ISFC logical link configuration with one shared FICON channel and "straight" FCTC connections

Observation

Figure 2 shows that the relocation and quiesce times improve about 30 %, as the number of FCTC connections is increased from one to four; if more than four FCTC connections are used, the times slightly degrade.

Conclusion

With four or more FCTC connections, no further improvements are achieved; in fact, with nine FCTC connections even a degradation of the relocation and quiesce times occurs.

If one shared FICON channel with "straight" FCTC connections is used, configuring four FCTC connections for the ISFC logical link provides for shortest relocation and quiesce times. Too many FCTC connections can have a negative impact.

Note that the cause of the improvements resulting from adding the third FCTC device differs from that of adding the second FCTC device. For explanation, recall that z/VM designates one FCTC connection for unidirectional use in each direction when between two and eight FCTC connections are configured for an ISFC logical link (see ISFC logical link configuration):

• With the addition of the second FCTC connection, the first FCTC connection is designated for use by the receiving LPAR2, and only the second FCTC connetion is used for transferring relocation data; however, now the write collisions – which inevitably occur in the one FCTC connection case – are avoided.
• With the addition of the third FCTC connection, still one FCTC connection is designated for unidirectional use, but the number of FCTC connections used for data transfer is doubled (two instead of one).