In recent years the capacity of mainframeclass servers has grown, and the quantity of data they are required to handle has grown with them. As a result, the existing S/390 ® I/O architecture required modifications to support an order of magnitude increase in the bandwidth. In addition, new Internet applications increased the demand for improved latency. Adapters were needed to support more users and a larger number of connections to consolidate the external network interfaces. The combination of all of the above requirements presented a unique challenge to server I/O subsystems. With the introduction of the zSeries TM comes an enhanced version of a new I/O architecture for the mainframe called queued direct I/O (QDIO). The architecture was initially exploited for Gigabit and Fast Ethernet adapters. More recently the architecture was exploited by the OSA-Express network adapter for Asynchronous Transfer Mode (ATM) and highspeed Token Ring connections, and it was exploited by HiperSockets for internal LPARto-LPAR connections. In each of these features, the TCP/IP stack is changed to tightly integrate the new I/O interface and to offload key TCP/IP functions to hardware facilities. For external communications, the offloaded functions are performed by the OSA-Express hardware microcode; for internal communications, the offloaded functions are performed in the zSeries Licensed Internal Code (LIC). The result is a significant improvement in both latency and bandwidth for sockets-based messaging which is transparent to the exploiting applications.

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