Improving log capacity

If you specify a capacity that is too small, Db2 might need to access data in the archive log during rollback, restart, and recovery. Accessing an archive takes a considerable amount of time.

The following subsystem parameters control the capacity of the active log. In each case, increasing the value that you specify for the parameter increases the capacity of the active log.

NUMBER OF LOGS field on installation panel DSNTIPL

Controls the number of active log data sets that you create. Having too many or too few active log data sets can each be problematic. This information is summarized in the following table.

Table 1. The effects of installation options on log data sets

Value for ARCHIVE LOG FREQ	Value for NUMBER OF LOGS	Result
Low	High	Many small data sets. Can cause operational problems when archiving to tape. Checkpoints occur too frequently.
High	Low	Few large data sets. Can result in a shortage of active log data sets.

You can use the change log inventory utility (DSNJU003) to add more active log data sets to the BSDS.

ARCHIVE LOG FREQ field on installation panel DSNTIPL

Where you provide an estimate of how often active log data sets are copied to the archive log.

UPDATE RATE field on installation panel DSNTIPL

Where you provide an estimate of how many database changes (inserts, update, and deletes) you expect per hour.

The Db2 installation CLIST

Uses UPDATE RATE and ARCHIVE LOG FREQ to calculate the data set size of each active log data set.

Certain subsystem parameters

The following subsystem parameters control the interval between checkpoints:

• CHKTYPE
• CHKFREQ
• CHKLOGR
• CHKMINS