Active Session History (ASH) Performed an Emergency Flush

Set proper ASH size to avoid emergency flushes.

The alert log with following message now and then :

"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 134217728 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;"

Query the current ASH size :

SQL> select total_size from v$ash_info;


Current size is 134M according both the query and the messages in alert.log. We can add another 50% by running the following sql. The size should be appropriate if without any more similar messages in alert.log. Otherwise increase it up to maximum 254M.

SQL>alter system set "_ash_size"=200M;