We tried to add io_uring support to libnbd (and indeed still hope to do so). There's a bit of an "impedance mismatch" between existing event-driven code and io

We tried to add io_uring support to libnbd (and indeed still hope to do so). There's a bit of an "impedance mismatch" between existing event-driven code and io_uring, not impossible, but also not completely easy to just convert code. Especially when you want to keep the non-io_uring code working (for old Linux, BSD etc). To give an example, libnbd does a lot of send and recv system calls interleaved, for sending requests and receiving replies, with a complicated state machine to keep track of when we would block. It's still not obvious how to convert this to an io_uring style where you have to add the send/recv requests to the submission queue and then separately pick up the responses from the completion queue. It's a particular problem dealing with possible errors and keeping the API the same (if you just call exit when you hit any error, it's a lot easier). If you're writing all new code then io_uring is a great choice.