I have created an OpenNebula storage driver for Saturnring open-source scale-out iSCSI-based block storage. It is open-sourced as part of Saturnring (Apache2 license). This could benefit users needing to deploy a block storage system with OpenNebula.
This OpenNebula driver supports persistent block devices, backed by Saturnring iSCSI storage targets, for OpenNebula VMs. This driver borrows ideas and code from the addon-iscsi sample (https://github.com/OpenNebula/addon-iscsi)
More information about the driver: https://github.com/sachinkagarwal/saturnring/blob/master/saturn-opennebula-driver/README.md