oracle db_cache_size的修改规则

Granule size is determined by total SGA size. On most platforms, the size of a granule is 4 MB if the total SGA size is less than 1 GB, and granule size is 16MB for larger
SGAs. Some platform dependencies arise. For example, on 32-bit Windows, the
granule size is 8 M for SGAs larger than 1 GB.

问题：db_cache_size在修改前的value是52M,执行了修改db_cache_size参数为50M后，再查看发现值还是52M未变成新值50M。

SQL> show parameter db_cache_size

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_cache_size                        big integer 52M
SQL> alter system set db_cache_size=50M;

System altered.

SQL> show parameter db_cache_size

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_cache_size                        big integer 52M
SQL>

问题分析：这是由于oracle中SGA大小是523108352 bytes=0.49G，因SGA小于1G，则db_cache_size的value需是4M的倍数，修改的db_cache_size=50M不是4M的倍数，因此修改未生效。

解决办法：修改db_cache_size的value为4M的倍数即可。

例：SQL> show parameter db_cache_size

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_cache_size                        big integer 52M
SQL> alter system set db_cache_size=56M;

System altered.

SQL> show parameter db_cache_size

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_cache_size                        big integer 56M
SQL>