Oracle 12cR1 RAC

col sid for a10
col name for a20
col value for a20
select sid,name,value from v$spparameter where name='undo_tablespace';

SID        NAME                 VALUE
---------- ------------------------------ --------------------
orcl2      undo_tablespace      UNDOTBS2
orcl1      undo_tablespace      UNDOTBS1

SYS@orcl1> show parameter create_file

NAME                   TYPE          VALUE
------------------------------------ ---------------------- ------------------------------
db_create_file_dest        string         +DATADG
SYS@orcl1> 
SYS@orcl1> show parameter undo

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

--不自动扩展，没有最大值
SYS@orcl1> alter tablespace undotbs1 add datafile  size 10m;
SYS@orcl1> alter tablespace undotbs2 add datafile  size 20m;
--扩容到30m
SYS@orcl1> alter database datafile '+DATADG/ORCL/DATAFILE/undotbs2.273.964970069' resize 30m;
--自动扩展，最大100M
SYS@orcl1> alter database datafile '+DATADG/ORCL/DATAFILE/undotbs2.273.964970069' autoextend on maxsize 100m;

col file_name for a50
set lines 200
select file_name,bytes/1024/1024 mb,maxbytes/1024/1024 max_mb from dba_data_files where tablespace_name like 'UNDOTBS%';
FILE_NAME                                                                            MB     MAX_MB
---------------------------------------------------------------------------- ------------- ----------
+DATADG/ORCL/DATAFILE/undotbs1.261.964970069      155 32767.9844
+DATADG/ORCL/DATAFILE/undotbs2.273.964970069      30        100
+DATADG/ORCL/DATAFILE/undotbs1.367.965681119      10          0
+DATADG/ORCL/DATAFILE/undotbs2.369.965681285      20          0