In InnoDB, redo log file operations unnecessarily go through the fil_io() API. It would be cleaner to just have the log file descriptors straight inside log_sys.log.
Most redo log writes are synchronous. The only exception is writing the checkpoint header block, which uses asynchronous I/O. It might be better to dedicate the io_handler_thread() for data file operations only, and not let them deal with the redo log checkpoint write completion.