```ublk-nbd[1] is available now. Basically it is one #nbd client, but totally implemented in userspace […] io handling is based on #liburing[3] […] ublk-nbd supports both tcp and unix socket, and allows to enable #io_uring
send zero copy via command line '--send_zc', see details in README[4]. […] ublk-nbd IOPS is higher by ~40% than […]```

https://lore.kernel.org/all/Y8lSYBU9q5fjs7jS@T590/

[1] https://github.com/ming1/ubdsrv/tree/master/nbd
[3] https://github.com/axboe/liburing
[4] https://github.com/ming1/ubdsrv/blob/master/nbd/README.rst