DBWR, Direct I/O and the Devil

  • From: "Don Seiler" <don@xxxxxxxxx>
  • To: "Oracle-L Freelists" <oracle-l@xxxxxxxxxxxxx>
  • Date: Wed, 28 Nov 2007 12:59:06 -0600

I'd like to lay bare some aspects of my instance and see what everyone
thinks.  I've recently been re-reading Kevin Closson's series on
over-configuring DBWRs [0], and had some questions.

First of all, this is on Oracle RDBMS 10.2.0.2 Enterprise Edition on
RHEL4.  Both DB and OS are 64-bit.  Processing is 4 dual-core 64-bit
CPUs.  Filesystem for all database files is Veritas (vxfs).

Second, based on a suggestion from a colleague, I set
filesystemio_options=directio.  We also mounted the vxfs drives with
the convosync=direct option.  However, disk_asynch_io is still true.
Is there a conflict here?

Third, we have db_writer_processes=4.  This was done a long time ago
and hasn't been looked at since.  I imagine it was done based on the
"1 DBWR for every CPU" line of thinking that Kevin spotlights in his
series.  Our database is a hybrid of OLTP data and bulk-loaded data
that is either direct-path sqlldr or INSERT/APPEND from external
tables.  Kevin mentioned that direct-path writes don't use the DBWR,
so that this instance *might* do perfectly well with just 1 DBWR.  I'm
wondering if using directio is also a factor in determining the proper
value of db_writer_processes.

Fourth, should I have even gone to directio in the first place?  I'd
like to know what people use to benchmark I/O throughput, similar to
what Kevin does in his tests.

[0] 
http://kevinclosson.wordpress.com/2007/08/10/learn-how-to-obliterate-processor-caches-configure-lots-and-lots-of-dbwr-processes/
-- 
Don Seiler
http://seilerwerks.wordpress.com
ultimate: http://www.mufc.us
--
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