I am totally new to oracle (just joined as a junior trainee DBA). I am slightly intimidated will the documentations I read everyday. Any kind of explanation will be highly appreciated.

I have a couple of instances each on 2 servers. Two 2 node RAC clusters on 2 servers. 10.2.0.2.0 on RHEL 3.

In database1:
TABLESPACE_NAME GB
---------- --------------------
UNDOTBS1 4.90625
UNDOTBS2 15.9136736

Instance1:
NAME TYPE VALUE
--------------------------- ----------- ---------
undo_management string AUTO
undo_retention integer 900
undo_tablespace string UNDOTBS1

Instance2:
NAME TYPE VALUE
------------------------- ----------- ---------
undo_management string AUTO
undo_retention integer 900
undo_tablespace string UNDOTBS2



In database2:
TABLESPACE_NAME GB
---------- ---------------------
UNDOTBS1 15.2107041
UNDOTBS2 11.0957684

Instance1:
NAME TYPE VALUE
----------------------------- --------------- ---------------
undo_management string AUTO
undo_retention integer 900
undo_tablespace string UNDOTBS1

Instance2:
NAME TYPE VALUE
--------------------------- ------------- -----------------
undo_management string AUTO
undo_retention integer 900
undo_tablespace string UNDOTBS2

From what I read last few weeks, in oracle 10g, if we use automatic undo management and undo retention, oracle will automatically allocate and deallocate segments when required and we do not need to bother about it growing once we set a proper size. But later read that if we do not set the MAX_FILE_SIZE (i think this is what it is), it will keep growing and allocate segments all over the file system and occupy space.

these are my questions:
1. When undo TS is 15GB, does it mean that these 15GB is taken from the disk space and sitting here "useless".
2. Do I have to shrink it (to re-claim space)?
3. What are the steps to do it? I read in one form thread that I can create a new tablespace and change that to be the undo tablespace and drop the old one.
4. what other information about my DB or server should I provide to complete my question?

Can someone kindly explain the procedure? Reference to a documentation will also help. But if someone can list the steps, that will be really kind.