So far I've talked about the "Big Three" (z/OS, CICS, DB2). Now, I will start to address the DASD and I/O monitoring considerations for some of the other OMEGAMON tools.
OMEGAMON XE for IMS provides DASD and I/O related information in several different areas. In the real time 3270 displays, OMEGAMON IMS provides IMS database I/O rates, IMS Log I/O rate info, Long and Short Message Queue rate info, Fast Path Database I/O info, plus information on the various OSAM and VSAM database buffer pools. This information is often useful from a diagnostic and tuning standpoint, and there are no real overhead concerns in terms of collecting the data.
There are a couple areas where DASD, I/O, and I/O related information can impact the cost of monitoring. One area is Bottleneck Analysis. Bottleneck Analysis is a very useful and powerful analysis tool for understanding where IMS workload is spending it's time. One of the sub-options of Bottleneck Analysis is a database switch (DBSW option). If you have Bottleneck Analysis ON, but the database switch option OFF, you will save some CPU in the OMEGAMON IMS collector task. Another consideration is Epilog history. Epilog does a nice job of gathering historical performance analysis information, but you can save some cost of collection by turning off DASD collection in the Epilog history options. This is done by specifying the NORESC(DAS,DEV) option.
Probably the biggie, related to database and I/O monitoring in OMEGAMON IMS is the Transaction Reporting Facility (TRF). If TRF is enabled, OMEGAMON IMS will typically generate records on transaction and database activity into the IMS log. This data is often useful for performance analysis and charge back, but it is potentially voluminous. If you turn it on, be aware of the options for TRF, and recognize that there will be costs in terms of additional CPU usage by the OMEGAMON collector task, and more data written to the IMS log files.