作者：eygle |English Version 【转载时请以超链接形式标明文章出处和作者信息及本声明】
链接：http://www.eygle.com/archives/2009/08/asmm_sga_tuning.html
链接：http://www.eygle.com/archives/2009/08/asmm_sga_tuning.html
我们知道，在Oracle10g自动SGA管理模式下，SGA的相关参数不受手动控制。
可是有些时候我们需要调整一下参数，如db_keep_cache_size，可能会遇到内存不足的错误：
SQL> alter system set db_keep_cache_size=3584M scope=both sid='*';
alter system set db_keep_cache_size=3584M scope=both sid='*'
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-00384: Insufficient memory to grow cache
在自动模式下，内存设置由双下划线参数控制，如__db_cache_size:
SQL> SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ
2 FROM SYS.x$ksppi x, SYS.x$ksppcv y
3 WHERE x.indx = y.indx AND x.ksppinm LIKE '%&par%'
4 /
Enter value for par: db_cache_size
old 3: WHERE x.indx = y.indx AND x.ksppinm LIKE '%&par%'
new 3: WHERE x.indx = y.indx AND x.ksppinm LIKE '%db_cache_size%'
NAME VALUE DESCRIB
------------------------------ -------------------- ------------------------------------------------------------
__db_cache_size 482344960 Actual size of DEFAULT buffer pool for standard block size
buffers
db_cache_size 0 Size of DEFAULT buffer pool for standard block size buffers
注意当前的SGA_TARGET和SGA_MAX_SIZE的设置：
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 680M
sga_target big integer 680M
我们可以先调节sga_target参数：
SQL> alter system set sga_target=400M;
System altered.
SQL> select * from v$sgainfo;
NAME BYTES RES
------------------------------ ---------- ---
Fixed SGA Size 1263848 No
Redo Buffers 7122944 No
Buffer Cache Size 205520896 Yes
Shared Pool Size 197132288 Yes
Large Pool Size 4194304 Yes
Java Pool Size 4194304 Yes
Streams Pool Size 0 Yes
Granule Size 4194304 No
Maximum SGA Size 713031680 No
Startup overhead in Shared Pool 41943040 No
Free SGA Memory Available 293601280
11 rows selected.
如果内存空闲，可以调节，那么可以看到参数的变化：
SQL> SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ
2 FROM SYS.x$ksppi x, SYS.x$ksppcv y
3 WHERE x.indx = y.indx AND x.ksppinm LIKE '%&par%'
4 /
Enter value for par: db_cache_size
old 3: WHERE x.indx = y.indx AND x.ksppinm LIKE '%&par%'
new 3: WHERE x.indx = y.indx AND x.ksppinm LIKE '%db_cache_size%'
NAME VALUE DESCRIB
------------------------------ -------------------- ------------------------------------------------------------
__db_cache_size 188743680 Actual size of DEFAULT buffer pool for standard block size
buffers
db_cache_size 0 Size of DEFAULT buffer pool for standard block size buffers
再调高sga_target：
SQL> alter system set sga_target=600M;
System altered.
SQL> SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ
2 FROM SYS.x$ksppi x, SYS.x$ksppcv y
3 WHERE x.indx = y.indx AND x.ksppinm LIKE '%&par%'
4 /
Enter value for par: db_cache
old 3: WHERE x.indx = y.indx AND x.ksppinm LIKE '%&par%'
new 3: WHERE x.indx = y.indx AND x.ksppinm LIKE '%db_cache%'
NAME VALUE DESCRIB
------------------------------ -------------------- ------------------------------------------------------------
__db_cache_size 398458880 Actual size of DEFAULT buffer pool for standard block size
buffers
db_cache_size 0 Size of DEFAULT buffer pool for standard block size buffers
_db_cache_pre_warm TRUE Buffer Cache Pre-Warm Enabled : hidden parameter
_db_cache_crx_check FALSE check for costly crx examination functions
_db_cache_process_cr_pin_max maximum number of cr pins a process may have
db_cache_advice ON Buffer cache sizing advisory
_db_cache_advice_sample_factor 4 cache advisory sampling factor
_db_cache_advice_batch_size 128 cache advisory simulation batch size
8 rows selected.
观察SGA组件的变化：
SQL> select * from v$sgainfo;
NAME BYTES RES
------------------------------ ---------- ---
Fixed SGA Size 1263848 No
Redo Buffers 7122944 No
Buffer Cache Size 415236096 Yes
Shared Pool Size 197132288 Yes
Large Pool Size 4194304 Yes
Java Pool Size 4194304 Yes
Streams Pool Size 0 Yes
Granule Size 4194304 No
Maximum SGA Size 713031680 No
Startup overhead in Shared Pool 41943040 No
Free SGA Memory Available 83886080
11 rows selected.
现在增加db_keep_cache_size，数据库会自动调整，去减少db_cache_size的内存分配：
SQL> show parameter keep
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
buffer_pool_keep string
control_file_record_keep_time integer 7
db_keep_cache_size big integer 16M
SQL> alter system set db_keep_cache_size=200M;
System altered.
SQL> alter system set db_keep_cache_size=300M;
System altered.
SQL> show parameter sga
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 680M
sga_target big integer 600M
SQL> SET linesize 120
SQL> COL name for a30
SQL> COL value for a20
SQL> COL describ for a60
SQL> SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ
2 FROM SYS.x$ksppi x, SYS.x$ksppcv y
3 WHERE x.indx = y.indx AND x.ksppinm LIKE '%&par%'
4 /
Enter value for par: db_cache_size
old 3: WHERE x.indx = y.indx AND x.ksppinm LIKE '%&par%'
new 3: WHERE x.indx = y.indx AND x.ksppinm LIKE '%cache_size%'
NAME VALUE DESCRIB
------------------------------ -------------------- ------------------------------------------------------------
__db_cache_size 100663296 Actual size of DEFAULT buffer pool for standard block size
buffers
db_cache_size 0 Size of DEFAULT buffer pool for standard block size buffers
这种曲线调节，关键是释放了buffer cache的内存，这取决于系统的压力、buffer使用情况，在无人使用时较易成功。
-The End-