Oracle各个命中率参数的总结及调优

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关于Oracle中各个命中率的计算以及相关的调优

 

1、Library Cache的命中率

SELECT SUM(pinhits)/sum(pins)  FROM V$LIBRARYCACHE;

通常在98%以上,否则,需要要考虑加大共享池,绑定变量,修改cursor_sharing等参数

2、计算共享池内存使用率

SELECT (1 - ROUND(BYTES / (&TSP_IN_M * 1024 * 1024), 2)) * 100 || '%'
FROM V$SGASTAT
WHERE NAME = 'free memory' AND POOL = 'shared pool';

其中: &TSP_IN_M是你的总的共享池的SIZE(M)

共享池内存使用率,应该稳定在75%-90%间,太小浪费内存,太大则内存不足

3、空闲的共享池内存

SELECT * FROM V$SGASTAT  WHERE NAME = 'free memory' AND POOL = 'shared pool';

4、db buffer cache命中率

#计算公式:Hit ratio = 1 - [physical reads/(block gets + consistent gets)]

SELECT NAME, PHYSICAL_READS, DB_BLOCK_GETS, CONSISTENT_GETS,
1 - (PHYSICAL_READS / (DB_BLOCK_GETS + CONSISTENT_GETS)) "Hit Ratio"
FROM V$BUFFER_POOL_STATISTICS
WHERE NAME='DEFAULT';

通常应在90%以上,否则,需要调整,加大DB_CACHE_SIZE

5、数据缓冲区命中率

SQL> select value from v$sysstat where name ='physical reads';

SQL> select value from v$sysstat where name ='physical reads direct';

SQL> select value from v$sysstat where name ='physical reads direct (lob)';

SQL> select value from v$sysstat where name ='consistent gets';

SQL> select value from v$sysstat where name = 'db block gets';

这里命中率的计算应该是

x = physical reads direct + physical reads direct (lob)

命中率 =100 - ( physical reads - x) / (consistent gets + db block gets - x)*100

通常如果发现命中率低于90%,则应该调整应用可可以考虑是否增大数据缓冲区

6、共享池的命中率(重要)

select sum(pinhits-reloads)/sum(pins)*100 "hit radio" from v$librarycache;

假如共享池的命中率低于95%,就要考虑调整应用(通常是没使用bind var )或者增加内存

7、计算在内存中排序的比率

SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (memory)';—查询内存排序数

SQL>SELECT * FROM v$sysstat t WHERE NAME='sorts (disk)';—查询磁盘排序数

SQL>SELECT round(&sort_in_memory/(&sort_in_memory+&sort_in_disk),4)*100||'%' FROM dual;

此比率越大越好,太小整要考虑调整,加大PGA

8、PGA的命中率

#计算公式:BP x 100 / (BP + EBP)
BP: bytes processed
EBP: extra bytes read/written

SQL>SELECT * FROM V$PGASTAT WHERE NAME='cache hit percentage';

或者从OEM的图形界面中查看

我们可以查看一个视图以获取Oracle的建议值:

SQL>SELECT round(PGA_TARGET_FOR_ESTIMATE/1024/1024) target_mb,
ESTD_PGA_CACHE_HIT_PERCENTAGE cache_hit_perc,
ESTD_OVERALLOC_COUNT
FROM V$PGA_TARGET_ADVICE;

The output of this query might look like the following:


TARGET_MB CACHE_HIT_PERC ESTD_OVERALLOC_COUNT

----------          --------------       --------------------

   63              23                   367

125              24                    30

250              30                     3

375              39                     0

500              58                     0

600              59                     0

700              59                     0

800              60                     0

900              60                     0

在此例中:PGA至少要分配375M

我个人认为PGA命中率不应该低于50%

9、共享区字典缓存区命中率

#计算公式:SUM(gets - getmisses - usage -fixed) / SUM(gets)
SQL>select sum(gets-getmisses-usage-fixed)/sum(gets)
from v$rowcache;

命中率应大于0.85

10、数据高速缓存区命中率

#计算公式:1-(physical reads / (db block gets + consistent gets))
SQL>select name,value
from v$sysstat
where name in ('physical reads','db block gets','consistent gets');

命中率应大于0.9最好

11、共享区库缓存区命中率

#计算公式:SUM(pins - reloads) / SUM(pins)
SQL>select sum(pins-reloads)/sum(pins)
from v$librarycache;

命中率应大于0.99

 

Honest1y
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