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Oracle Advice on the data Buffer Cache

The old rule of thumb said that if the buffer cache hit ratios were depressed (typically below 90%), you should increase the db_block_buffer/db_cache_size parameter until the ratio improved.  We have already mentioned that this practice may not yield better overall performance (it will not help excessive logical I/O caused by inefficient SQL calls), but we hasten to add that increasing the cache memory can indeed improve response times for many databases suffering from improper cache sizes.

Of course, memory should be added to Oracle intelligently, which means keeping an eye on server page and swap activity.  However, how will you know the amount of extra memory that the cache needs?  If the truth were known, most DBAs simply begin edging the memory setting higher with no rhyme or reason, and hope to stop the dial on just the right amount. 

With Oracle9i and above, you can use the db_cache_advice parameter to help predict the benefit of adding additional memory to the buffer/data cache.  Setting this value to ON tells Oracle to begin collecting I/O statistics that can be used to assist you in intelligently assigning additional RAM to Oracle, while not giving more than is actually needed.  The technique generally involves setting the db_cache_advice parameter to ON during a time that represents the normal workload for the database. 

Predict oracle cache use

Once the database has been stressed, you can examine the prediction results by using the cacheadvice.sql script:

See Code depot for complete script

 

select
       size_for_estimate,
       buffers_for_estimate,
. . .

from
       sys.v$db_cache_advice
where
       name = 'DEFAULT'and
. . .

;

Figure 5.5 – Examining Oracle’s prediction for adding or subtracting RAM from the data cache

In the above example, Oracle is telling us that for this small 9.2 instance, anything above 16MB for the buffer cache will be wasted RAM. 

As a general guideline, all memory available on the host should be tuned, and the db_cache_size should be allocating RAM resources up to the point of diminishing returns (Figure 5.6).  This is the point where additional buffer blocks do not significantly improve the buffer hit ratio.

Figure 5.6- The optimal size of the RAM data buffer

The new v$db_cache_advice view is similar to an Oracle7 utility that also predicted the benefit of adding data buffers.  The Oracle7 utility used the x$kcbrbh view to track buffer hits and the x$kcbcbh view to track buffer misses.


The above is an excerpt from Oracle Performance Troubleshooting by Robin Schumacher.

It's only $19.95 and you can order it and get instant access to the Oracle scripts here:

http://www.rampant-books.com/book_2003_1_perf.htm

 

 

 

 
 
 

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