Active Session History (ASH) Performed An Emergency Flush Messages In The Alert Log

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APPLIES TO:

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 <some value> 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.

Then increase the value for _ash_size by some value, like 50% more than what is currently allocated.  For example if total_size = 16MB, then an increase of 50% more would be (16MB + (16MB * 50%)) = 24MB. 

You can verify the change using the following select: