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Technical Notes › Storage Usage › A Scenario

A Scenario

Upon returning to work after a weekend, you may find that the CA OPS/MVS working set, or the sum of real and expanded storage, is large (it may exceed 100 MB). After examining the situation, you will find that the real storage usage is not excessive but the expanded storage component of the working set is extremely large (it may exceed 200 MB). At most sites, the largest area of CA OPS/MVS virtual storage consists of the OPSLOG and is controlled by the BROWSEMAX parameter. At every OPSLOG checkpoint interval (see the description of the BROWSEINTERVAL parameter in the Parameter Reference), CA OPS/MVS explicitly releases all real storage that is used to contain OPSLOG messages that are outside the current buffer. This keeps the usage of real storage under control; however, there is no equivalent mechanism for explicitly controlling expanded storage.

Over the weekend, messages and other events continue to be collected in the OPSLOG. However, there is typically a very limited demand for expanded storage and the main expanded storage indicator, which is migration age, rises. You can use the OPS/REXX OPSSRM('MIGRATIONAGE') function or your performance monitor to determine the migration age. The operating system continues to build up pages in the expanded storage area used by the OPSLOG, even though the real storage has been explicitly freed. This occurs because there is no other demand for expanded storage. In other words, z/OS will not take expanded storage pages away from an address space unless there is a demand for those pages. Therefore, no overhead is expended discarding unneeded expanded storage pages. Also, since these pages are already backed by DIV, they can be reclaimed for use without being written to page data sets, indicating that these pages can be stolen when demand increases.

On Monday morning, you will see that as the workload increases, the expanded storage migration age decreases and the CA OPS/MVS OPSLOG-related expanded storage pages are the first pages to become eligible for stealing. By the time the system reaches its normal state, the CA OPS/MVS working set is back to its usual size.