ASH buffer size

An undocumented feature is shown in modern Oracle version alert log
For example:
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 33554432 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;