nbdkit, our high performance, portable Network Block Device server has now been ported to macOS. It’s a command line tool and macOS is sufficiently FreeBSD-like that the port wasn’t very hard. It’s relatively full featured, including a large portion of the plugins and filters, a brand new exit-with-parent implementation, and almost all tests passing.
However one larger problem remains (for performance) which is the lack of atomic CLOEXEC when opening pipes or sockets. Linux has pipe2 and accept4. I wasn’t able to find any good equivalent on macOS, and hence most of the time we are limited to serializing some requests that could otherwise run in parallel.