Today we had to wrangle a database that was having a problem, and going through the log for the instance I found this gem:
Active Session History (ASH) performed an emergency flush. This may mean that ASH is undersized. If emergency flushes are a recurring issue, you may consider increasing ASH size by setting the value of _ASH_SIZE to a sufficiently large value. Currently, ASH size is (setting) bytes. Both ASH size and the total number of emergency flushes since instance startup can be monitored by running the following query:
select total_size,awr_flush_emergency_count from v$ash_info;
While I knew that ASH did get flushed, I did not know about an emergency flush for ASH! Maybe that is the parameter that Tim Gorman shared during the Hotsos Symposium this year. So not only do we know this can pop into the database log but we are given the exact table so we can build an automated alert to tell us when ASH is flushed and how many emergencies have happened.