enq: FB - contention

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#enq FB - contention #等待事件 #format block

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本文深入探讨了在Oracle数据库中使用SQL查询来识别与Format Block Enqueue（enq:FB-contention）相关的等待类型及参数，解释其在ASSM表空间中的作用，并提供了在不同查询场景下的分析建议。

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select wait_class,name from v$event_name where name = 'enq: FB - contention';

WAIT_CLASS NAME
---------------------------- ----------------------------------------------------------------
Other enq: FB - contention

SQL> select name,PARAMETER1,PARAMETER2,PARAMETER3 from v$event_name where name = 'enq: FB - contention';

NAME PARAMETER1 PARAMETER2 PARAMETER3
-------------------------------- -------------------------------- -------------------------------- --------------------------------
enq: FB - contention name|mode tablespace # dba

"This is the Format Block enqueue, used only when data blocks are using ASSM (Automatic Segment Space Management or bitmapped freelists). As we might expect, common FB enqueue relate to buffer busy conditions, especially since ASSM tends to cause performance problems under heavily DML loads."

http://www.dba-oracle.com/t_enqueue_wait_types_.htm

"well the format block enqueue ("enq: fb contention") is related to grabbing
new blocks in ASSM tablespaces, during an insert operation for example. It
might be a good idea at this point to find out what the 'query' is that you
are actually doing and if it is happening in parallel etc. Incidentally are
both of these queries on the one system, or is one PROD and one UAT?"

http://www.freelists.org/post/oracle-l/Execution-time-in-SHARED-SERVER-vs-DEDICATED-SERVER-RAC-10gR2,12

转载：http://blog.chinaunix.net/uid-499812-id-3046288.html