Which two statements about the background process of the database writer are true?

Database Writer (DBWn) can write data to the file contents of the buffer. DBWn process is responsible for the database buffer cache modified buffer (gray data buffer) is written to disk. While for most systems, a database writer process (DBW0) is sufficient, but if you frequently need to modify the system data, you can configure additional processes (DBW1 to DBW9 and DBWa to DBWj) to improve write performance (A correct, you can have many a dbwr process). These additional DBWn processes in a single-processor system to no avail.
When the database buffer cache in a buffer is modified, it is marked as gray data buffer, and added to the order by the SCN stored in the data buffer ash LRUW (LRU write) list. Therefore, the order is written to the redo log buffers order for these changes to redo the match. When the number of available buffer cache buffer is below a certain threshold inside (to the extent of the server process finds difficult to obtain available buffers) when, DBWn ash will write data buffers to the data file, and writes the order change the order of the same gray data buffer in the order LRUW list.
SGA memory structure contains redo stream having a location redo byte address (RBA), when an instance fails, will begin to recover the position. This structure acts as a pointer pointing to redo, and written by the CKPT process frequency control in accordance with the file once every three seconds. Since DBWn gray write data buffer in accordance with SCN order, and redo performed in SCN order, so whenever DBWn from LRUW gray list write data buffer, but also to maintain the SGA memory structure pointer forward, so that instance recovery (if needed) from approximately correct position to start reading redo, and to avoid unnecessary I / O. This is called “incremental checkpoints.”
NOTE: There are other DBWn implementation may write, for example, when the table space is set to read-only or offline when placed. In these cases, it does not appear incremental checkpoints, because only the data files belonging to the corresponding gray-independent data buffer in order of SCN is sequentially written into the database.
LRU algorithm more frequently accessed blocks stored in the buffer cache, so that, when the buffer is written to disk, the buffer will soon be unlikely to contain useful data.
DB_WRITER_PROCESSES DBWn initialization parameter specifies the number of processes. The maximum number is 20 DBWn process.
If the user does not specify the number of processes during startup, Oracle DB will be based on the number of CPU and processor groups to determine how to set DB_WRITER_PROCESSES.
In the following cases, DBWn process will gray data buffer is written to disk:
• When the server process after scanning a threshold number of buffers can not find a clean reusable buffer, the notice DBWn write. DBWn while performing other processing, the gray data buffer is written to disk asynchronously.
• DBWn regularly write buffers to advance the checkpoint. Check Point is the location of the redo thread (log) the executed instance recovery. This log position is determined by the buffer cache buffer oldest gray data.
In all cases, DBWn are performing batched (multiple) writes to improve efficiency. Number block blocks written by write operation on the operating system.
 
 
C Answer: Yes lgwr process
 
 
D answer: LGWR first record, after DBWR to write.
 
 
LGWR usually write speed fast enough to ensure that the buffer is always space for new items to use, even for the redo log views is large as well. LGWR a continuous portion of the buffer is written to disk.
LGWR write operation is performed in the following cases:
• When a user process commits a transaction
• When one-third of the redo log buffer is full
• Redo the redo log buffer entries reach 1M
• written to disk (if desired) before the modified buffer DBWn process
• every 3 seconds
After the buffer must change all the associated redo records written to disk, DBWn can be written to the disk buffer is modified (written agreement in advance).

The checkpoint is responsible for signalling DBWn at checkpoints and updating all the datafiles and control files of the database to indicate the most recent checkpoint.

The database writer process (DBWn) writes the contents of buffers to datafiles. The DBWn processes are responsible for writing modified (dirty) buffers in the database buffer cache to disk.