Oracle supports multiple block sizes in a database. The standard block size is used for
the SYSTEM tablespace. You specify the standard block size by setting the initialization
parameter DB_BLOCK_SIZE. Legitimate values are from 2K to 32K.
Optionally, you can also set the size for two additional buffer pools, KEEP and
RECYCLE, by setting DB_KEEP_CACHE_SIZE and DB_RECYCLE_CACHE_SIZE. These
three parameters are independent of one another.
The cache has a limited size, so not all the data on disk can fit in the cache. When the
cache is full, subsequent cache misses cause Oracle to write dirty data already in the
cache to disk to make room for the new data. (If a buffer is not dirty, it does not need to
be written to disk before a new block can be read into the buffer.) Subsequent access to
any data that was written to disk results in additional cache misses.
The size of the cache affects the likelihood that a request for data results in a cache hit.
If the cache is large, it is more likely to contain the data that is requested. Increasing
the size of a cache increases the percentage of data requests that result in cache hits.
You can change the size of the buffer cache while the instance is running, without
having to shut down the database. Do this with the ALTER SYSTEM statement.
Use the fixed view V$BUFFER_POOL to track the sizes of the different cache
components and any pending resize operations.
Oracle支持多个块值 ，标准块大小由DB_BLOCK_SIZE指定 ， 合法的值从2k至32K.
DB_KEEP_CACHE_SIZE 指定KEEP池大小 ， DB_RECYCLE_CACHE_SIZE指定RECYCLE池大小
增大cache的大小， 增加了请求数据的cache命中率 ， 可以使用alter system来动态改变cache的大小， v$buffer_pool列出了cache组件的大小等信息。[@more@]
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