here is an excellent discussion  [URL]
We have a CX4-120 and it has 4 FC shelves.  We just installed a shelf of all SATA II 1TB drives.  Is there any performance hit by mixing SATA and FC on the same BUS ?  We only have one bus, bus 0.  Can someone tell me about this?
Adding SATA Shelf to otherwise all FiberChannel Shelves