等待事件 enq: KO - fast object checkpoint

素颜马尾好姑娘i 2022-01-07 02:25 207阅读 0赞

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You see this event because object level checkpoint which happens when you run direct path reads on a segment (like with serial direct read full [**table**][table] scan or parallel execution full segment scans).  
Now the question is how much of your sessions response time is spent waiting for that event - and whether the winning in query speed due direct read/parallel execution outweighs the time spent waiting for this checkpoint.  
Waiting for KO enqueue - fast object checkpoint means that your session has sent CKPT process a message with instruction to do object level checkpoint and is waiting for a reply.  
CKPT in turn asks DBWR process to perform. the checkpoint and may wait for response.  
So you should check what the DBWR and CKPT processes are doing - whether they're stuck waiting for something (such controlfile enqueue) or doing IO or completely idle.

该等待事件是由于当进行TABLE FULL SCAN或并行查询整个段时，对象级别发生checkpoint,由于direct path read必须要从磁盘中读入到PGA中，因此必须等待checkpoint完成，将脏块写回磁盘。相当于写阻塞了读。当发生该等待事件，一个简单的查询就将变得非常慢。

1.查看检查点及写入进程

select sid,program from v$session where program like '%CKPT%' or program like '%DBW%';

2.查看这两个进程等待的内容

select sid,event,state,p1,p2,p3,seconds\_in\_wait from v$session\_wait where sid in (select sid  from v$session where program like '%CKPT%' or program like '%DBW%');

3.发现 db file async I/O submit 等待事件

解决方法:从metalink 文档来看，即使disk\_asynch\_io参数设置为true，操作系统本身也支持AIO；也并不代表oracle就能进行AIO操作。可以看到，还必须设置filesystem\_options参数，建议设置为ASYNCH或setall即可

4.查看参数 show parameter filesystemio\_options 和 show parameter disk,并修改参数

附加解决方法:1.db\_writer\_processes

2.增加REDO GROUP

[table]: http://space.itpub.net/15480802/viewspace-706375