application.ex

defmodule Logging21.Application do
  @moduledoc false

  use Application

  def start(_type, _args) do
    :logger.add_handlers(:logging_21)

    opts = [strategy: :one_for_one, name: Logging21.Supervisor]
    Supervisor.start_link([], opts)
  end
end

config.exs

import Config

config :logger,
  handle_otp_reports: false,
  handle_sasl_reports: false

config :logging_21, :logger, [
  {
    :handler,
    :my_hacky_handler,
    :logger_disk_log_h,
    %{
      config: %{
        :file => './my_disk_log',
        :type => :wrap,
        :max_no_files => 4,
        :max_no_bytes => 100,
        :filesync_repeat_interval => 1000
      }
    }
  }
]