When Does Database Writer Write?

The DBWn background process writes to the datafiles whenever one of the following events occurs:
 A user’s Server Process has searched too long for a free buffer when reading a buffer into the Buffer Cache.
 The number of modified and committed, but unwritten, buffers in the Database Buffer
Cache is too large.
 At a database Checkpoint event. See Chapters 10 and 11 for information on checkpoints.
 The instance is shut down using any method other than a shutdown abort.
 A tablespace is placed into backup mode.
 A tablespace is taken offline to make it unavailable or changed to READ ONLY.
 A segment is dropped.

When Does Log Writer Write?

The LGWR background process writes to the current redo log group whenever one of the
following database events occurs:
 Every three seconds.
 A user commits a transaction.
 The Redo Log Buffer is one-third full.
 The Redo Log Buffer contains 1MB worth of redo information.
 Before the DBWn process whenever a database checkpoint occurs.