ASA Database Administration Guide
Backup and Data Recovery
Backup and recovery internals
The priority of writing dirty pages to the disk increases as the time and the amount of work since the last checkpoint grows. The priority is determined by the following factors:
Checkpoint Urgency The time that has elapsed since the last checkpoint, as a percentage of the checkpoint time setting of the database. The server -gc option controls the maximum desired time, in minutes, between checkpoints. You can also set the desired time using the CHECKPOINT_TIME option.
For more information, see -gc server option.
Recovery Urgency A heuristic to estimate the amount of time required to recover the database if it fails right now. The server -gr option controls the maximum desired time, in minutes, for recovery in the event of system failure. You can also set the desired time using the RECOVERY_TIME option.
For more information, see -gr server option.
The checkpoint and recovery urgencies are important only if the server does not have enough idle time to write dirty pages.
Frequent checkpoints make recovery quicker, but also create work for the server writing out dirty pages.
There are two database options that allow you to control the frequency of checkpoints. CHECKPOINT_TIME controls the maximum desired time between checkpoints and RECOVERY_TIME controls the maximum desired time for recovery in the event of system failure.
The writing of dirty pages to disk is carried out by a task within the server called the idle I/O task. This task shares processing time with other database tasks.
There is a threshold for the number of dirty pages, below which writing of database pages does not take place.
When the database is busy, the urgency is low, and the cache only has a few dirty pages, the idle I/O task runs at a very low priority and no writing of dirty pages takes place.
Once the urgency exceeds 30%, the priority of the idle I/O task increases. At intervals, the priority is increased again. As the urgency becomes high, the engine shifts its primary focus to writing dirty pages until the number gets below the threshold again. However, the engine only writes out pages during the idle I/O task if the number of dirty pages is greater than the threshold.
If, because of other activity in the database, the number of dirty pages falls to zero, and if the urgency is 50% or more, then a checkpoint takes place automatically since it is a convenient time.
Both the checkpoint urgency and recovery urgency values increase in value until the checkpoint occurs, at which point they drop to zero. They do not decrease otherwise.