Google Compute Image

A customized NixOS image module for Google Compute Engine.

The difference to upstreams module is that we are using systemd-repart to generate disk images, while upstream uses make-disk-image.nix.

By doing so, we loose support for legacy boot, but gain more flexibility with partition layouts and filesystems used as well as performance improvements of the image build. Performance concerns were the primary motivation for this project, as we were seeing excessive memory usage when building larger (~40GB) images with make-disk-image.nix.

Other then the upstream image, which consists of a single ext4 partition, we pack /nix/store into a read-only erofs partition and mount a writable overlay-store at run-time.

There's an effort to upstream this in NixOS/nixpkgs#463135, but currently waiting for a good way to override system.build.image. See NixOS/nixpkgs#463249 for discussion.

This work is sponsored by replit

Usage

├───nixosConfigurations
│   └───default: NixOS configuration
└───nixosModules
├───nixosModules
│   ├───bloat: NixOS module
│   ├───debug: NixOS module
│   ├───google-compute-repart: NixOS module
│   └───image-script: NixOS module
└───packages
    └───x86_64-linux
        ├───image: package 'google-compute-image-25.11.20251117.89c2b23'
        └───imageScript: package 'build-disk-image'

• packages.x86_64-linux.image produces an image of nixosConfigurations.default, an example configuration of the repositories modules. That image still needs to be renamed to disk.raw and put into a .tar.gz before it's ready to be uploaded. imageScript below already includes that step, but we've decided to wait for a solution of NixOS/nixpkgs#463249 before adding it to image.

• packages.x86_64-linux.imageScript generates a shell script that builds the same image, but without storing the resulting image in /nix/store. The image is written to the current working directory instead.

• nixosModules.google-compute-repart reuses upstreams google-compute-image.nix, but asserts EFI boot, puts /nix/store into erofs and uses systemd-repart to generate the image.

• nixosModules.image-script adds the system.build.imageScript attribute to the NixOS closure. nixosModules.debug and nixosModules.bloat are optional modules to aid debugging of early boot and performance testing for larger images. Neither is active by default.

To run the example configuration Google cloud:

# Build the image & note the file name
image=$(nix run .#imageScript)

# Upload it to a bucket
gsutil cp $image "gs://$BUCKET_NAME/"

# Create a VM image from it & mark it UEFI-compatible
gcloud compute images create "$IMAGE_NAME" \
    --source-uri="gs://$BUCKET_NAME/$(basename $image)" \
    --project="$PROJECT_ID" \
    --guest-os-features=UEFI_COMPATIBLE