One day suddenly, I got a warning on my Warehouse system that,
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 16777216 bytes. Both ASH size and the total number of emergency flushes since instance start-up can be monitored by running the following query:

SQL>    select total_size,awr_flush_emergency_count from v$ash_info;

TOTAL_SIZE AWR_FLUSH_EMERGENCY_COUNT
———- ————————-
33554432                         8

Reasons:
Typically some activity on system causes more active sessions, therefore filling the ASH buffers faster than usual causing this message to be displayed. This indicates the buffers might need to be increased to support peak activity on the database.
It is not a problem per session, just indicates the buffers might need to be increased to support peak activity on the database.

Fix:
Increase the size of ASH (Consider increasing around 50%) — (current_value+50% of current_value);

SQL> alter system set “_ASH_SIZE”=25165824 scope=both;
System altered.