HP 1606 FICON Administrator's Guide v6.4.0 (53-1001771-01, June 2010) - Page 50

4 FICON emulation overview XRC emulation The eXtended Remote Copy (XRC) application is a direct attached storage device (DASD) application that implements disk mirroring, as supported by the disk hardware architecture and a host software component called System Data Mover (SDM). Within this application a specific application channel program invokes a well constrained type of channel program called a Read Record Set (RRS) channel program. This channel program is used to read only updates to virtual disk volumes (record sets) after they have been brought into synchronization with the aim of writing only record updates to a mirrored volume. The RRS channel program accesses primary volumes from a remote host for the purpose of reading these record sets (updates) that is supported by XRC emulation. The emulation feature thus allows the primary volume to be located at a distance from its mirrored secondary without encountering performance degradation associated with a specific FICON facility called IU pacing. FIGURE 10 XRC emulation Figure 10 shows how the primary volume and the secondary mirrored volume may be geographically distant across an IP WAN. The latency introduced by greater distance creates delays in anticipated responses to certain commands. The FICON pacing mechanism may interpret delays as an indication of a large data transfer that could monopolize a shared resource, and react by throttling the I/O. XRC emulation provides local responses to remote hosts, eliminating distance related delays. You can use the Brocade 7500, 7800, an FR4-18i, or an FX8-24 blade with FICON emulation. Tape Pipelining Tape pipelining refers to the concept of maintaining a series of I/O operations across a host-WAN-device environment and should not be confused with the normal FICON streaming of CCWs and data in a single command chain. Normally tape access methods can be expected to read data sequentially until they reach the end-of-file delimiters (tape marks) or to write data sequentially until either the data set is closed or an end-of-tape condition occurs (multi-volume file). The emulation design strategy attempts to optimize performance for sequential reads and writes, while accommodating any other non-conforming conditions in a lower performance non-emulating frame shuttle. Since write operations can be expected to comprise the larger percentage of I/O operations for tape devices (for archival purposes) they are addressed first. 38 FICON Administrator's Guide 53-1001771-01