If you implement database writer I/O slaves by setting the
dbwr_io_slaves parameter, you
configure a single (master) DBWR process that has slave processes
that are subservient to perform asynchronous I/O calls. I/O slaves
can also be used to simulate asynchronous I/O on platforms that do
not support asynchronous I/O or implement it inefficiently.
You can't activate both multiple DBWRs and I/O
slaves. If both parameters are set in the parameter file,
dbwr_io_slaves will take precedence.
To determine whether to use multiple DBWn
processes or database slaves, follow these guidelines:
Use db_writer_processes for most write
intensive applications. One per CPU is the recommended setting.
Use db_writer_processes for databases
that have a large data buffer cache.
Use dbwr_io_slaves for applications that
are not write intensive and run on operating systems that support
asynchronous I/O.
Use dbwr_io_slaves on platforms that do
no support asynchronous I/O.
Use dbwr_io_slaves on single CPU
systems. Multiple DBWR processes are CPU intensive.
The above text is
an excerpt from:
OCP Instructors Guide for Oracle DBA Certification
A Study Guide to Advanced Oracle Certified Professional Database
Administration Techniques
