"Why would you want to use an NBD rather than a more common
file-sharing protocol? Several scenarios come to mind:
"If client can provide better tools for low-level maintenance of
disks (newer versions of fsck, for instance) than the server,
providing NBD access may make sense.
"The client may have need of expanded network disk space, but a
traditional network filesystem may not be adequate. For instance,
you might be testing new filesystem drivers or need features of a
particular filesystem and those features might not be properly
handled by NFS or SMB/CIFS.
"The server might not support the filesystem or data structures
on the device you wish to export"