ASM Buffer Cache是用來做什麼的！

10g在檢視v$sga_dynamic_components中我們注意到多了一個component：ASM Buffer Cache，並且在instance_type=asm的例項中該引數有值，也就是說該引數是為asm例項所使用，它是用來做什麼，在doc中搜了一下沒找到，我猜測應該和db_cache_size的作用差不多，估計是用來cache disk header資訊的。

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SQL> select component,current_size from v$sga_dynamic_components;

COMPONENT CURRENT_SIZE
------------------------------ ------------
shared pool 41943040
large pool 12582912
java pool 0
streams pool 0
DEFAULT buffer cache 0
KEEP buffer cache 0
RECYCLE buffer cache 0
DEFAULT 2K buffer cache 0
DEFAULT 4K buffer cache 0
DEFAULT 8K buffer cache 0
DEFAULT 16K buffer cache 0

COMPONENT CURRENT_SIZE
------------------------------ ------------
DEFAULT 32K buffer cache 0
ASM Buffer Cache 25165824

13 rows selected.

SQL> alter system set sga_target=80m;
alter system set sga_target=80m
*
ERROR at line 1:
ORA-15021: parameter "sga_target" is not valid in asm instance

--無意中發現sga_target不適合asm例項型別

SQL> show parameter instance_type

NAME TYPE VALUE
---------------------------------------------------------
instance_type string asm
SQL>