet-deploy with the normal deploy-rs target syntax when additional fleet nodes have been added to flake/deploy.nix:

fleet-build
fleet-build x1g13 galleria
fleet-deploy .#nix-builder