Paraphrased from the Oracle Database Performance Tuning Guide for 10g
Configure multiple database writer processes, or using I/O slaves, is useful when the transaction rates are high or when the buffer cache is so large that a single DBWn process cannot keep up with the load. Multiple DB writer processes are recommended for systems with multiple CPUs (at least one DB writer for every 8 CPUs) – configure with DB_WRITER_PROCESSES.

If not practical to use multiple DBWR processes, use DBWR_IO_SLAVES to distribute the load over multiple slave processes – configure
DBWR_IO_SLAVES. DBWR_IO_SLAVES is intended for scenarios where multiple DB_WRITER_PROCESSES cannot be used, for example in systems with a single CPU. I/O slaves are also useful when asynchronous IO is not available, because the multiple I/O slaves simulate non-blocking, asynchronous requests by freeing DBWR to continue identifying blocks in the cache to be written.
Implementing DBWR_IO_SLAVES forces only one DBWR process to start.

If the system does not support asynchronous I/O, or if asynchronous I/O is already configured and there is still a DBWR bottleneck, configure multiple DBWR processes. Multiple DBWr processes should deliver more throughput than the same number of I/O slaves. I/O slaves have been depreciated.

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