How DBWn Works

Question

Hi,
I want to know how DBWn works
SAy; There is a tablespace DATASR3.
This tablespace consists of 10 Datafiles.
6 Datafiles are already filled. (Reached to its MAXLIMIT.)
and Other 4 datafiles is in the preocess of getting the data.
Im interesting in why its happens like this?... How DBWn will going to add/write data to datafiles.
Is it in a circular fashion OR randomly it (DBwn) decides to which datafiles it goes to write.
If its circular fashion then all datafiles must filled...according to order. (then it is not possible to full-fill only 6 Datafiles?)
DBWn and LGWR works in the same way? .... Writing to files one after the other!
Regards,

Answer 1

Hi user2734911,
Did you search via google?
Database writer process (DBWn) is a background process that writes buffers in the database buffer cache to data files.
Modified or new data is not necessarily written to a datafile immediately. To reduce the amount of disk access and to increase performance, data is pooled in memory and written to the appropriate data files all at once (in bulk mode), as determined by the background process database writer process (DBWn).
The disk write can happen before or after the commit. See Database Buffer Cache.
After a COMMIT, the database writes the redo buffers to disk but does not immediately write data blocks to disk. Instead, database writer (DBWn) performs lazy writes in the background.
The database writer (DBWn) process periodically writes cold, dirty buffers to disk. DBWn writes buffers in the following circumstances:
* A server process cannot find clean buffers for reading new blocks into the database buffer cache.
As buffers are dirtied, the number of free buffers decreases. If the number drops below an internal threshold, and if clean buffers are required, then server processes signal DBWn to write.
The database uses the LRU to determine which dirty buffers to write. When dirty buffers reach the cold end of the LRU, the database moves them off the LRU to a write queue. DBWn writes buffers in the queue to disk, using multiblock writes if possible. This mechanism prevents the end of the LRU from becoming clogged with dirty buffers and allows clean buffers to be found for reuse.
* The database must advance the checkpoint, which is the position in the redo thread from which instance recovery must begin.
* Tablespaces are changed to read-only status or taken offline.
DBWn and LGWR works in the same way? .... Writing to files one after the other!*
LGWR does sequential writes - so it takes lesser time to do its job.
It needs to keep a record of what operations were peformed on the data and hence doesn't have a requirement to post the data to the actual data blocks of interest.
However DBWn needs to write data to appropriate data blocks - which means it need to first find where to write and then do the writing job.
Since a sequential write takes lesser time, it is advantageous to have separate LGWR and DBWn processes.
The user gets better performance because LGWR does it faster sequential job while DBWn does its slow job in the background*.