Allow DSMAX to be set by Db2 command

Our installation has repeatedly had trouble with Db2 shutting down in a timely fashion for IPLs. We have taken dumps and sent them to IBM and IBM has stated repeatedly that the delay in shutdown involves dataset close. We set DSMAX very high and have many open datasets.

We were considering setting DSMAX to a lower number immediately prior to an IPL to allow Db2 time to close most of the datasets asynchronously, in advance, while still allowing the subsystem to remain in service until IPL-time.

The main issue with this approach is that it requires a separate ZPARM module with the different DSMAX setting to be maintained, and our Automation package to issue SET SYSPARM commands to move between DSMAX settings.

Instead, we would like to see DSMAX eligible to be set by a Db2 command, for the current subsystem-session only. This would allow us to reduce DSMAX prior to shutdown and proactively close datasets that may not be in-use, thus accelerating the actual Db2 shutdown, without actually shutting down Db2 service entirely.

This would also allow shops to increase DSMAX dynamically without modifying ZPARMs or needing access to APF libraries, should they reach a point where the current setting is leading to excessive dataset thrashing.

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| Jul 28, 2022

A further thought while considering this idea...even notwithstanding the APF-library access issues associated with changing the ZPARM using SET SYSPARM RELOAD, the dynamic decrease of DSMAX presents an additional problem, in that the ZPARM setting would take effect immediately.
While this WOULD cause Db2 to start closing open datasets, it also creates a window where the subsystem is technically OVER the DSMAX limit, so if more datasets needed to be opened for a new process, they may get 00C20113 errors.
A dynamic INCREASE of DSMAX via COMMAND would likely work the same way as doing it through ZPARM. A DECREASE, however, would require Db2 to close datasets up to the 3% margin from the NEW value of DSMAX BEFORE enforcing the new limit, to avoid any outage or RESOURCE UNAVAILABLE condition.

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Allow DSMAX to be set by Db2 command