Wednesday, September 28, 2011

Converting from Rollback to Undo

This procedure will drop the existing rollback segments and tablespace, and replace them with an auto-managed undo tablespace.

  • 1. Identify the rollback segments and tablespaces to be removed:
    select segment_name
    , tablespace_name
    , status
    from dba_rollback_segs
    where tablespace_name != 'SYSTEM'
    /
  • 2. Produce offline and drop statements for each rollback segment:
    select 'alter rollback segment ' || segment_name || ' offline;'
    from dba_rollback_segs
    where tablespace_name = '&TS'
    and tablespace_name != 'SYSTEM'
    /

    select 'drop rollback segment ' || segment_name || ';'
    from dba_rollback_segs
    where tablespace_name = '&TS'
    and tablespace_name != 'SYSTEM'
    /
    Review the output and, if you are happy with it, cut and paste it back into sqlplus.

  • 3. Make a note of the old tablespace's size and location, then drop it.
    drop tablespace <tablespace_name> including contents and datafiles
    /
  • 4. Create a new 'undo' tablespace in place of the old one.
    create undo tablespace undo
    datafile '<path/file>' size <size>
    extent management local
    /
  • 5. Update the initialisation parameters
    If you are using a spfile (and you should be!) run the following commands:
    alter system reset rollback_segments scope=spfile sid='*';
    alter system set undo_management=auto scope=spfile;
    alter system set undo_tablespace=undo scope=spfile;
     If you are still using a pfile (init.ora) then do the following:
    Remove the following settings:
    rollback_segments=...
    Add/alter these two lines:
    undo_management=auto
    undo_tablespace=undo
  • 6. Restart the instance
    Note the location of the alert log. If there is a problem during start-up, you will probably receive nothing more than a message saying 'instance aborted'. The alert log will contain a slightly more detailed explanation.

    Restart the instance:
    shutdown immediate
    startup
    Rerun the query from step 1. There will be a whole load of rollback segments with names like '_SYSSMU1$'. This is an indictaion that the instance is now using auto undo. 

Sukhwinder Singh
Apps DBA

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