Buffer Hit Ratio (BuffHitRatio)

The BuffHitRatio parameter shows the ratio of physical data blocks read to requests for data blocks. The formula below is used:

sga_consistent_gets + sga_db_block_gets - sga_physical_reads

sga_consistent_gets + sga_db_block_gets

This measures how many times Oracle went to read something and found it already in memory.

• 100% would mean everything Oracle read was already in memory.
• 0% would mean that nothing was in memory.
• A value between 1 and 99% means that only that percentage of the time Oracle issued a read and find what it needed in memory. The rest of the time a physical read had to take place.

The annotation for this parameter displays the values and the time interval.

Recommendations
This parameter alarms when too few of the data requests accessed the buffer cache; too many requests accessed the disk.

• If the ratio is < 95%, you should increase the value of the init.ora db_block_buffers parameter or, for Oracle 11g and later, increase the init.ora_db_cache_size. Increasing these values decreases I/O by increasing the probability that a required block is in memory. However, setting db_block_buffers or db_cache_size too high could require that paging of the SGA, degrading performance.
• Do tuning analysis using the Oracle db_block_lru_statistics and db_block_lru_extended_statistics parameters in V$CURRENT_BUCKET view.
   

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