When integrating with off the shelf storage engines that do I/O synchronously (including via memory mapped files) I think some effort may be needed to get decent performance. FoundationDB uses a single-threaded asynchronous execution model and blocking that thread to read from disk will be disastrous for throughput and latency.
Our adaptation of sqlite uses coroutines to let it use our asynchronous I/O abstraction. That approach won’t work for memory mapped I/O. You could use a real thread pool but I’m not sure how performance will be.
A storage engine that doesn’t use our I/O abstractions also won’t be simulatable, so I will be somewhat nervous about correctness.