OpenSANd is a server system that allows access to disk storage over the NBD protocol. It is designed to work particularly well with VirtualControl and thus allow KVM machines to use disk partitions over Ethernet, similar to expensive, commercial SAN systems using FibreChannel or iSCSI.
OpenSANd is part of the Kumulus software stack. If you have no clue on how to use this software, you might want to start there.
OpenSANd is hosted on sourceforge