FICON Express2 Channel Performance Version 1.0 - IBM

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FICON Express2 Channel Performance Version 1.0 Page 27 can look at the FICON processor, bus and link utilizations. If any one of these utilizations is above 50% then overuse of the FICON channel could be contributing to additional PEND and CONNECT time delays. If, on the other hand, PEND and CONNECT times are high and FICON channel utilizations are less than 50%, then overuse of a FICON director port or control unit port could be contributing factors. If FICON channels from multiple CECs are connected to the same director destination port, then one must add up the activity from all the CECs to determine the total destination port activity. This total activity level should be less than the “knee of the curve” points depicted in the measurement results that appear in the white papers for the specific native FICON DASD or TAPE product that is being used. One of the basic differences between native FICON and ESCON channel performance is the CONNECT time component of response time. Since an ESCON channel is only capable of executing one I/O at a time, the amount of time that it takes to execute the protocol + data transfer components of CONNECT time is relatively constant from one I/O operation to the next with the same exact channel program. With FICON however, CONNECT time can vary from one execution of a channel program to another. This is a side effect of the multiplexing capability of FICON. Since both the channel and the control unit can be concurrently executing multiple I/O operations, the individual data transfer frames of one I/O operation might get queued up behind the data transfer frames of another I/O operation. So, the CONNECT time of an I/O with FICON is dependent upon the number of I/O operations that are concurrently active on the same FICON channel, link and control unit connection. Multiplexing also means that the start and end of the CONNECT time for one native FICON I/O operation can overlap the start and end of the CONNECT time for several other native FICON I/O operations. But AVG CONN TIME for large block size transfers should be significantly less for native FICON channels than for the same transfer size on ESCON or FICON Bridge channels due to the much faster (2 Gbps or 200 MB/sec) link transfer speeds of native FICON vs. the 20 MB/sec link transfer speed of ESCON. Several examples of CONNECT times at various levels of FICON Express2 channel processor, bus and link utilizations are shown in the FICON Express2 benchmark measurement results displayed in Figures 5 through 17 of this paper. Little’s Law can be used to estimate the average number of open exchanges or simultaneously active I/O’s or multiplexing level for both a FICON channel and a control unit port for a given RMF interval. This formula is essentially a variation of the formula for calculating I/O intensity levels which has been used for years to identify “hot spots” in an I/O configuration. I/O intensity levels are calculated by multiplying total response times by activity rates. The number of I/O’s that are simultaneously active and transferring data between the channel and the control unit can be determined by multiplying the CONNECT time component of response time (in units of seconds or milliseconds(ms) times 0.001) by the activity rate (in
FICON Express2 Channel Performance Version 1.0 Page 28 units of I/Os per second). If there is only 1 LCU (logical control unit) connected to a single set of FICON channels, then the average number of open exchanges can be calculated by multiplying the activity rate for that LCU by the sum of the “CMR + CONN + DISC” times for that LCU divided by the number of channels in the path group for that LCU. If there are multiple LCUs connected to a set of FICON channels, then the results of this calculation needs to be summed for all these LCUs. Similarly, to determine the average number of exchanges for a given physical CU port if there are multiple sets of channels from multiple LPARs on multiple CECs connected to the same set of CU ports, this calculation needs to be done for each LCU for each LPAR and then summed to get the total for the CU port. In any case, if the result of this calculation is a higher than normal value for your workload, then one must look at each of the components of the formula to determine the cause of the high number of open exchanges. AVG CMR DLY or “command response” delay time is a new field that has been added to the RMF Device Activity report for FICON. An example of this is displayed in Figure 23 above. AVG CMR DLY time is a subset of PEND time. As shown in Figure 24, when a channel opens a new exchange with a control unit by sending the first command in the channel program to the control unit, the control unit responds with a CMR. Architecturally, the official end to PEND time (for both FICON and ESCON) is designated by the time when the channel receives the CMR signal from the control unit. FICON Command/Data Transfer CCW=Channel Control Word CE=Channel End DE=Device End CMR = Command Response FICON Express2 Channel total pend time ssch CCW1 CCW2 CCW3 CE/DE Control Unit cmr time...subset of pend CCW1 CMR Figure 24 CMR cmd End cmd End cp ---> sap ---> channel ---> cu port ---> channel Device CMR time begins when exchange begins & ends when pend time ends
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