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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