Freeing Shared Memory with onmode -F

The database server does not automatically free the shared-memory segments that it adds during its operations. Once memory has been allocated to the database server virtual portion, the memory remains unavailable for use by other processes running on the host computer. When the database server runs a large decision-support query, it might acquire a large amount of shared memory. After the query completes, the database server no longer requires that shared memory. However, the shared memory that the database server allocated to service the query remains assigned to the virtual portion even though it is no longer needed.

The onmode -F command locates and returns unused 8-kilobyte blocks of shared memory that the database server still holds. Although this command runs only briefly (one or two seconds), onmode -F dramatically inhibits user activity while it runs. Systems with multiple CPUs and CPU VPs typically experience less degradation while this utility runs.

It is recommended that you run onmode -F during slack periods with an operating-system scheduling facility (such as cron on UNIX). In addition, consider running this utility after you perform any task that substantially increases the size of database server shared memory, such as large decision-support queries, index builds, sorts, or backup operations. For additional information on the onmode utility, see the IBM Informix: Dynamic Server Administrator's Reference.