How frequently data blocks are accessed from the buffer cache (Block Buffer Hit Ratio)

Oracle database maintains dynamic performance view V$BUFFER_POOL_STATISTICS with overall buffer usage statistics. This view maintains the following counts every time a data block is accessed either from the block buffers or from the disk:

NAME – Name of the buffer pool
PHYSICAL_READS – Number of physical reads
DB_BLOCK_GETS – Number of reads for INSERT, UPDATE and DELETE
CONSISTENT_GETS – Number of reads for SELECT

DB_BLOCK_GETS + CONSISTENT_GETS = Total Number of reads

Based on above statistics we can calculate the percentage of data blocks being accessed from the memory to that of the disk (block buffer hit ratio). The following SQL statement will return the block buffer hit ratio:

SELECT NAME, 100 – round ((PHYSICAL_READS / (DB_BLOCK_GETS + CONSISTENT_GETS))*100,2) HitRatio
FROM V$BUFFER_POOL_STATISTICS;

A hit ratio of 95% or greater is considered to be a good hit ratio for OLTP systems. The hit ratio for DSS (Decision Support System) may vary depending on the database load. A lower hit ratio means Oracle is performing more disk IO on the server. In such a situation, you can increase the size of database block buffers to increase the database performance. You may have to increase the physical memory on the server if the server starts swapping after increasing block buffers.