SYMPTOMS
Below error is seen in 12.1.0.2 database alert log:
---
Tue Sep 26 08:01:33 2017
Error ORA-235 occurred during an un-locked control file transaction. This
error can be ignored. The control file transaction will be retried.
---
CAUSE
Expected behavior
SOLUTION
Starting from Oracle 12c, controlfile transactions are enhanced. We are no longer request a controlfile enqueue before scanning a particular controlfile section.
This is done to minimize locking issues and improve performance and scalability. Due to this change, the process reading a controlfile section may sometimes encounter ORA-235
if the same section is being modified by another process. When this occurs, the reading process will simply re-read the section again. You can safely ignore this message, it's not an issue.
The errors cannot be suppressed at database level by setting any event or initialization parameter. The reason is that although the errors are harmless, they do give an idea of the concurrency on
the controlfile and can help debug issues relating to slow controlfile i/o etc so, this enhancement has been added specifically by development. It's best to modify any monitoring scripts to ignore/exclude this error.
Since this is working as designed and these are simply informational, the messages can simply be ignored.
按照ORACLE的说法，这个ora错误是由于12c及以上版本数据库对控制文件的读取做了改进，不在需要像oracle11g或者以下版本一样在扫描特定的控制文件之前先请求一个控制文件队列（这样做是为了更好的性能和可用性，减小锁的影响），但是这样也可能出现在读取控制文件时，刚好其他进程在修改控制文件，则会出现此报错，但是此报错可以忽略，因为这个进程会再次读取控制文件。
可以看出，如果偶然出现这个报错，是可以忽略的，但是如果一直大面积出现，还是要注意是否有进程一直在修改控制文件引起其他进程无法正常读取导致这个报错。
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