Recommendations

Edit online

For the configuration of an ISFC logical link, in general, we confirm the recommendations of the z/VM performance report (see ISFC Improvements) which states that four FCTC devices per FICON channel should be configured.

However, in addition, the effects resulting from z/VM's convention from using certain FCTC devices unidirectionally (see Avoidance of write collisions) should be taken into consideration.

For that reason, we extend the recommendation of the z/VM performance report, and recommend using he following procedure to determine the set of FCTC devices used for an ISFC logical link:

  1. Determine NC, the number of FICON channels that are available for being used in context of the ISFC logical link
  2. Determine NDC, the number of devices per FICON channel to be used, as follows:
    • Initially, assume NDC = 4
    • If NDC * NC > 14, reduce NDC such that NDC * NC <= 14
  3. Configure the IOCDS for the FCTCs, as follows:
    • Select one FICON channel to hold the “unidirectional” devices
    • Determine a device number range that is large enough to provide device numbers for all needed FCTC devices. Unless device number constraints exist in your installation, consider 16 devices for each “normal” FICON channel, and two times 16 devices for the FICON channel selected to hold the “unidirectional” devices. While only few of all these FCTC devices will actually be used for the ISFC logical link, the subdivision into subranges provides for a better structure of the IOCDS configuration.
    • For the unidirectional FICON channel, configure two ranges of FCTC devices, with one “low” range at the start, and one “high” range at the end of the determined device number range
    • For each of the other FICON channels, configure one range of FCTC devices, with device numbers from within the determined device number range
  4. In z/VM, configure the ISFC logical link, as follows:
    • Configure 2 FCTC devices from the “low” range(s) configured in step 3.
    • Configure NDC FCTC devices for each of the other FICON channels
    • Configure 2 FCTC devices from the “high” range(s) configured in step 3.
    • If NDC is less than four, and not yet sixteen FCTC devices are configured, add additional FCTC devices in a symmetrical manner, until sixteen FCTC devices are configured.

As an example, consider the use of four FICON channels with CHPIDs 4A, 4B, 4C and 4D:

  • Select the FICON channel with CHPID 4A to hold the unidirectional devices
  • Determine NDC as 3 (because 3 * 4 = 12 < 14)
  • In the IOCDS, for example define FCTC devices as follows:
  • D000 to D00F on 4A
  • D010 to D01F on 4B
  • D020 to D02F on 4C
  • D030 to D03F on 4D
  • D040 to D04F on 4A
  • Configure the ISFC logical link, in a first step with 3 FCTC devices per FICON channel:
    D000 D001 
D010 D011 D012 
D020 D021 D022 
D030 D031 D032 
D040 D041
  • Because not yet sixteen FCTC devices are configured, and for some FICON channels less than four FCTC devices are usable in either direction, add more FCTC devices in a symmetrical and equal manner: 
    D000 D001 D002
D010 D011 D012
D020 D021 D022 D023
D030 D031 D032 
D040 D041 D042
  • With this ISFC logical link configuration, in either direction four FCTC devices are available data transfer from each of the FICON channels with CHPIDs 4A and 4C, while three FCTC devices are available from each of the FICON channels with CHPIDs 4B and 4D. The device numbers in italics are the newly added ones, the underlined devices numbers are those of the FCTC devices selected by z/VM for unidirectional use.

As another example, consider the use of two FICON channels with CHPIDs 4A and 4B:

  • Select 4A to hold the unidirectional devices
  • Determine NDC as 4 (because 4 * 2 = 8 < 14)
  • In the IOCDS, for example define FCTC devices as follows:
  • D000 to D00F on 4A
  • D010 to D01F on 4B
  • D020 to D02F on 4A
  • Configure the ISFC logical link, in a first step with 4 FCTC devices per FICON channel: 
    D000 D001 
D010 D011 D012 D013
D020 D021
  • Because not yet sixteen FCTC devices are configured, and for some FICON channels less than four FCTC devices are usable in either direction, add more FCTC devices in a symmetrical and equal manner: 
    D000 D001 D002
D010 D011 D012 D013
D020 D021 D022
  • With this ISFC logical link configuration, in either direction four FCTC devices are available for data transfer from each of FICON channels 4A and 4B. The device numbers in italics are newly added, the underlined devices numbers are those of the FCTC devices selected by z/VM for unidirectional use.

Another recommendation applies if the connected LPARs reside on the same CEC, and FCTC devices from more than one FICON channel are configured for the connecting ISFC logical link. In this case n ISFC logical link configuration with "straight" (see ISFC logical link configurations with "straight" FCTC connections) or "crossed" (see ISFC logical link configurations with "crossed" FCTC connections) FCTC connections is possible in the IOCDS. Because our measurements show that "straight" FCTC connections result in slightly better transfer times and data rates than "crossed" FCTC connections, our recommendation is to use "straight" FCTC connections.