10200(consistent read buffer status)内部诊断事件可以用于探测一致性读CR(consistent read)块的访问情况，虽然cr读的统计信息可以从v$sysstat或AWR/statspack中获取，但是10200 event还是我们研究Consistent Read一致性读的有力工具。该事件可以通过在会话session级别设置ALTER SESSION SET EVENTS 或 DBMS_SYSTEM.SET_EV. Set 来开启，一般调用级别为Level 10。 该事件返回的trace跟踪文件，记录了为了实现一致性读的目的，哪些数据块以及这些块的各历史版本在执行过程中被创建(CR block creation)并检验(CR block inspection)，以找出Best CR block满足Consistent一致性。 注意10200 Internal Event主要是被ktrgtc和ktrget(call ktrget to get one block ->calling KTR layer to apply RBS to have consistent read Block;)这2个Oracle内核功能函数触发，这2个内部函数Internal Function的主要作用： ktrget:

• Initializes a buffer cache CR scan request
• Calls kcbgtcr for the best resident buffer to start from to build the CR buffer
• Calls ktrgcm to build the CR buffer by applying undo
• Returns CR buffer to the requestor

kcbgtcr:

• If successful, returns the “best” candidate(performed by ktrexf or examination function)
• Scans the hash bucket for the DBA for buffers that may be used to build a CR buffer
• If not successful, calls kcbget

  10200 event trace example:  

Consistent read started for block 0 : 0040081a

本文转自maclean_007 51CTO博客，原文链接:
http://blog.51cto.com/maclean/1278093