SGA_MAX_SIZE是從9i以來就有的作爲設置SGA大小的一個參數，而SGA_TARGET則是從10g纔有的一個新參數，作爲配合10g自動管理SGA而出現的，下面以實驗的方式，深入解析這2個參數的區別和作用
[oracle@bak ~]$ sqlplus / as sysdba
SQL*Plus: Release 10.2.0.1.0 - Production on Mon Oct 13 01:59:11 2014
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 272M
sga_target big integer 272M
SQL> desc v$sgainfo
Name Null? Type
----------------------------------------- -------- ----------------------------
NAME VARCHAR2(32)
BYTES NUMBER
RESIZEABLE VARCHAR2(3)
SQL> set pages 100
SQL> select name,bytes/1024/1024 "size(MB)",resizeable from v$sgainfo;
NAME size(MB) RES
-------------------------------- ---------- ---
Fixed SGA Size 1.16254425 No
Redo Buffers 2.8359375 No
Buffer Cache Size 172 Yes
Shared Pool Size 80 Yes
Large Pool Size 4 Yes
Java Pool Size 4 Yes
Streams Pool Size 8 Yes
Granule Size 4 No
Maximum SGA Size 272 No
--SGA_MAX_SIZE對應的值
Startup overhead in Shared Pool 36 No
Free SGA Memory Available 0
11 rows selected.
注意，resizeable值爲NO的，都是不可動態調整的值，並且由於SGA_TARGET和SGA_MAX_SIZE的大小一致，因此Free
SGA Memory Available=0，而所有可動態調整的幾個內存組件再加上一個4M Granule Size，則正好等於SGA_TARGET的值
SQL> select (172 + 80 + 4 + 4 + 8 + 4) "sga_target(MB)" from dual;
sga_target(MB)
--------------
272
我們也可以從v$sga_dynamic_components這個視圖瞭解各內存組件的內存分配情況，和v$sgainfo內容基本是一致的，除了沒有展示Gradual Size這一項
SQL> select component,current_size/1024/1024 "size(MB)" from v$sga_dynamic_components;
COMPONENT size(MB)
---------------------------------------------------------------- ----------
shared pool 80
large pool 4
java pool 4
streams pool 8
DEFAULT buffer cache 172
KEEP buffer cache 0
RECYCLE buffer cache 0
DEFAULT 2K buffer cache 0
DEFAULT 4K buffer cache 0
DEFAULT 8K buffer cache 0
DEFAULT 16K buffer cache 0
DEFAULT 32K buffer cache 0
ASM Buffer Cache 0
13 rows selected.
SGA = shared pool + large pool + java pool + streams pool + DEFAULT buffer cache = 80 +4 + 4 + 8 + 172 =268M
這個算出來的SGA值比SGA_TARGET的值少了4M，其實就是那個Gradual Size的值
SQL> col name for a35
SQL> select name,issys_modifiable from v$parameter where name like '%sga%';
NAME ISSYS_MOD
----------------------------------- ---------
sga_max_size FALSE
pre_page_sga FALSE
lock_sga FALSE
sga_target IMMEDIATE
ISSYS_MODIFIABLE列的值爲FALSE的，就是不能動態修改的參數，需重啓生效，爲IMMEDIATE的，即可以動態修改
SQL> alter system set sga_target=350m;
alter system set sga_target=350m
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-00823: Specified value of sga_target greater than sga_max_size
如果不帶scope參數，默認爲both，但要在內存中生效，必須滿足SGA_TARGET<SGA_MAX_SIZE的條件，因此必須指定scope=spfile,因此雖然是動態參數，但是依舊要重啓後生效
SQL> alter system set sga_target=350m scope=spfile;
System altered.
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 369098752 bytes
Fixed Size 1219472 bytes
Variable Size 100664432 bytes
Database Buffers 264241152 bytes
Redo Buffers 2973696 bytes
Database mounted.
Database opened.
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 352M
sga_target big integer 352M
可以看到，當調整了SGA_TARGET參數後，儘管沒有去手動調整SGA_MAX_SIZE的值，但現在它自動和修改後的SGA_TARGET的值匹配了，從272M調整爲352M，這裏還有個比較有意思的地方，就是指定了350M，爲何會變爲352M，這多出來的2M又是什麼呢？
SQL> select 350/4 "Granual Count" from dual;
Granual Count
-------------
87.5
SQL> select 88*4 "SGA_SIZE(MB)" from dual;
SGA_SIZE(MB)
------------
352
其實很容易理解，剛纔說了Granual Size的大小爲4M，由於Granual必須完整地分配，因此會分配給SGA共88個Granual，即88*4=352M,也就是多給了2M
再來看一下剛纔那些內存組件的內存分配情況
SQL> select name,bytes/1024/1024 "size(MB)",resizeable from v$sgainfo;
NAME size(MB) RES
----------------------------------- ---------- ---
Fixed SGA Size 1.16297913 No
Redo Buffers 2.8359375 No
Buffer Cache Size 252 Yes
Shared Pool Size 80 Yes
Large Pool Size 4 Yes
Java Pool Size 4 Yes
Streams Pool Size 8 Yes
Granule Size 4 No
Maximum SGA Size 352 No
Startup overhead in Shared Pool 36 No
Free SGA Memory Available 0
11 rows selected.
SQL> select component,current_size/1024/1024 "size(MB)" from v$sga_dynamic_components;
COMPONENT size(MB)
---------------------------------------------------------------- ----------
shared pool 80
large pool 4
java pool 4
streams pool 8
DEFAULT buffer cache 252
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
DEFAULT 32K buffer cache 0
ASM Buffer Cache 0
13 rows selected.
SQL>
當我們把SGA_TARGET從272M調整爲350M時，由於Gruanual的存在，Oracle實際給SGA分配了352M的內存，而這352M內存中，其他內存組件的大小都沒有發生變化，僅僅只是db buffer cache的值由之前的172M調整到了252M
之所以db buffer cache size隨着SGA_TARGET的值自動調整，是因爲這時10g的新特性，只要SGA_TARGET爲非零值，那麼內存組件就是採用動態分配原則，由Oracle自動調整各內存組件的大小。不過有個地方挺奇怪的，雖然從啓動實例時分配地內存也好，從v$sgainfo以及v$sga_dynamic_components中也好，看到的值都是252M，而從參數看db_cache_size的值，仍然是172M，不知何故，難道是bug?
SQL> show parameter db_cache
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_cache_advice string ON
db_cache_size big integer 172M
剛纔是直接修改SGA_TARGET而不修改SGA_MAX_SIZE的情況，我們再來看一下反過來的情況，先把SGA_TARGET調整回272M
SQL> alter system set sga_target=270m scope=spfile;
System altered.
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 285212672 bytes
Fixed Size 1219016 bytes
Variable Size 100664888 bytes
Database Buffers 180355072 bytes
Redo Buffers 2973696 bytes
Database mounted.
Database opened.
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 272M
sga_target big integer 272M
注意，一旦SGA_TARGET動了，重啓數據庫後，SGA_MAX_SIZE會跟着調整，也一起回到了272M的狀態，但與增大SGA_TARGET時的情況不同，也可以選擇不重啓數據庫，那麼SGA_MAX_SIZE的值就不會做調整，而內存組件的值依然會跟着SGA_TARGET走（體現動態調整）
SQL> alter system set sga_target=270m;
System altered.
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 352M
sga_target big integer 272M
由於是減小SGA_TARGET的值，因此不受必須比TARGET_MAX_SIZE小的這個條件限制，因此數據庫無需重啓
SQL> select name,bytes/1024/1024 "size(MB)",resizeable from v$sgainfo;
NAME size(MB) RES
----------------------------------- ---------- ---
Fixed SGA Size 1.16297913 No
Redo Buffers 2.8359375 No
Buffer Cache Size 172 Yes
--僅僅調整了db buffer cache,由252M變爲172M
Shared Pool Size 80 Yes
Large Pool Size 4 Yes
Java Pool Size 4 Yes
Streams Pool Size 8 Yes
Granule Size 4 No
Maximum SGA Size 352 No
Startup overhead in Shared Pool 36 No
Free SGA Memory Available 80
11 rows selected.
剛纔是直接調整SGA_TARGET,我們再來看一下只調整SGA_MAX_SIZE的情況，先重啓數據庫，使SGA_MAX_SIZE的值回到272M
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 285212672 bytes
Fixed Size 1219016 bytes
Variable Size 100664888 bytes
Database Buffers 180355072 bytes
Redo Buffers 2973696 bytes
Database mounted.
Database opened.
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 272M
sga_target big integer 272M
SQL> alter system set sga_max_size=350m scope=spfile;
System altered.
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 369098752 bytes
Fixed Size 1219496 bytes
Variable Size 184550488 bytes
Database Buffers 180355072 bytes
Redo Buffers 2973696 bytes
Database mounted.
Database opened.
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 352M
sga_target big integer 272M
SQL> select name,bytes/1024/1024 "size(MB)",resizeable from v$sgainfo;
NAME size(MB) RES
----------------------------------- ---------- ---
Fixed SGA Size 1.16300201 No
Redo Buffers 2.8359375 No
Buffer Cache Size 172 Yes
Shared Pool Size 80 Yes
Large Pool Size 4 Yes
Java Pool Size 4 Yes
Streams Pool Size 8 Yes
Granule Size 4 No
Maximum SGA Size 352 No
Startup overhead in Shared Pool 36 No
Free SGA Memory Available 80
11 rows selected.
注意，僅僅調整SGA_MAX_SIZE後，SGA_TARGET並不會跟着調整，因此各內存組件的值也不會調整，而這時我們發現Free SGA Memory Available的值爲80M，而不再是之前的0了，調整SGA_MAX_SIZE爲SGA_TARGET增加了80M的上限，這80就是352-272得到的，相當於做了個預留，告訴SGA，你可以現在只用172M，但如果想增加，可以再多用80M，達到352M。而這個時候只要SGA_TARGET重新設置的值沒有超過SGA_MAX_SIZE的值，就可以在不停庫的情況下增加SGA大小，這樣做是有好處的
最後，我們還可以把SGA_TARGET設置爲0，即表示禁用10g的新特性——自動分配內存，這樣我們可以對各個內存組件的值進行單獨設置，對於特殊的應用場景，有時候也是需要的，因爲自動管理只是oracle提供的一種便利行，但並不意味着自動調整就一定萬事皆OK，11g的自動內存管理就一度被詬病，這裏不討論
SQL> alter system set sga_target=0;
System altered.
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 352M
sga_target big integer 0
SQL> select name,bytes/1024/1024 "size(MB)",resizeable from v$sgainfo;
NAME size(MB) RES
----------------------------------- ---------- ---
Fixed SGA Size 1.16300201 No
Redo Buffers 2.8359375 No
Buffer Cache Size 172 Yes
Shared Pool Size 80 Yes
Large Pool Size 4 Yes
Java Pool Size 4 Yes
Streams Pool Size 8 Yes
Granule Size 4 No
Maximum SGA Size 352 No
Startup overhead in Shared Pool 36 No
Free SGA Memory Available 80
設置SGA_TARGET=0以後，已經分配的內存組件的值不會變化，如果不設置新值，那麼依舊保持原來的值，除非單獨設置，即使從起數據庫實例也一樣，SGA_MAX_TARGET依然不會再根據SGA_TARGET調整，因爲SGA_MAX_TARGET我們是給過它確定的值的
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 369098752 bytes
Fixed Size 1219496 bytes
Variable Size 184550488 bytes
Database Buffers 180355072 bytes
Redo Buffers 2973696 bytes
Database mounted.
Database opened.
SQL> show parameter sge
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 352M
--重啓後依然是352M，並不會變爲0
sga_target big integer 0
SQL> select name,bytes/1024/1024 "size(MB)",resizeable from v$sgainfo;
NAME size(MB) RES
----------------------------------- ---------- ---
Fixed SGA Size 1.16300201 No
Redo Buffers 2.8359375 No
Buffer Cache Size 172 Yes
Shared Pool Size 80 Yes
Large Pool Size 4 Yes
Java Pool Size 4 Yes
Streams Pool Size 8 Yes
Granule Size 4 No
Maximum SGA Size 352 No
Startup overhead in Shared Pool 36 No
Free SGA Memory Available 80
11 rows selected.
SQL> alter system set sga_target=270m;
System altered.
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 369098752 bytes
Fixed Size 1219496 bytes
Variable Size 184550488 bytes
Database Buffers 180355072 bytes
Redo Buffers 2973696 bytes
Database mounted.
Database opened.
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 352M
sga_target big integer 272M
重啓數據庫實例後，SGA_MAX_SIZE的值沒有跟着SGA_TARGET做調整，除非再次增大SGA_TARGET的大小並超過SGA_MAX_SIZE的上限，那麼下次重啓後，SGA_MAX_SIZE又會再一次跟着SGA_TARGET調整
SQL> alter system set sga_target=380m scope=spfile;
System altered.
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area 398458880 bytes
Fixed Size 1219640 bytes
Variable Size 100664264 bytes
Database Buffers 293601280 bytes
Redo Buffers 2973696 bytes
Database mounted.
Database opened.
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 380M
sga_target big integer 380M
SGA_MAX_SIZE又再一次和SGA_TARGET的值一致了，並且380正好可以除盡4，共分配95個Granual，因此不會有多2M的問題
總結：
1.SGA_MAX_SIZE是靜態參數，而SGA_TARGET可以動態修改，當要改的SGA_TARGET值超過SGA_MAX_SIZE的值時，因爲不允許在內存中直接生效，即scope=both或memory都不行，因此必須指定scope=spfile,重啓後才能修改成功。如果此時沒有設置過SGA_MAX_SIZE得值，那麼無論是改大還是改小，重啓數據庫後，SGA_MAX_SIZE都回跟着SGA_TARGET做調整。
2.當先給SGA_MAX_SIZE設置了一個較大的值，重啓數據庫後，SGA_TARGET可以依然保持原有大小，相當於給SGA_TARGET先預留了一些內存，預先設置好可SGA可分配內存，等到數據庫出現壓力導致SGA內存不足，可以直接通過SGA_TARGET動態修改到這一上限而無需停庫。
3.當給SGA_TARGET設置非零值時，表示採用內存組件內存由oracle動態調整，如shared
pool,db buffer cache等，這些內存組件只會跟着SGA的大小動態進行調整（增大或減小），與其他值無關。
4.當SGA_TARGET設置爲零時，表示禁用內存組件由SGA自動管理，如果未做單獨設置，那麼保持原有值。此時可以根據具體場景需求，單獨
爲某個內存組件配置合理的內存大小。
5.如果是先設置了SGA_MAX_SIZE的值，再設置了SGA_TARGET，那麼只有當SGA_TARGET設置的值超過SGA_MAX_SIZE的值時，SGA_MAX_SIZE纔會在重啓生效後，調整到與SGA_TARGET的值一致，反之則不會改變。