ASH buffers are stored in the shared pool. Oracle wants to keep around 30 minutes of history, but the more active sessions, combined with the shared pool size and the sample frequency, will obviously influence how far back in time ASH activity is available. ASH space could be as little as 1MB or as much as 30MB.14 To see the current memory dedicated to ASH, execute the following:

You may be able to specify memory allocated to ASH by setting the _ash_size (in bytes) instance parameter, but experience has shown Oracle does not always respect this setting.

By default, ASH is enabled on all Oracle Database 10g and above systems. With the instance parameter statistics_level=typical (default), ASH will be enabled. You can also directly turn ASH on or off by setting the instance parameter _ash_enable to either true or false. By default, when ASH buffers are written to the AWR tables, only one out of every ten samples is written. This is controlled by the _ash_disk_filter_ratio parameter, which has a default value of 10.