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select size_for_estimate, buffers_for_estimate ,ESTD_PHYSICAL_READ_factor,ESTD_PHYSICAL_READS from v$db_cache_advice
2 where block_size=‘8192‘ and advice_status=‘ON‘;
中，size_for_estimate表示预计的数据库高速缓冲区的大小（即：db_cache_size的值）。ESTD_PHYSICAL_READS表示预计的物理读。
当增加数据库高速缓冲区的的时候，物理读（ESTD_PHYSICAL_READS）越少，说明命中率越高。
1.优化缓冲区大小、提高服务器的命中率
db_cache_size big integer 838860800
2.查看缓冲区命中率是否需要调优.
select 1 - ((physical.value - direct.value - lobs.value) / logical.value)
"Buffer Cache Hit Ratio"
from v$sysstat physical,v$sysstat direct,v$sysstat lobs,v$sysstat logical
where physical.name = ‘physical reads‘
and direct.name=‘physical reads direct‘
and lobs.name=‘physical reads direct (lob)‘
and logical.name=‘session logical reads‘;
当命中率>90%说明命中率很高了
3。获取推荐的值
select name,size_for_estimate,v$db_cache_advice.ESTD_PHYSICAL_READS from v$db_cache_advice
where block_size=‘8192‘ and advice_status=‘ON‘;
set linesize 1000
---20100619Oracle9i数据 Solaris 9i操作系统
SQL> set linesize 1000
SQL> select size_for_estimate, buffers_for_estimate ,ESTD_PHYSICAL_READ_factor,ESTD_PHYSICAL_READS from v$db_cache_advice
where block_size=‘8192‘ and advice_status=‘ON‘;
SIZE_FOR_ESTIMATE BUFFERS_FOR_ESTIMATE ESTD_PHYSICAL_READ_FACTOR ESTD_PHYSICAL_READS
----------------- -------------------- ------------------------- -------------------
80 9925 28.9757 1436311200
160 19850 2.1053 104360120
240 29775 1.5819 78413087
320 39700 1.4262 70693980
400 49625 1.3543 67131735
480 59550 1.278 63349434
560 69475 1.1893 58954568
640 79400 1.1325 56135206
720 89325 1.0762 53347837
800 99250 1 49569438
880 109175 .7067 35030953
SIZE_FOR_ESTIMATE BUFFERS_FOR_ESTIMATE ESTD_PHYSICAL_READ_FACTOR ESTD_PHYSICAL_READS
----------------- -------------------- ------------------------- -------------------
960 119100 .3991 19784701
1040 129025 .2305 11423374
1120 138950 .1927 9552903
1200 148875 .1506 7466278
1280 158800 .1501 7438186
1360 168725 .1501 7438186
1440 178650 .1501 7438186
1520 188575 .1501 7438186
1600 198500 .1501 7438186
SIZE_FOR_ESTIMATE M 为单位：
当SIZE_FOR_ESTIMATE=80M 的时候 ESTD_PHYSICAL_READS=1436311200
当SIZE_FOR_ESTIMATE=1120M 的时候 ESTD_PHYSICAL_READS=9552903
当SIZE_FOR_ESTIMATE=1280M 的时候 ESTD_PHYSICAL_READS=7438186
之后ESTD_PHYSICAL_READS固定了
所以应该过大db_cache_size=1120M的值使得
4.修改发现DB_cache_size太大了。过大SGA区域解决
alter system set db_cache_size=1120M
--sga设置太小了导致
SQL> alter system set db_cache_size=1120M;
alter system set db_cache_size=1120M
*
ERROR 位于第 1 行:
ORA-02097: 无法修改参数，因为指定的值无效
ORA-00384: 没有足够的内存来增加高速缓存的大小
之前的值
sga_max_size big integer 1494715120
SQL>alter system set SGA_MAX_SIZE=3500M scope=spfile;
系统已更改。
重新启动与关闭解决
SQL> shutdown immediate
数据库已经关闭。
已经卸载数据库。
ORACLE 例程已经关闭。
SQL> startup
ORACLE 例程已经启动。
Total System Global Area 3675756336 bytes
Fixed Size 735024 bytes
Variable Size 2835349504 bytes
Database Buffers 838860800 bytes
Redo Buffers 811008 bytes
数据库装载完毕。
数据库已经打开。
查看命中率\当前只有80的满足要求
SQL> select size_for_estimate, buffers_for_estimate ,ESTD_PHYSICAL_READ_factor,ESTD_PHYSICAL_READS from v$db_cache_advice
2 where block_size=‘8192‘ and advice_status=‘ON‘;
SIZE_FOR_ESTIMATE BUFFERS_FOR_ESTIMATE ESTD_PHYSICAL_READ_FACTOR ESTD_PHYSICAL_READS
----------------- -------------------- ------------------------- -------------------
80 9925 1 1528
160 19850 1 1528
240 29775 1 1528
320 39700 1 1528
400 49625 1 1528
480 59550 1 1528
560 69475 1 1528
640 79400 1 1528
720 89325 1 1528
800 99250 1 1528
880 109175 1 1528
SIZE_FOR_ESTIMATE BUFFERS_FOR_ESTIMATE ESTD_PHYSICAL_READ_FACTOR ESTD_PHYSICAL_READS
----------------- -------------------- ------------------------- -------------------
960 119100 1 1528
1040 129025 1 1528
1120 138950 1 1528
1200 148875 1 1528
1280 158800 1 1528
1360 168725 1 1528
1440 178650 1 1528
1520 188575 1 1528
1600 198500 1 1528
命中率降低了？
SQL> select 1 - ((physical.value - direct.value - lobs.value) / logical.value)
2 "Buffer Cache Hit Ratio"
3 from v$sysstat physical,v$sysstat direct,v$sysstat lobs,v$sysstat logical
4 where physical.name = ‘physical reads‘
5 and direct.name=‘physical reads direct‘
6 and lobs.name=‘physical reads direct (lob)‘
7 and logical.name=‘session logical reads‘;
Buffer Cache Hit Ratio
----------------------
.906673167
继续缩小SGA大小
SQL>alter system set SGA_MAX_SIZE=3000M scope=spfile;
SQL> shutdown immediate
数据库已经关闭。
已经卸载数据库。
ORACLE 例程已经关闭。
SQL> startup
ORACLE 例程已经启动。
Total System Global Area 3155661888 bytes
Fixed Size 734272 bytes
Variable Size 2315255808 bytes
Database Buffers 838860800 bytes
Redo Buffers 811008 bytes
数据库装载完毕。
数据库已经打开。
select size_for_estimate, buffers_for_estimate ,ESTD_PHYSICAL_READ_factor,ESTD_PHYSICAL_READS from v$db_cache_advice
where block_size=‘8192‘ and advice_status=‘ON‘;
继续测试：等待30分钟之后测试
SQL> alter system set db_cache_size=1120M;
select 1 - ((physical.value - direct.value - lobs.value) / logical.value)
"Buffer Cache Hit Ratio"
from v$sysstat physical,v$sysstat direct,v$sysstat lobs,v$sysstat logical
where physical.name = ‘physical reads‘
and direct.name=‘physical reads direct‘
and lobs.name=‘physical reads direct (lob)‘
and logical.name=‘session logical reads‘;
---命中率逐渐的提高了
---半个小时之后查询命中率是98%
SQL> select 1 - ((physical.value - direct.value - lobs.value) / logical.value)
2 "Buffer Cache Hit Ratio"
3 from v$sysstat physical,v$sysstat direct,v$sysstat lobs,v$sysstat logical
4 where physical.name = ‘physical reads‘
5 and direct.name=‘physical reads direct‘
6 and lobs.name=‘physical reads direct (lob)‘
7 and logical.name=‘session logical reads‘;
Buffer Cache Hit Ratio
----------------------
.980309028
-----查询推荐的值
show parameter db_block_size
8192
select name,size_for_estimate,v$db_cache_advice.ESTD_PHYSICAL_READS from
v$db_cache_advice where block_size=‘8192‘ and advice_status=‘ON‘;
NAME SIZE_FOR_ESTIMATE ESTD_PHYSICAL_READS
-------------------- ----------------- -------------------
DEFAULT 112 9581
DEFAULT 224 9514
DEFAULT 336 9514
DEFAULT 448 9514
DEFAULT 560 9514
DEFAULT 672 9514
DEFAULT 784 9514
DEFAULT 896 9514
DEFAULT 1008 9514
DEFAULT 1120 9514
DEFAULT 1232 9514
NAME SIZE_FOR_ESTIMATE ESTD_PHYSICAL_READS
-------------------- ----------------- -------------------
DEFAULT 1344 9514
DEFAULT 1456 9514
DEFAULT 1568 9514
DEFAULT 1680 9514
DEFAULT 1792 9514
DEFAULT 1904 9514
DEFAULT 2016 9514
DEFAULT 2128 9514
DEFAULT 2240 9514
显示只要112M大小的空间就可以稳定降低ESTD_PHYSICAL_READS
alter system set db_cache_size=112M
NAME SIZE_FOR_ESTIMATE ESTD_PHYSICAL_READS
-------------------- ----------------- -------------------
DEFAULT 16 0
DEFAULT 32 0
DEFAULT 48 0
DEFAULT 64 0
DEFAULT 80 0
DEFAULT 96 0
DEFAULT 112 0
DEFAULT 128 0
DEFAULT 144 0
DEFAULT 160 0
DEFAULT 176 0
NAME SIZE_FOR_ESTIMATE ESTD_PHYSICAL_READS
-------------------- ----------------- -------------------
DEFAULT 192 0
DEFAULT 208 0
DEFAULT 224 0
DEFAULT 240 0
DEFAULT 256 0
DEFAULT 272 0
DEFAULT 288 0
DEFAULT 304 0
DEFAULT 320 0
---查看命中率
select 1 - ((physical.value - direct.value - lobs.value) / logical.value)
"Buffer Cache Hit Ratio"
from v$sysstat physical,v$sysstat direct,v$sysstat lobs,v$sysstat logical
where physical.name = ‘physical reads‘
and direct.name=‘physical reads direct‘
and lobs.name=‘physical reads direct (lob)‘
and logical.name=‘session logical reads‘;
Buffer Cache Hit Ratio
----------------------
.982795739
说明Oracle更换峰值的情况决定db_cache_size的大小。
所以设置最大峰值满足的情况。
alter system set db_cache_size=1120M;满足峰值的时候最大值
select name,size_for_estimate,v$db_cache_advice.ESTD_PHYSICAL_READS from
v$db_cache_advice where block_size=‘8192‘ and advice_status=‘ON‘;
oracle10g Solaris 10 sparc系统调整
--查
select name,size_for_estimate,v$db_cache_advice.ESTD_PHYSICAL_READS from
v$db_cache_advice where block_size=‘8192‘ and advice_status=‘ON‘;
-------------------- ----------------- -------------------
DEFAULT 128 1206647
DEFAULT 256 1067789
DEFAULT 384 987752
DEFAULT 512 965164
DEFAULT 640 913815
DEFAULT 768 893485
DEFAULT 896 867321
DEFAULT 1024 808884
DEFAULT 1152 599909
DEFAULT 1280 344618
--此时开始保持在一个稳定的读取值
alter system set db_cache_size=1196M
DEFAULT 1296 321763
NAME SIZE_FOR_ESTIMATE ESTD_PHYSICAL_READS
-------------------- ----------------- -------------------
DEFAULT 1408 285030
DEFAULT 1536 280506
DEFAULT 1664 280325
DEFAULT 1792 279816
DEFAULT 1920 266871
DEFAULT 2048 235838
DEFAULT 2176 231298
DEFAULT 2304 228483
DEFAULT 2432 224971
DEFAULT 2560 219467
--修改
alter system set db_cache_size=1196M
--查看命中率
select 1 - ((physical.value - direct.value - lobs.value) / logical.value)
"Buffer Cache Hit Ratio"
from v$sysstat physical,v$sysstat direct,v$sysstat lobs,v$sysstat logical
where physical.name = ‘physical reads‘
and direct.name=‘physical reads direct‘
and lobs.name=‘physical reads direct (lob)‘
and logical.name=‘session logical reads‘;
Oracle优化 -- 关于Database Buffer Cache相关参数DB_CACHE_SIZE的优化设置
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原文地址：http://www.cnblogs.com/login2012/p/5752687.html
