Local Undo in 12.2 PDB in RAC

In 12.1 all PDBs shared a single undo tablespace. With 12.2 oracle introduced local undo, where each container has it's own undo tablespace on each instance it is opened. Which mode is currently used could be checked with following.

SQL> select * from database_properties where property_name='LOCAL_UNDO_ENABLED';

PROPERTY_NAME        PROPERTY_VALUE  DESCRIPTION
-------------------- --------------- ------------------------------
LOCAL_UNDO_ENABLED   TRUE            true if local undo is enabled

It is possible to move from local undo mode to shared undo mode and vice versa.

In RAC instances this means there would be undo tablespaces per instance and per PDB. However, due to automatic naming convention used this may not be obvious until datafile path is examined. Below example was on a base release of 12.2 with Patch 26124078 applied [ID 2497491.1].

RAC CDB consists of two instances.

SQL> select inst_id,instance_name from gv$instance;

   INST_ID INSTANCE_NAME
---------- ----------------
         1 cdb12c1
         2 cdb12c2

Currently there are no PDBs except the seed.

SQL> show pdbs

    CON_ID CON_NAME                       OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
         2 PDB$SEED                       READ ONLY  NO

The current undo tablespaces are their datafile locations are as follows

    CON_ID TBLSPACE   NAME
---------- ---------- --------------------------------------------------------------------------------
         1 UNDOTBS1   +DATA/ORCL/DATAFILE/undotbs1.279.1000054779
         1 UNDOTBS2   +DATA/ORCL/DATAFILE/undotbs2.289.1000054817
         2 UNDOTBS1   +DATA/ORCL/81B657D10A1B2EE8E0535D00A8C08053/DATAFILE/undotbs1.295.1000054779

Each instance has it's own undo tablespace which is the expected setup on RAC.

SQL> select inst_id,con_id,name,value from gv$parameter where name='undo_tablespace' order by 1;

   INST_ID     CON_ID NAME                      VALUE
---------- ---------- ------------------------- --------------------
         1          1 undo_tablespace           UNDOTBS1
         2          1 undo_tablespace           UNDOTBS2

When creating a PDB and opening it which will create the undo tablespace for remote instance's PDB.

SQL> create pluggable database pdb3 admin user pdb1 identified by pdb1 default tablespace users ;

Pluggable database created.

SQL> alter pluggable database pdb3 open instances=all;

Pluggable database altered.

The alert log on the remote instance shows the creation of this local undo tablespace for the PDB on that instance.

PDB3(3):Opening pdb with no Resource Manager plan active
PDB3(3):CREATE SMALLFILE UNDO TABLESPACE undo_2 DATAFILE '+DATA' SIZE 193986560 AUTOEXTEND ON NEXT 5242880 MAXSIZE 34359721984 ONLINE
2019-02-13T11:35:09.410262+00:00
PDB3(3):[4337] Successfully onlined Undo Tablespace 5.
PDB3(3):Completed: CREATE SMALLFILE UNDO TABLESPACE undo_2 DATAFILE '+DATA' SIZE 193986560 AUTOEXTEND ON NEXT 5242880 MAXSIZE 34359721984 ONLINE
Pluggable database PDB3 opened read write

Login to the PDB and query the instance specific local undo tablespaces.

SQL> alter session set container=pdb3;

Session altered.

SQL> select inst_id,con_id,name,value from gv$parameter where name='undo_tablespace' order by 1;

   INST_ID     CON_ID NAME                      VALUE
---------- ---------- ------------------------- ----------
         1          3 undo_tablespace           UNDOTBS1
         2          3 undo_tablespace           UNDO_2

It appears that the local undo tablespace for PDB on instance 1 uses the same as the CDB$root's undo tablespaces "UNDOTBS1" this is not the case. The datafile for this undo tablesapce shows it's different to root's undo tablespace.

SQL>  select t.con_id,t.name tblspace,d.name from  v$tablespace t, v$datafile d where  d.con_id=t.con_id and t.ts#=d.ts# and t.name like 'UND%' order by 1,2;

    CON_ID TBLSPACE   NAME
---------- ---------- -------------------------------------------------------------------------------
         3 UNDOTBS1   +DATA/ORCL/81C59E3616323828E0535D00A8C059DD/DATAFILE/undotbs1.288.1000121653
         3 UNDO_2     +DATA/ORCL/81C59E3616323828E0535D00A8C059DD/DATAFILE/undo_2.285.1000121707

This shows that each PDB in each instance gets a local UNDO tablespace.

Useful metalink notes
Undo Modes in 12.2 Multitenant Databases - Local and Shared Modes [ID 2169828.1]
Different levels of encrypted undo tablespace across instances PDB In a 12.2 RAC - OCI . [ID 2497491.1]