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; TOTAL_SIZE ---------- 134217728
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;