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"Dino Hsu" <dino1_at_ms1.hinet.net> wrote in message
news:d4jvgtch2rslse2tmd01752id3tk5eelc9_at_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
>
Asynchronous I/O is any I/O where the requestor doesn't wait for completion
of the I/O request.
Whether or not it is available is O/S dependent.
Sometimes it will work on RAW devices only.
On several systems (AIX) you need to configure it.
If you are using asynchronous I/O you shouldn't need multiple database
writers and/or IO slaves (that is, as far as I read the Oracle docs on this)
If you can't use async I/O (e.g. when you don't want to use/can't use raw
devices) in that case you need to configure I/O slaves.
I assume the number 4 is just some 'sensible' default.
Hth,
Sybrand Bakker, Oracle DBA Received on Sat May 26 2001 - 12:48:57 CDT