Capture program startup parameters important to minimize CPU usage

Certain Capture program startup parameters affect replication performance. Use recommendations detailed in this section to minimize CPU usage.

For each of the startup parameters, there is a corresponding value in the IBMSNAP_CAPPARMS table, although those values are not listed here. All startup parameters apply to both the asncap and STRDPRCAP commands, unless otherwise stated.

commit_interval (for asncap command) or FRCFRQ (STRDPRCAP command)

Set commit_interval or FRCFRQ to a high value between 30 and 60 seconds.

This parameter specifies the approximate number of seconds that will elapse between Capture commit points. The smaller the value that you specify, the sooner changed data will be available to the Apply program, which fetches only data that has been committed. However, the smaller this value is, the greater the amount of overhead that is consumed by the Capture program. In addition to the DB2® commit itself, which causes a forced log write, there is the cost of re-preparing dynamic SQL statements. This cost increases as you register more tables for replication and thus use more CD tables. Higher values cause the Capture program to behave like a long-running batch job, which can impact DB2 restart should the Capture program fail.

Important: Set this value to a value less than the DB2 lockout value to prevent contention between the Capture worker and pruning threads.

IBM® i: This parameter tells Capture how often to write rows to the CD tables, which is especially important for programs that do not use commit control.

monitor_interval (for asncap command) or MONITV (for STRDPRCAP command)

Set monitor_interval to a high value.

This parameter specifies how frequently (in seconds) the Capture program inserts rows into the Capture monitor (IBMSNAP_CAPMON) table. Setting a high value reduces the amount of overhead for gathering Monitor statistics.

IBM i: An interval of less than 120 seconds is not supported in a System i® environment. Slow performance could result if the interval is too small.

prune_interval (for asncap command)

Set prune_interval to a high value.

This parameter specifies how frequently (in seconds) the CD, unit-of-work (UOW), Capture monitor (IBMSNAP_CAPMON), Capture trace (IBMSNAP_CAPTRACE), and signal (IBMSNAP_SIGNAL) tables are pruned. Setting a high value reduces the overhead required for processing pruning queries during each pruning interval.

sleep_interval (for asncap command)

Set sleep_interval to a moderate value between 5 and 10 seconds.

Linux, UNIX, Windows: This parameter specifies the number of seconds that the Capture program sleeps when it finishes processing the active log and determines that the buffer is empty. Set this parameter to a moderate value to minimize CPU usage for lower volume environments.

z/OS®: For z/OS data-sharing environments, this parameter specifies the number of seconds that the Capture program sleeps after the buffer returns less than half full.