Traditionally, large scale-up servers used cache-coherent buses for inter-processor communications.  These proprietary buses and servers are very costly and power-hungry. Today’s powerful x86 servers replace proprietary scale-up architectures with low-cost machines connected through high-speed, low-latency clustered interconnects.  This article will take an in-depth view of their cost and power benefits compared to scale-up architectures, and explain that Ethernet can be tunneled through a PCI Express (PCIe) fabric to provide a very-high-performance, low-cost cluster interconnect suitable for storage IO.