DBWn defers writing to the data files until one of the following events occurs:
1. Incremental or normal checkpoint
2.The number of dirty buffers reaches a threshold value
3. A process scans a specified number of blocks when scanning for free buffers and cannot find any
4. Timeout occurs
5.A ping request in Real Application Clusters (RAC) environment
6.Placing a normal or temporary tablespace offline
7.Placing a tablespace in read-only mode
8.Dropping or truncating a table
9. ALTER TABLESPACE tablespace name BEGIN BACKUP
LGWR performs sequential writes from the Redo Log Buffer to the online redo log file under the following situations:
1. When a transaction commits
2.When the Redo Log Buffer is one-third full
3.When there is more than 1 MB of changes recorded in the Redo Log Buffer
4.Before DBWn writes modified blocks in the Database Buffer Cache to the data files
5. Every three seconds