## D.1 Overview 

As the volume of data that is generated and transmitted by technologies that are driven by cloud, mobile, analytics, and social computing applications grows, pressure increases on business IT organizations to provide fast access to that data across the web, application, and database tiers that comprise most enterprise workloads. SMCs helps to access data faster and with less latency. They also reduce CPU resource consumption over traditional TCP/IP for communications. 

## D.2 Shared Memory Communication over RDMA 

IBM z14 delivers two new RoCE features: 25GbE RoCE Express2 (FC 0430) and 10GbE RoCE Express2 (FC 0412). The 10GbE RoCE Express (FC 0411) feature can be carried forward to z14 (from z13/zEC12). SMC-R (RoCE) includes the following features: 

GLYPH<SM590000> Remote Direct Memory Access (RDMA) technology provides low latency, high bandwidth, high throughput, and low processor utilization attachment between hosts. 

GLYPH<SM590000> SMC-R is a protocol that allows TCP applications to benefit transparently from RDMA for transferring data. Consider the following points: 

– SMC-R uses RoCE Express/Express2 adapter as the physical transport layer. 

– Initial deployment is limited to z/OS to z/OS communications with a goal to expand usage to more operating systems, and possibly appliances and accelerators. 

GLYPH<SM590000> Single Root I/O Virtualization (SR-IOV) technology provides the capability to share the RoCE ExpressExpress2 adapter between logical partitions (LPARs). Consider the following points: 

– RoCE Express2 supports 63 Virtual Functions (VFs) per physical port for a total of 126 VFs per PCHID. 

– RoCE Express supports 31 VFs per PCHID. 

GLYPH<SM590000> The maximum number of RoCE Express and RoCE Express2 features that is supported per z14 is eight (combined). 

## D.2.1 RDMA technology overview 

RDMA over Converged Ethernet (RoCE) is part of the InfiniBand Architecture Specification that provides InfiniBand transport over Ethernet fabrics. It encapsulates InfiniBand transport headers into Ethernet frames by using an IEEE-assigned Ethertype. One of the key InfiniBand transport mechanisms is RDMA, which is designed to allow the transfer of data to or from memory on a remote system with low latency, high throughput, and low CPU utilization. 

Traditional Ethernet transports, such as TCP/IP, typically use software-based mechanisms for error detection and recovery. They also are based on the underlying Ethernet fabric that uses a “best-effort” policy. With the traditional policy, the switches typically discard packets that are in congestion and rely on the upper-level transport for packet retransmission. 

However, RoCE uses hardware-based error detection and recovery mechanisms that are defined by the InfiniBand specification. A RoCE transport performs best when the underlying Ethernet fabric provides a lossless capability, where packets are not routinely dropped. 

