ISFC logical link configuration with three shared FICON channels and "crossed" FCTC connections
Figure 1 shows an ISFC logical link configuration with three shared FICON® channels and "crossed" FCTC connections.
Figure 2 shows how the relocation and the quiesce times scale in this case when the amount of FCTC connections configured for the ISFC logical link is varied.
Observation
When three FICON channels are used in a crossed manner, the shortest relocation and quiesce times are already achieved with two and with four FCTC connections per FICON channel. Compared to the corresponding "straight" setup (compare Figure 2 with Figure 2), the relocation times are about 11% longer, except for the case with one FCTC connection per FICON channel, which is 8% shorter.
Conclusion
An explanation for the generally better results of the "straight" configurations is given in ISFC logical link configuration with two shared FICON channels and "crossed" FCTC connections.
The absence of an improvement when adding the third FCTC connection per FICON channel probably again is caused by the way z/VM® designates certain FCTC connections for unidirectional use (see Avoidance of write collisions).
The FCTC connection usage for the cases “6 (2)” and “9 (3)” is detailed Table 1. The relationship between FCTC device numbers and FCTC connections can be tracked by means of Figure 1.
| Configuration | FCTC connections | |||||
|---|---|---|---|---|---|---|
| # configured per | designated for use by LPAR1 | used for data transfer LPAR2 LPAR1 | ||||
| Link | CHPID | 3D/3C | 3E/3D | 3C/3E | ||
| "6 (2)" | 6 | 2 | D010 | D011 | D060, D061 | D080, D081 |
| "9 (3)" | 9 | 3 | D010, D011 | D012 | D060, D061, D061 | D080, D081, D082 |
As described in Avoidance of write collisions, z/VM dedicates one or two of the FCTC connections with the lowest device numbers for use by the system with the lexicographically lower name, which is LPAR2 in this case. With more than eight FCTC connections, two FCTC connections are designated for use by LPAR1.
Table 6 shows the effect of this approach by example of the configurations “6 (2)” and “9 (3)”:
- With configuration “6 (2)”, from each of the FCTC connections on FICON channels 3E/3D and 3C/3E, two are used for relocation data transfer
- With configuration “9 (3)”, from each of the FCTC connections on FICON channels 3E/3D and 3C/3E, three are used for relocation data transfer
- In both cases, from the FCTC connection on FICON channels 3D/3C, only one FCTC connection is used for relocation data transfer, leaving about one 15 to 20 % of those FICON channels capacity unused.
Apparently, the additional third FCTC connection on FICON channels 3E/3D and 3C/3E did not have a positive effect.