FCTC connection I/O rates

Figure 1 shows the I/O rates per FCTC connection achieved during the relocation process for the case that two shared FICON® channels and five "straight" FCTC connections are used. The FCTC connection I/O rates are measured in SSCH/s (Start Subchannel per second). The I/O rate data values were obtained using the “General I/O Device Performance” log (DEVLOG, FCX168) of the z/VM® Performance Toolkit.

Observation

The first two FCTC connections (coinciding two bottom blue graphs, lowest device numbers D050 and D051) are not used for relocation data transfer in this case because of z/VM dedicating these FCTC connections for unidirectional use (see Avoidance of write collisions).

The remaining three FCTC connections (coinciding three topmost blue graphs, device numbers D052 to D054) on the same FICON channel (CHPID 3D) are loaded with a higher I/O rate than the FCTC connections (coinciding five middle green graphs, device numbers D0A0 to D0A4) on the other the FICONchannel (CHPID 3E).

Conclusion

The relationship between the lower and the higher I/O rate per FCTC connection is inversely proportional to the number of FCTC connections effectively used (here 5:3). The average I/O rate per CHPID is 270 SSCH/s for both FICON channels (CHPID 3D: 3 * 90 SSCH/s; CHPID 3E: 5 * 54 SSCH/s).

For an explanation, recall that the test workload causes a CPU heavy load situation, but no shortage of memory or paging resources in both the sending and the receiving z/VM system. It appears that in this situation z/VM tries to issue I/O operations evenly across the FICON channels of which FCTC connections are configured for an ISFC logical link, regardless of how many FCTC connections are actually used for data transfer. In this case, even while the three FCTC connections on the FICON channel with CHPID 3D are loaded with a higher data rate, this does not cause a significant reduction of the overall ISFC logical link performance.