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Adapt the read stream logic for real AIO: - If AIO is enabled, we shouldn't issue advice, but if it isn't, we should continue issuing advice - AIO benefits from reading ahead with direct IO - If effective_io_concurrency=0, pass READ_BUFFERS_SYNCHRONOUSLY to StartReadBuffers() to ensure synchronous IO execution There are further improvements we should consider: - While in read_stream_look_ahead(), we can use AIO batch submission mode for increased efficiency. That however requires care to avoid deadlocks and thus done separately. - It can be beneficial to defer starting new IOs until we can issue multiple IOs at once. That however requires non-trivial heuristics to decide when to do so. Reviewed-by: Noah Misch <noah@leadboat.com> Co-authored-by: Andres Freund <andres@anarazel.de> Co-authored-by: Thomas Munro <thomas.munro@gmail.com>