SPDK's RocksDB integration really hasn't gotten a lot of love. There's really two main challenges we hit and then never revisited.

First, the IO threads in RocksDB are a thread pool that assume they perform blocking operations. That doesn't jive with SPDK's async model (nor io_uring's). We're having to message pass to an async thread and block on semaphores in the IO threads.

Second, RocksDB was heavily reliant on the kernel page cache to make it fast.

Both of these things could have changed since we last worked on the integration. I haven't kept up with RocksDB development recently.

source: am SPDK maintainer