Maintenance windows

A maintenance window is a time period that you define for the running of automatic maintenance activities, which are backups, statistics collection, statistics profiling, and reorganizations. An offline window might be the time period when access to a database is unavailable. An online window might be the time period when users are permitted to connect to a database.

A maintenance window is different from a task schedule. During a maintenance window, each automatic maintenance activity is not necessarily run. Instead, the database manager evaluates the system to determine the need for each maintenance activity to be run. If the maintenance requirements are not met, the maintenance activity is run. If the database is already well maintained, the maintenance activity is not run.

Think about when you want the automatic maintenance activities to be run. Automatic maintenance activities consume resources on your system and might affect the performance of your database when the activities are run. Some of these activities also restrict access to tables, indexes, and databases. Therefore, you must provide appropriate windows when the database manager can run maintenance activities.

Offline maintenance activities

Offline maintenance activities (offline database backups and table and index reorganizations) are maintenance activities that can occur only in the offline maintenance window. The extent to which user access is affected depends on which maintenance activity is running:

• During an offline backup, no applications can connect to the database. Any currently connected applications are forced off.
• During an offline table or index reorganization (data defragmentation), applications can access but not update the data in tables.

Offline maintenance activities run to completion even if they go beyond the window specified. Over time, the internal scheduling mechanism learns how to best estimate job completion times. If the offline maintenance window is too small for a particular database backup or reorganization activity, the scheduler will not start the job the next time and relies on the health monitor to provide notification of the need to increase the offline maintenance window.

Online maintenance activities

Online maintenance activities (automatic statistics collection and profiling, online index reorganizations, and online database backups) are maintenance activities that can occur only in the online maintenance window. When online maintenance activities run, any currently connected applications are allowed to remain connected, and new connections can be established. To minimize the impact on the system, online database backups and automatic statistics collection and profiling are throttled by the adaptive utility throttling mechanism.

Online maintenance activities run to completion even if they go beyond the window specified.