OCP 1Z0 052 19

19. Which two statements about the background process of the database writer are true? (Choose two.) 
A.It is possible to have multiple database writers in an Oracle instance.  
B.It writes dirty buffers, if any, to data files whenever a checkpoint occurs. 
C.It writes dirty buffers, if any, to data files whenever a transaction commits. 
D.It writes dirty buffers, if any, to data files before the log writer (LGWR) writes. 
Answer: AB

The checkpoint process (CKPT) updates the control file and data file headers with checkpoint information and signals DBWn to write blocks to disk

you can configure additional processes—DBW1 through DBW9 and DBWa through DBWj—to improve write performance if your system modifies data heavily. 
The DBWn process writes dirty buffers to disk under the following conditions:
When a server process cannot find a clean reusable buffer after scanning a threshold number of buffers, it signals DBWn to write. DBWn writes dirty buffers to disk asynchronously if possible while performing other processing.
DBWn periodically writes buffers to advance the checkpoint, which is the position in the redo thread from which instance recovery begins (see "Overview of Checkpoints"). The log position of the checkpoint is determined by the oldest dirty buffer in the buffer cache.

C 不对 database writer 可以在发生成commit 之前或之后，commit只是以写入redo log为准。

D 不对 相对于database writer 来说 LGWR发生的更频繁，以保证redo log及时记录。