The NBD is one great project :-)
Currently there are many Distributed Storages projects such as the Gluster and Ceph are widely use, but they couldn't support NBD directly. For example, for the Gluster, if we want to export the volume file as one NBD block device we must mount the volume first and then use the nbd utils to export it again, which a little ugly.
To make it more efficient, we also need one common and generic utils, something like , to support them directly. And good news is that I have a working code with most basic things @nbd-runner project. It is hosted under the gluster/ currently and I'd like to request a repository under  to host this.
Now the Gluster handler coding is done, the Ceph and Azure is in progress now.