Size of the Database Buffer Cache (82)

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支援多個塊值 ，標準塊大小由DB_BLOCK_SIZE指定 ， 合法的值從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， 增加了請求資料的cache命中率 ， 可以使用alter system來

動態改變cache的大小， v$buffer_pool列出了cache元件的大小等資訊。[@more@]