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undo_management parameter Tips

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

The initialization parameters shown in Table 8.6 are relevant for undo management.




If AUTO, use automatic undo management mode. If MANUAL, use manual undo management mode.


A dynamic parameter specifying the name of an undo tablespace to use.


A dynamic parameter specifying the length of time to retain undo. Default is 900 seconds.


Set to TRUE to suppress error messages generated by manual undo SQL statements when operating in automatic undo management mode. If set to FALSE, error messages are issued. This is a dynamic parameter.

Table 8.6: Undo Initialization Parameters

To view the existing parameters, use the command:

SQL>  show parameter undo

NAME                             TYPE        VALUE
-------------------------------- ----------- ----------
undo_management                  string      AUTO
undo_retention                   integer     900
undo_suppress_errors             boolean     FALSE
undo_tablespace                  string      UNDOTBS1  

UNDO Tablespace Features

Some of the general features of undo tablespaces include:

* They are locally managed with system extent allocation.

* The UNDO tablespace cannot be used for any purpose other than UNDO segments and no operations can be performed on system-generated undo segments. When trying to create a table using the following statement:

create table salesman(
   name varchar2(4),
   salesid number(6) )
   undo_rbs1 ;

The following message will be generated: ORA-30022: Cannot create segments in undo tablespace

* Only one UNDO tablespace can be used at the instance level.

* UNDO segments are created automatically and are owned by PUBLIC. They are named as _syssmun$ and are not manually manageable. The number of segments depends on the sessions parameter.

* When Automatic undo management (AUM) is chosen, it will not be possible to manage an undo or rollback.

For example, if this statement is issued:

create public rollback segment rsg1 tablespace rbs1 ;

The following error will be generated: ORA-30019: Illegal rollback segment operation in automatic mode

* Only the undo segments of the active UNDO tablespace and the SYSTEM rollback segments are kept online.

* All the instances in RAC must run in the same undo mode (i.e. AUTO or MANUAL).

Table 8.7 shows the views that provide undo information.




Contains statistics for monitoring and tuning undo space. Use this view to help estimate the amount of undo space required for the current workload.


For automatic undo management mode, information reflects behavior of the undo segments in the undo tablespace.


Contains undo segment information.


Shows the commit time for each extent in the undo.

Table 8.7: Dynamic performance views related to undo

System Rollback Segment

The system rollback segment is shared, and is used by all instances for transactions involving system objects. Even when using the automatic undo management method, Oracle can still use the system rollback segment. There is only one system rollback segment for the entire database. It resides in the SYSTEM tablespace.

Using Rollback Segments ? Manual Method

Even though the recommended method is to use automatic undo management (AUM), the traditional method of manually creating rollback segments is allowed. In such a case, set the initialization parameter undo_management=MANUAL. When operating in manual mode, each instance needs at least one rollback segment to start successfully.

A rollback segment may be acquired by only one instance. Once an instance acquires a particular rollback segment, it is used exclusively by that instance. No other instance can write to it, although they can read from it for the purpose of read-consistent images.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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