Type of signaling path

A CTC provides a direct path between two systems, while sending a message through a coupling facility is a two step, push-pull process. Thus, depending on message size and the type of coupling facility link, CTCs are sometimes faster than using coupling facility structures.

These are examples of XCF response time, (MXFER TIME), from controlled experiments in a test environment. The unidirectional CTCs have a single PATHIN or PATHOUT per physical CTC. The bi-directional CTCs have a pair of PATHIN AND PATHOUT defined for physical CTC. The 4 bi-directional CTCs have 4 pairs of PATHIN and PATHOUT per physical CTC.

XCF internally times the various signals and gives preference to the faster paths. For example, in Figure 1, compare the number of requests for DEFSMALL which were sent through the structure to the number which were sent through the CTCs. It should be noted XCF does not attempt to balance the workload across paths; once it finds a fast path, it continues to use it. APAR OW38138 describes changes which improves the path distribution.

Figure 1. Example: XCF Path Statistics

                    XCF PATH STATISTICS
-------------------------------------------------------------------
                      OUTBOUND FROM JA0                     
-------------------------------------------------------------------
        T FROM/TO                                 T FROM/TO
TO      Y DEVICE, OR      TRANSPORT     REQ   AVG Q
SYSTEM  P STRUCTURE       CLASS         OUT   LNGTH    AVAIL   BUSY  RETRY  
JC0     S IXCPLEX_PATH1   DEFAULT      1,744   0.00    1,176      0      0
        S IXCPLEX_PATH2   DEFSMALL     8,582   0.01    8,362    220      0 
        C C600 TO C614    DEFSMALL   20,223    0.01   20,160     63      0 
        C C601 TO C615    DEFSMALL   23,248    0.01   23,229     19      0 
        C C602 TO C616    DEFSMALL   23,582    0.01   23,568     14      0

In many environments, the difference in response time between CTCs and coupling facility structures is indiscernible and using coupling facility structures certainly simplifies management of the configuration.