FCTC connection I/O rates for non-shared ISFC logical link configurations

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This section shows FCTC I/O rates for one, two and three non-shared FICON® channels.

One non-shared FICON channels with twelve FCTC connections each

Figure 1 shows the I/O rates per FCTC connection for the one FICON channel that is used by the sending LPAR2. The receiving LPAR1 uses a different FICON channel. The color coding for FICON channel with CHPID 3E is orange.

Figure 1. One non-shared FICON channels with twelve FCTC connections each: FCTC connection I/O rates
Graph showing One non-shared FICON channels with twelve FCTC devices each: FCTC device I/O rates

Observation

Two FCTC connections are designated by z/VM® for unidirectional use (lower orange graphs). The other FCTC connections uniformly exhibit I/O rates of about 45 SSCH/s (coinciding orange graphs), resulting in a combined I/O rate for the ISFC logical link of about 450 SSCH/s (10 * 45 SSCH/s). The link data rate is about 342 MB/s.

Two non-shared FICON channels with six FCTC connections each

Figure 2 shows the I/O rates per FCTC connection for the two FICON channels that are used by the sending LPAR2. The receiving LPAR1 uses two different FICON channels. The color coding for FICON channel with CHPID 3E is orange, that for the FICON channel with CHPID 4C is blue.

Figure 2. Two non-shared FICON channels with six FCTC connections each: FCTC connection I/O rates

Graph showing two non-shared FICON channels with six FCTC devices each: FCTC device I/O rates

Observation

Two FCTC connections are designated by z/VM for unidirectional use (lower orange graphs), both on the FICON channel with CHPID 3E. The other FCTC connections on that FICON channel uniformly exhibit I/O rates of about 117 SSCH/s each (coinciding topmost orange graphs), resulting in a combined I/O rate for the FICON channel with CHPID 3E of about 468 SSCH/s (4 * 117 SSCH/s). The FCTC connections on the FICON channel with CHPID 4C uniformly exhibit I/O rates of about 78 SSCH/s each (coinciding middle blue graphs), resulting in a combined I/O rate for the FICON channel with CHPID 4C of about 468 SSCH/s (6 * 78 SSCH/s). The combined I/O rate of the ISFC logical link is 936 SSCH/s, which is a little bit more than twice the I/O rate achieved with only one non-shared FICON channel. The link data rate is about 708 MB/s, which again is a little bit more than twice the data rate achieved with only one non-shared FICON channel.

Three non-shared FICON channels

Figure 3 shows the I/O rates per connection for the three FICON channels that are used by the sending LPAR2. The receiving LPAR1 uses three different FICON channels. The color coding for FICON channel with CHPID 3E is orange, that for the FICON channel with CHPID 4C is blue, and that for the FICON channel with CHPID 4D is green.

Figure 3. Three non-shared FICON channels: FCTC connection I/O rates
Graph showing Three non-shared FICON channels: FCTC connection I/O rates

Observation

Two FCTC connections are designated by z/VM for unidirectional use (lower orange graphs), both on the FICON channel with CHPID 3E. The other FCTC connections on that FICON channel uniformly exhibit I/O rates of about 214 SSCH/s (coinciding middle blue graphs), resulting in a combined I/O rate for the FICON channel with CHPID 3E of about 428 SSCH/s (2 * 214 SSCH/s). The FCTC connections on the FICON channels with CHPID 4C and 4D (coinciding middle blue and green graphs) exhibit I/O rates of about 115 SSCH/s each (coinciding middle blue graphs), resulting in a combined I/O rate for each of the FICON channel with CHPID 4Cand 4D of about 460 SSCH/s (4 * 115 SSCH/s). The combined I/O rate of the ISFC logical link is 1348 SSCH/s, which is about three times the I/O rate achieved with only one non-shared FICON channel. The link data rate is about 1017 MB/s, which is a little bit more than three times the data rate achieved with only one non-shared FICON channel.

Conclusion

As with their shared counterparts, the I/O rates and data rates for non-shared ISFC logical link configurations scale linearly with the amount of FICON channels in use. The effects resulting from some FCTC connections being designated for unidirectional use by z/VM are similar.

Given a fixed amount of FICON channels being used for an ISFC logical link, the data rates for a “remote” link (the z/VM members reside in LPARs on different CECs), are almost double as high as those for a “local” link (the z/VM members reside in LPARs on the same CEC).

For example, compare Figure 3 (non-shared, three FICON channels) with Figure 2 (shared, three FICON channels): In both cases, three FICON channels are used for the ISFC logical link configuration. The averaged data rate per FICON channel is about 339 MB/s (1017/3 MB/s) for the non-shared, send only case, and averaged send data rate per FICON channel for the shared case is 197 MB/s (595/3 MB/s).

Possible reasons for the non-shared case not reaching twice the data rate of the shared case might be

  • For the configuration with shared FICON channels and "straight" FCTC connections, that the FICON channels might optimize at least some of the data transfer by transferring data directly and not passing it through the switch
  • That the FICON channels in the non-shared case are used almost only in one direction (send only), while in the shared case the FICON channels are used equally in either direction (send and receive).

Insofar, the results observed here are similar to those published in the “IBM System z9® I/O and FICON Express4 Channel Performance” paper (see FICON Express4 Channel Performance). There a data rate of 350 MB/s per FICON channel was observed for a use-case exhibiting a “large sequential read/write mix” (bi-directional), as opposed to 330 MB/s per FICON channel for a use-case exhibiting “Large sequential all reads or all writes” (uni-directional).