Brother-Eagle™ Oracle Performance
Advice: Buffer Hit Ratio

The buffer hit ratio has proven to give some insight into the performance of the database buffer cache.  The idea is that if my query can access data directly from the buffer, then I avoided an expensive visit to disk.  There's a lot that goes into determining the optimal buffer size but at the end of the day, not all of the data in the database can reside in memory.

This parameter is collected at each interval. Therefore, if you specified a 60 second refresh rate, the value displayed applies to the last 60 seconds.  The formula is:

1 - (( physical reads) / (consistent gets + db block gets)

Consider using the DB_CACHE_ADVICE feature within Oracle to help you with this determination.  The view v$db_cache_advice will show the different values for physical data block reads given different sizes of the cache.

Hit ratios should be used sparingly and should not be the main performance metric reviewed.  Ratios should simply be used as performance indicators which may warrant further investigation into other areas.

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