Parameter - _DBWR_TRACING

This parameter enables tracing the DBWR process(es)

Version Type Default Session
Modifiable		 System
Modifiable
8.1.5 Integer Yes Immediate
8.1.6 Integer Yes Immediate
8.1.7 Integer Yes Immediate
9.0.1 Integer Yes Immediate
9.2.0 Integer Yes Immediate

For example in the init.ora file 

_dbwr_tracing = 10;

This parameter can also be set using the ALTER SYSTEM command e.g. 

ALTER SYSTEM SET "_dbwr_tracing" = 10;