Database Writer (DBWn) The server process records changes to undo and data blocks in the Database Buffer Cache. DBWn writes the dirty buffers from the Database Buffer Cache to the data files. It ensures that a sufficient number of free buffers (buffers that can be overwritten when server processes need to read in blocks from the data files) are available in the Database Buffer Cache. Database performance is improved because server processes make changes only in the Database Buffer Cache. DBWn defers writing to the data files until one of the following events occurs: Incremental or normal checkpoint The number of dirty buffers reaches a threshold value A process scans a specified number of blocks when scanning for free buffers and cannot find any Timeout occurs A ping request in Real Application Clusters (RAC) environment Placing a normal or temporary tablespace offline Placing a tablespace in read-only mode Dropping or truncating a table ALTER TABLESPACE tablespace name BEGIN BACKUP Log Writer (LGWR) LGWR performs sequential writes from the Redo Log Buffer to the online redo log file under the following situations: When a transaction commits When the Redo Log Buffer is one-third full When there is more than 1 MB of changes recorded in the Redo Log Buffer Before DBWn writes modified blocks in the Database Buffer Cache to the data files Every three seconds Because the redo is needed for recovery, LGWR confirms the commit operation only after the redo is written to disk. LGWR can also call on DBWn to write to the data files.[@more@]