Tuning to remove I/O resource contention
A performance problem can occasionally be traced to I/O contention. Tuning activities for I/O devices, DASD storage access, channel usage, or control unit contention should not be confined to system-related devices. For IMS™, an important factor is the volume of I/O activity, because reduced I/O activity reduces contention.
See I/O subsystem configuration and the considerations for database I/O analysis included in Program execution times.
Dynamic monitoring of the I/O subsystem
Typically, the I/O subsystem is not monitored dynamically, because corrective action normally requires reconfiguration of devices or data sets. This takes some time to plan and implement. However, if RMF is being run, the RMF Channel Activity and Direct Access Activity can be used to check against your objectives for I/O times, channel utilization, and device utilization.
Immediate action might not be possible. However, it might be possible to move data sets or reconfigure devices to correct the problem before the next IMS restart.
Daily monitoring
Examination of the RMF Direct Access Device Activity and Channel Activity reports, together with knowledge of the I/O configuration and data set placement, might suggest a possible tuning action along the lines of those described in Guidelines for IMS system data set placement and Dynamic monitoring of the I/O subsystem.
Detailed monitoring
If additional data is required, plan to run GTF and obtain Volume Map, Seek Histogram, and System/Job Summary (EXCP-SIO-IO data) reports. These reports can be used to analyze I/O activity in great detail.