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

 
 Home
 E-mail Us
 Oracle Articles


 Oracle Training
 Oracle Tips

 Oracle Forum
 Class Catalog


 Remote DBA
 Oracle Tuning
 Emergency 911
 RAC Support
 Apps Support
 Analysis
 Design
 Implementation
 Oracle Support


 SQL Tuning
 Security

 Oracle UNIX
 Oracle Linux
 Monitoring
 Remote s
upport
 Remote plans
 Remote
services
 Application Server

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

 Remote S
upport  
 Development  

 Implementation


 Consulting Staff
 Consulting Prices
 Help Wanted!

 


 Oracle Posters
 Oracle Books

 Oracle Scripts
 Ion
 Excel-DB  

Don Burleson Blog 


 

 

 


 

 

 

 

 

Oracle Automatic undo management and transactions_per_rollback_segment

Oracle Tips by Burleson Consulting
 

Iíve been doing a lot of database health checks on 9i and now 10g databases recently. Most are using the automatic undo management feature and on the whole it does a pretty good job of managing the undo segments (for you other old timers, rollback segments).

However, I have been noticing, shall I say, some rather retro behavior as a result of the automatic undo management. In the bad old days when we managed the undo segments manually we would tune to reduce extends and the subsequent shrinks which resulted. The shrinks would cause waits on the next session to use the segment as the segment shrank back to the ďoptimalĒ setting.

The usual method to set the initial, next and optimal was to examine the rollback segment views and determine such values as average transaction size, max transaction size and also, determine the number of active DML and DDL statements (SELECT didnít and doesnít really count for much in rollback/undo activity, generally speaking). From these values we could set initial, next and optimal to reduce over extending the segments and reduce the subsequent shrinks and waits as well as the needed number of segments.

What seems to be happening is that Oracle looks at two basic parameters, TRANSACTIONS (based on 1.1*SESSIONS) and TRANSACTIONS_PER_ROLLBACK_SEGMENT, and then uses an internal algorithm to determine the number of undo segments to create in the undo tablespace. The size seems to be determined by the number created and the overall size of the tablespace. So, if you set up for 300 SESSIONS this usually means about 330 TRANSACTIONS, the TRANSACTIONS_PER_ROLLBACK_SEGMENT defaults to 5 so Oracle right from the gate assumes you will ultimately need 66 undo segments. Seems they forgot that generally speaking, only about 1 in 10 ďtransactionsĒ in most databases actually do DML/DDL and that 90% are usually SELECT. I have seen in almost all Oracle databases with automatic undo used, that reach near the setting of SESSIONS number of actual connected users, that Oracle over allocates the number of undo segments leaving sometimes dozens offline and never used.

The other thing I see a great deal of is the old extends, shrinks and waits we used to spend so much time tuning away. In many cases I also see the old ORA-01555 (snapshot too old) errors coming back. If the undo segment tablespace is too small and Oracle creates too many small segments, then it is quite easy to see why.

So, am I saying donít use automatic undo? No, not at all. I say use the automatic undo, but understand how to use it wisely. Essentially, utilize the TRANSACTIONS_PER_ROLLBACK_SEGMENT to control the number of segments created, and size the undo tablespace large enough that the segments are sized appropriately. In addition, if you are not going to use 300 sessions, donít set the SESSIONS to 300! Make sure to align the SESSIONS parameter to the proper number of expected sessions.

If you need to change the undo segment configuration in your environment (look at the v$rollstat view to see if you have excessive waits, shrinks and extends) you will need to alter the parameters, configure a second undo segment tablespace, and then restart the database (if you changed SESSIONS or TRANSACTIONS_PER_ROLLBACK_SEGMENT) to utilize the new settings.

What seems to be happening, is that as a start the Oracle algorithm will create 10 active undo segments and sets the MAX_ROLLBACK_SEGMENTS parameter equal to the value TRANSACTIONS/TRANSACTIONS_PER_ROLLBACK_SEGMENT, as the number of session increases, Oracle adds a new segment at each increment of TRANSACTIONS_PER_ROLLBACK_SEGMENT above 10*TRANSACTIONS_PER_ROLLBACK_SEGMENT that your user count reaches. It doesnít seem to care if the session is doing anything, it just has to be present. Oracle leaves the new segment offline, just taking up space, unless the user does DML or DDL. The minimum setting Oracle seems to utilize is 30 for MAX_ROLLBACK_SEGMENTS. For example, with a SESSIONS setting of 300, this resulted in a TRANSACTIONS setting of 330, with a default TRANSACTIONS_PER_ROLLBACK_SEGMENT of 5, the MAX_ROLLBACK_SEGMENTS parameter was set to 66. With a setting of 20, instead of a new setting of 17 (330/20 rounded up) we get a setting of 30. If we set it to 10, we get a setting of 33. Note that even with manually setting the parameter MAX_ROLLBACK_SEGMENTS, if automatic UNDO management is turned on, your setting will be overridden with the calculated one.

So watch the settings of SESSIONS, TRANSACTIONS_PER_ROLLBACK_SEGMENT and the size of the undo tablespace to properly use the automatic undo feature in Oracle9i and 10g.

 


 

 

  
 
  
 
 
 

 
 
 

Oracle training Excel
 
Oracle performance tuning software 
 

 

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 -  2014

All rights reserved by Burleson

Oracle © is the registered trademark of Oracle Corporation.