FICON Express2 Channel Performance Version 1.0 - IBM

FICON Express2 Channel Performance Version 1.0
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Each time a SAP encounters a busy and has to retry another path for a SSCH, additional SAP
cycles are consumed and the %IOP busy or SAP utilization will increase. One of the benefits
of native FICON is that it makes SAPs or IOPs more productive due to the reduction in
busies or RETRIES/SSCH. For the same activity rate, one should see less IOP utilization %
busy with native FICON, FICON Express and FICON Express2 channels than with ESCON
channels. One must be careful not to misinterpret IOP utilization %’s however. High IOP
utilization %’s are usually an indicator of contention especially with ESCON channels,
directors and control units. Adding additional IOPs will NOT help reduce channel
configuration contention. One must identify the source of the configuration contention and
fix it. Migrating from ESCON to native FICON configurations is a natural solution to this
problem. Figures 27 and 28 represent a dramatic example of this. Figure 27 is from a z900
ESCON configuration with a lot of contention. Specifically, for the time interval reported,
there were a total of 4.73 retries per SSCH. 4.19 of these were channel path busies. This
means that when the SAP tried to start a new I/O operation on an ESCON channel, that
channel was already busy processing another I/O and the SAP had to try to find another
ESCON channel path that was available for this I/O and on the average it did this 4.19 times
per SSCH. This means that either the ESCON channels were operating at high utilizations or
there were not enough paths per LCU defined to handle the number of I/O operations that
were being issued simultaneously to the total number of LCU’s that shared the same set of
ESCON paths. This can happen when there is a burst of activity during a subset of the total
RMF interval, e.g. for a few minutes out of a 30 minute or longer interval. There was also an
average of 0.54 director port busies per SSCH during this time interval. This means that 2 or
more ESCON channel paths most likely from multiple CECs in the same sysplex were trying
to connect to the same director port and control unit port at the same time and with ESCON
only 1 I/O operation to a given director port or CU port can be active at once. The other
I/O’s that attempt to use the same destination port will get DP busy signals. The rules of
thumb available for these statistics are:
1. keep SAP utilization or %IOP BUSY below 70%,
2. AVG Q LNGTH should be less than 1 and
3. Total RETRIES/SSCH should be less than 2 with the sum of DP, CU and DV busies per
SSCH less than 1.