nixos-system-config/modules/nixos/core/impermanence.nix

{ config, lib, ... }:

{
  boot.initrd.postDeviceCommands = lib.mkAfter ''
    mkdir /btrfs_tmp
    mount ${config.fileSystems."/".device} /btrfs_tmp
    if [[ -e /btrfs_tmp/root ]]; then
        mkdir -p /btrfs_tmp/old_roots
        timestamp=$(date --date="@$(stat -c %Y /btrfs_tmp/root)" "+%Y-%m-%-d_%H:%M:%S")
        mv /btrfs_tmp/root "/btrfs_tmp/old_roots/$timestamp"
    fi

    delete_subvolume_recursively() {
        IFS=$'\n'
        for i in $(btrfs subvolume list -o "$1" | cut -f 9- -d ' '); do
            delete_subvolume_recursively "/btrfs_tmp/$i"
        done
        btrfs subvolume delete "$1"
    }

    for i in $(find /btrfs_tmp/old_roots/ -maxdepth 1 -mtime +30); do
        delete_subvolume_recursively "$i"
    done

    btrfs subvolume create /btrfs_tmp/root
    umount /btrfs_tmp
  '';

  environment.persistence."/nix/persist" = {
    hideMounts = true;
    directories = [
      "/var/log" 
      "/var/lib/nixos"
      "/var/lib/AccountsService"
    ];
    files = [
      "/etc/machine-id"
    ];
  };
}