Or more specific asnwer to anand's Q about oracle's write ahead logging logic.

The database writer writes modified blocks from the database buffer cache to the datafiles. Since Oracle uses write-ahead logging, DBWn does not need to write blocks when a transaction commits. Instead, DBWn is designed to perform batched writes with high efficiency. In the most common case, DBWn writes only when more data needs to be read into the system global area and too few database buffers are free. The least recently used data is written to the datafiles first.