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Performance tuning the db_writer_process in Oracle v11g

I recently updated my system to Oracle v11g from oracle 9i.  In Oracle 9, the db_writer_process was set by default to 1, but in v11 db_writer_process is set to 4.  

Also with v11 i get a message in my alert logs that reads Private strand flush not complete which i have read as being a normal message for oracle 11.  Oracle also says that i can get rid of this message by increasing the value of db_writer_process.

Does anyone have any information on how performance is affect by increasing or decreasing the db_writer_process parameter?

Any help is greatly appropriated, Thank YOu
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Alex Matzinger
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Alex Matzinger
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Aaron ShiloCommented:
Increasing this parameter is also likely to have minimal effect on single-CPU systems.  Increasing this parameter could, in fact, reduce performance on systems where the CPU’s are already over burdened.  In cases where the main performance bottleneck is that a single DBWR process cannot keep up with the work load, then increasing the value for DB_WRITER_PROCESSES may improve performance.

When increasing DB_WRITER_PROCESSES it may also be necessary to increase the DB_BLOCK_LRU_LATCHES parameter, as each DBWR process requires an LRU latch.

Default value of DB_WRITER_PROCESSES is 1 or CPU_COUNT / 8, whichever is greater. If the number of processor groups is less than 36 but greater than the number of DB writer processes, then the number of DB writer processes is adjusted to be a multiple of the number of processor groups. If the number of DB writer processes is greater than or equal to the number of processor groups, then there is no adjustment.

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Alex MatzingerAuthor Commented:
Thank you for your response, Is there any tool i can use to tune this feature in oracle.  I have two very similair systems, but one has 4 log writers, and the other has 20 log writers, is there a way i can tell the difference.  I don't want to increase the number of log writers, and have it negatively affect my system.  

Adidtion note:  My system has 32 CPU's
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Alex MatzingerAuthor Commented:
I would also like to note that my system is using Async IO.
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Alex MatzingerAuthor Commented:
Thank You, your info was spot on.  Ended up testing several different db_writer values to find the best fit.  Thank You
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