DASD is a major area of interest when we are talking about information needed for performance analysis on z/OS, relative to considering the cost of monitoring on z/OS. When you look at the OMEGAMON suite on z/OS consider the following: OMEGAMON z/OS monitors DASD, OMEGAMON Storage monitors DASD, OMEGAMON DB2 monitors DASD, OMEGAMON CICS monitors DASD, OMEGAMON IMS monitors DASD. See a potential pattern here? Each of the core OMEGAMON monitors provides some form of DASD or I/O analysis, and this is for very good reasons. Understanding I/O is an essential component of understanding the performance of the system and the workload. The trade-off comes with understanding the optimal method to monitor DASD relative to the cost.
As the prior paragraph implies, if you have the full suite of OMEGAMON tools, you have the potential to monitor the same string of DASD devices multiple times. So that brings us to the first recommendation: monitor DASD once, not multiple times. If you use OMEGAMON z/OS as your primary DASD analysis tool, then you can consider being more selective when enabling DASD analysis in OMEGAMON CICS or OMEGAMON DB2. In a prior post I mentioned how you can leave Object Analysis in OMEGAMON DB2 off by default, but turn on as needed. That is a good strategy since there are times where the Object Analysis data is useful, but you probably don't need to collect it on an ongoing basis.
Now let's consider OMEGAMON CICS. If you have OMEGAMON z/OS, then you don't really need to set DASD exceptions in OMEGAMON CICS, plus you may want to carefully consider enabling I/O clocks and counters for CICS task history (ONDV - online data viewing). When setting up task history parms in the KC2GLBxx module, consider your options related to I/O. DB2_CLOCKS_AND_COUNTERS, as well as VSAM, ADABASE, DLI, etc. are potentially expensive data collection options. If you don't really need it, then don't turn it on.
There are more considerations specific to OMEGAMON Storage and OMEGAMON z/OS, and I will cover this in a later post.