ExaFusion is Direct-to-Wire protocol which allows DATABASE processes send messages directly over the Infiniband network bypassing the OS kernel. This means database processes make no syscal to OS. Database process directly write data to IB HCA bypassing normal networking software stack. This is the one of reasons why IB improves the response time in Oracle Exadata Database Machine. Data is transferred directly from user space of one process on one node to user space to other process on other node (no context switch to kernel space). But, ExaFusion is disabled by default ! To enable ExaFusion look to the documentation:
http://docs.oracle.com/database/121/REFRN/GUID-A0612249-0A65-476D-A2B8-FA94DF5EEC7F.htm#REFRN-GUID-A0612249-0A65-476D-A2B8-FA94DF5EEC7F You should to setEXAFUSION_ENABLED=1 in the database parameter file.
As we know in previous editions 1/8 Exadata = 1/4 Exadata by HW.
This means that storage servers had a 12 hard disks and 4 flash cards, but 6 hard disks and 2 flash cards were disabled. But they still were installed in the server.
In the X6-2 we have a new feature from Oracle: "Eighth Rack HC storage servers have half the cores enabled and half the disks and flash cards removed."
This means that X6-2 1/8 contain 6 hard disks and 2 flash cards.
The EF edition has the half flash cards disabled, but installed in the storage server.