DFS lock handle in RAC

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RAC环境中非常频繁的使用序列(sequence)。则'DFS lock handle'等待事件经常会出现。

通常的办法是增大序列缓存，且不使用排序选项即NOORDER。

创建序列时默认的cache是20，非排序NOORDER.

通过如下语句更新序列缓存:

alter sequence SEQ_name cache 200;

RAC and sequences [ID 853652.1]中的一段话：

When caching + ordering is used and RAC is enabled (cluster_database = true),
then the session wanting to get the NEXTVAL of a sequence need to get
an exclusive instance SV lock before inserting or updating the sequence values
in the the shared pool. When multiple sessions want the nextval of the same sequence,
then some sessions will wait on 'DFS lock handle' waitevent with id1 equal to
the sequence number.

参考文档：
http://twit88.com/blog/2009/08/28/oracle-performance-dfs-lock-handle-and-oracle-sequence/

2011-03-03