The database buffer cache
If youdecide to disable SGA_TARGET by setting it to 0, youwill need to manually
set initialization parameters for the memory pools (unless you want to use previous
sizes). For the database buffer cache, you would assess the percentage of the database
blocks requested by users read from the cache versus from the disk. This
percentage is termed the hit ratio. If response times are too high and this ratio is
lower than 90% (as a rule of thumb), increasing the value of the initialization parameter
DB_CACHE_SIZE can increase performance.
You can use Oracle Enterprise Manager to get information about the
cache hit ratio.
It??™s tempting to assume that continually increasing the size of the database buffer
cache will translate into better performance. However, this is true only if the database
blocks in the cache are actually being reused. Most OLTP systems have a
relatively small set of core tables that are heavily used (for example, lookup tables for
things such as valid codes). The rest of the I/O tends to be random, accessing a row
or two in various database blocks in the course of the transaction.
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