1，問題描述：

SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
PL/SQL Release 11.2.0.4.0 - Production
CORE 11.2.0.4.0 Production
TNS for Solaris: Version 11.2.0.4.0 - Production
NLSRTL Version 11.2.0.4.0 - Production

alter日誌報下面內容：
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 266338304 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;

SQL> select total_size/1024/1024,awr_flush_emergency_count from v$ash_info;

TOTAL_SIZE/1024/1024 AWR_FLUSH_EMERGENCY_COUNT
-------------------- -------------------------
250 2

2，問題解決：

Applies to:
Oracle Server - Enterprise Edition - Version: 11.2.0.2 and later [Release: 11.2 and later ]
Information in this document applies to any platform.

Symptoms
Periodically see the following message in the alert log.

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

Solution
The current ASH size is displayed in the message in the alert log, or can be found using the following SQL statement.
select total_size from v$ash_info;

Then increase the value for _ash_size by some value, like 50% more than what is currently allocated. For example if total_size = 250MB, then an increase of 50% more would be (250MB + (250MB * 50%)) = 375MB.

sqlplus / as sysdba
alter system set "_ash_size"=393216000 scope=spfile;

Active Session History (ASH) performed an emergency flush

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