Call now: 252-767-6166  
Oracle Training Oracle Support Development Oracle Apps

 E-mail Us
 Oracle Articles
New Oracle Articles

 Oracle Training
 Oracle Tips

 Oracle Forum
 Class Catalog

 Remote DBA
 Oracle Tuning
 Emergency 911
 RAC Support
 Apps Support
 Oracle Support

 SQL Tuning

 Oracle UNIX
 Oracle Linux
 Remote s
 Remote plans
 Application Server

 Oracle Forms
 Oracle Portal
 App Upgrades
 SQL Server
 Oracle Concepts
 Software Support

 Remote S


 Consulting Staff
 Consulting Prices
 Help Wanted!


 Oracle Posters
 Oracle Books

 Oracle Scripts

Don Burleson Blog 







_ash_size and ASH data buffer size

Oracle Database Tips by Donald BurlesonApril 19,  2015

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. 

Oracle Training from Don Burleson 

The best on site "Oracle training classes" are just a phone call away! You can get personalized Oracle training by Donald Burleson, right at your shop!

Oracle training



Oracle Training at Sea
oracle dba poster

Follow us on Twitter 
Oracle performance tuning software 
Oracle Linux poster


Burleson is the American Team

Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

Verify experience! Anyone considering using the services of an Oracle support expert should independently investigate their credentials and experience, and not rely on advertisements and self-proclaimed expertise. All legitimate Oracle experts publish their Oracle qualifications.

Errata?  Oracle technology is changing and we strive to update our BC Oracle support information.  If you find an error or have a suggestion for improving our content, we would appreciate your feedback.  Just  e-mail:  

and include the URL for the page.


Burleson Consulting

The Oracle of Database Support

Oracle Performance Tuning

Remote DBA Services


Copyright © 1996 -  2020

All rights reserved by Burleson

Oracle ® is the registered trademark of Oracle Corporation.