Just some weeks ago we’ve learned that setting the hidden underscore parameter:
may cause trouble during upgrade. This parameter is used in very rare cases to have under all circumstances and situations this specified number of UNDO’s online. Now during upgrade this may result in massive latch contention – and there’s a patch available as well. Recommendation is to unset it during upgrade.
I don’t think that many people will hit this as I personally haven’t seen databases with this underscore in their init.ora or spfiles. So take this post more or less as a reminder for myself 🙂
Edit: I removed the mentioned bug number as it was incorrect (Dec 4, 2017)
Further Edit: As this seems to cause misunderstandings: This blog post is from 2012 and the issue described is first of all very rare, and second happened with upgrade to 188.8.131.52 only!!! (Feb 28, 2018)
Please remove this blog ASAP as its not correct and creating confusion for lot of customers.
There is no such issue for this parameter!!!!!
would you please mind to drop me an email instead of commenting in such a rude tone?
The blog post is 6 years old and first of all it says “This parameter is used in very rare cases”.
Second it says as well: “So take this post more or less as a reminder for myself”.