preservation-disko

Sane defaults for disko + preservation integration.

Quick Start

# flake.nix
{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    disko.url = "github:nix-community/disko";
    preservation-disko.url = "github:visualphoenix/preservation-disko";
  };
}

Start with disko's hybrid-tmpfs-on-root example, then add a persist partition:

# disko-config.nix
{ config, ... }:
{
  disko.devices = {
    disk.main = {
      # ... ESP and nix partitions from hybrid-tmpfs-on-root.nix ...

      content.partitions.persist = {
        size = "100%";
        content = {
          type = "filesystem";
          format = "ext4";
          mountpoint = "/persist";
          # This connects preservation to disko:
          postMountHook = config.preservation.diskoSetupCommands;
        };
      };
    };

    # tmpfs root from hybrid-tmpfs-on-root.nix
    nodev."/" = {
      fsType = "tmpfs";
      mountOptions = [ "defaults" "size=2G" "mode=755" ];
    };
  };

  fileSystems."/persist".neededForBoot = true;
}

# configuration.nix
{ inputs, ... }:
{
  imports = [
    inputs.disko.nixosModules.disko
    inputs.preservation-disko.nixosModules.default
    ./disko-config.nix
  ];
}

That's it. Run disko and nixos-install as normal.

What You Get

Out of the box, the module:

Feature	Details
Persisted directories	/var/lib/nixos, /var/lib/systemd, /var/log
Persisted files	/etc/machine-id (with proper initrd handling)
Disko integration	Auto-generated postMountHook commands
Initrd support	Enables boot.initrd.systemd automatically
Clan auto-detection	If using clan, automatically preserves secrets (not required)

Extending

Add more directories or files using standard NixOS module merging:

{ config, ... }:
let
  persistPath = config.preservation.persistentStoragePath;  # "/persist" by default
in
{
  preservation.preserveAt.${persistPath} = {
    directories = [
      "/var/lib/postgresql"
      "/var/lib/private/myapp"
    ];
    files = [
      { file = "/etc/ssh/ssh_host_ed25519_key"; }
      { file = "/etc/ssh/ssh_host_ed25519_key.pub"; }
    ];
  };
}

For directories needed before systemd starts (secrets, early-boot state):

{
  preservation.extraBindMounts."/var/lib/myapp-secrets".neededForBoot = true;
}

Note: /var/lib/sops-nix is handled automatically for clan users.

Options

Option	Default	Description
preservation.persistentStoragePath	"/persist"	Where persistent storage is mounted
preservation.installMountPoint	"/mnt"	Root mount point during installation
preservation.machineIdMode	"symlink"	How to persist /etc/machine-id: "symlink" or "bindmount"
preservation.extraBindMounts	{}	Additional bind mounts with neededForBoot support
preservation.diskoSetupCommands	(generated)	Shell commands for disko's postMountHook

How It Works

When you run disko, the postMountHook executes after mounting your persist partition. The generated commands:

1. Create directories on persistent storage (/mnt/persist/var/lib/nixos, etc.)
2. Create mount points on the root filesystem (/mnt/var/lib/nixos, etc.)
3. Set up bind mounts so nixos-install writes to the persistent locations

At runtime, preservation handles the bind mounts via initrd systemd.

Clan Integration

Clan is not required. This module works standalone with any NixOS + disko setup.

If you are using clan-core, the module automatically detects it and:

• Preserves clan.core.facts secretUploadDirectory
• Preserves /var/lib/sops-nix when clan.core.vars generators are defined

No configuration needed - it just works.

Troubleshooting

"Directory doesn't exist" during nixos-install

Make sure postMountHook = config.preservation.diskoSetupCommands; is on your persist partition content.

Machine-ID regenerates every boot

The module handles this by default. If you're overriding /etc/machine-id preservation, ensure inInitrd = true.

Secrets not available at boot (non-clan sops-nix users)

Clan users get /var/lib/sops-nix automatically. For standalone sops-nix:

preservation.extraBindMounts."/var/lib/sops-nix".neededForBoot = true;

References

• nix-community/preservation
• nix-community/disko
• NixOS PR #351151 - machine-id changes
• preservation#22 - machine-id discussion