InfiniBand is an interface; just like Fibre Channel (FC) or Ethernet. As an interface, InfiniBand carries different protocols. For example, the SCSI command set can run on InfiniBand but the SCSI command set can also run on top of FC. iSCSI-carries the SCSI command set on IP (Ethernet). By comparison, SAS is an interface for attaching disk drives to servers, storage arrays to servers and to attach disk drives within storage arrays.
You can have a server with an InfiniBand host channel adapter (HCA) that attaches to an InfiniBand disk array. That InfiniBand connection can run SCSI, iSCSI or some other protocol. In turn, this can be talking to an array that has SAS disk drives attached to it. In a small environment, you might use SAS to attach a local SAS drive in a 1U blade server. Alternately, you might have SAS in a disk array coexisting with SATA drives. Otherwise, you might have network storage with iSCSI to access a disk array with SAS disk drives in it.
These technologies all work together in a complementary fashion even though they are typically positioned against each other. For example, you might hear an argument for InfiniBand vs. iSCSI the same way that FC is normally positioned against iSCSI -- they are apples and oranges. InfiniBand is an interface, so it should be compared to FC or Ethernet. Then, compare your protocols, such as iSCSI vs. "SCSI on FC" or "SCSI on InfiniBand." InfiniBand, SAS, iSCSI are all very much complementary.
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