The size of the internal data buffer is important for Oracle's active session history because it influences the amount of data that is displayed for "active" sessions".  The size of the ASH buffer is governed by the hidden parm _ash_size.

But what is the relationship between _ash_size and the ASH data buffer size?

Oracle Press notes that the hidden parameter _ash_size is set to 30 megabytes by default and that you can increase it at-will with an alter system statement:

SQL> alter system set "_ash_size"=200m

They also state that the maximum ASH size equation is:

max (min (#of CPUs * 2MB, 5% of SHARED_POOL_SIZE, 30MB), 1MB) 

However, Indepal Johal says that it the max size of the ASH buffer is as follows:

Max( Min ( No. of CPU * 2 M, 5% of SHARED_POOL_SIZE, 2% of  SGA_TARGET) ,1M)

What governs the max size of the ASH buffer?

This discussion shows some interesting anomalies in the ASH sizes, and that while the default is 30m, there is a hard-coded upper limit of 250m for _ash_size in 10g and 11g. 

Milen Kulev notes this hard limit for _ash_size appears to be 250 meg:

SQL>   select  POOL , NAME , BYTES   from  V$SGASTAT where NAME='ASH buffers' ;

POOL         NAME                            BYTES
------------ -------------------------- ----------
shared pool  ASH buffers                 266338304

See MOSC Note 1385872.1 for more information on setting the _ash_size hidden parameter. 

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