DBWR_IO_SLAVES and Asynchronous I/O

From: Dino Hsu <dino1_at_ms1.hinet.net>
Date: Sat, 26 May 2001 23:51:10 +0800
Message-ID: <d4jvgtch2rslse2tmd01752id3tk5eelc9@4ax.com>

Dear all,

In a book, it says: (See note for source) "...As of 8.1, ORACLE uses the DBWR_IO_SLAVES setting to determine how many LGWR and ARCH I/O slaves to start; setting DBWR_IO_SLAVES to a value greater than 0 sets the corresponding LGWR_IO_SLAVES and ARCH_IO_SLAVES settings each to 4. If the option is not available on your system, then you may be able to use asynchronous I/O to reduce internal DBWR contention. With asynchronous I/O, only one DBWR process is started since the I/O processing is performed asynchronously."

My questions:
1.What is the definition of asynchronous I/O? Is it defined by DBWR_PROCESSES=1? Is it related to particular OS configurations? 2.Why do LGWR_IO_SLAVES and ARCH_IO_SLAVES become 4 instead of 3 or 5 when DBWR_IO_SLAVES is non-zero?
3."If the option is not available..." What does it mean by this?

Thanks in advance, Dino

Note: source information:
p.102, Kevin Leney, Oracle 8i DBA Handbook Ch4. Physical Database Layouts->
Concurrent I/O Among Background Processes Received on Sat May 26 2001 - 10:51:10 CDT