Could someone explain why/how NBD is better than just using a linux host as an iscsi target? Googling NBD vs iSCSI shows old articles with no real solid conclusion.
NBD is an extremely simple protocol. Read range, write range, delete range, sync -- that's it. If you want to implement an NBD server from scratch, you can totally do so in an afternoon. I have done this and use it in production: https://github.com/sandstorm-io/blackrock/blob/master/src/bl...
iSCSI is comparatively far more complex. It's a TCP-based adaption of the SCSI protocol, which has existed for decades as a way to talk to hard drives. As I understand it, you can pass arbitrary SCSI commands over iSCSI; see: https://en.wikipedia.org/wiki/SCSI_command iSCSI is enterprise-y and has a bigger ecosystem. You can netboot a diskless machine into Windows over iSCSI (I do this: http://kentonsprojects.blogspot.com/2011/12/lan-party-house-...).
Personally I like NBD a lot better because the simplicity means you can build new, cool things with it. But there are others who would say that NBD is a toy compared to iSCSI.
Just because `write()` returned successfully does not mean that the data has been written to disk (whether you're using NBD or otherwise). The application needs to call `fsync()` to force writes to disk and get confirmation of success. An `fsync()` will send all pending NBD_CMD_WRITEs followed by NBD_CMD_FLUSH and will only return success when all of these have completed successfully.
I would apologise for the code, but "how small can I make this" was sort of the point...