Determining actions to resolve exceptions

After you identify the possible causes of the exceptions, you can determine the best approach for tuning the areas.

Before you begin

Ensure that you have completed the steps in Understanding the status of DEDB areas and that you have identified the possible causes of the database exceptions.

About this task

In this task, you will determine the best approach for resolving exceptional states of a DEDB area. Key decision indicators are the database record profile of the area, the distribution of randomizing synonyms, and their effects on the root I/Os and database record I/Os.

By completing this task, you will be able to determine whether reorganizing the free space in the area will resolve the exceptional state or whether elaborate tuning tasks, such as tuning database definitions or randomizer, are required.

Procedure

The approach for database tuning varies depending on whether the record profile and the number of database records have changed or not.
If the record profile and the number of database records have been relatively unchanged
If exceptions that are related to free space fragmentation, free space usage, or overflow usage were reported earlier than you expected, consider reviewing the current criteria for reorganization or the current reorganization schedule, and refine them if necessary. If the exceptions are related to free space fragmentation, free space usage, or overflow usage, and if the number of database records in the area has been relatively unchanged, consider reorganizing free space, which does not require changes to the DBD definitions.
Free space reorganization is especially recommended if a free space fragmentation exception was reported or if the Freespace Analysis report indicates either or both of the following conditions:
  • Free space elements are fragmented, and the number of free space fragments that are smaller than the segment size has increased
  • The percentage of CIs that have enough total free space but that have many references to other sections of the database has increased
See Planning free space reorganization and usage monitoring for information about performing free space reorganization.
If the record profile or the number of database records have significantly changed
Determine if tuning of the DBD definitions or the randomizer is required.
As data is added and the packing density of RAA BASE increases, root segments or database records are likely to overflow to DOVF and IOVF. Overflow to DOVF and IOVF can increase the average number of physical I/Os that are required to retrieve a root or a record.
If the average database record length or the number of database records in the area has significantly increased, and the amount of free space in any section of the area has fallen below a predetermined threshold, consider expanding the area. When you expand the area, consider expanding a RAP CI or increasing the number of RAP CIs. When you increase the number of RAP CIs, also consider tuning the randomizer because the randomizing might change.
Database records might overflow from RAA BASE to DOVF or from UOW to IOVF when root segments that have unexpected sequence keys are inserted or when exceptionally large database records exist. The number of physical I/Os that are required to retrieve a root segment can increase if the root segment is in a long synonym chain. The number of physical I/Os that are required to read a database record can increase if the database record has become large.
Determine if the record profile, including key distribution, or the number of database records in the area has significantly changed from what you expected when you defined the area and when you selected the current randomizer. If the record profile or number of database records in the area have significantly changed, consider tuning the randomizer.
Tip: Tuning of a randomizer is difficult if exceptionally large database records exist in a specific UOW or UOWs. When these records exist, and if your database management policy permits, consider defining new areas that have enough CI size, and moving the large database records to the new areas. The new randomizer might need to be a two-stage randomizer because the randomizer must determine the area from a given root sequence key value before calculating a RAP number.
For details, see Tuning DEDB areas.