Hi,all . From the document ,i had confused about the following .
Automatic Undo Management in Oracle RAC
url >> http://docs.oracle.com/cd/B19306_01/rac.102/b28759/adminrac.htm#CHDGAIFJ
Oracle automatically manages undo segments within a specific undo tablespace that is assigned to an instance. Only the instance assigned to the undo tablespace can modify the contents of that tablespace. However, each instance can read the undo data blocks created by any instance. Also, when performing transaction recovery, any instance can update any undo tablespace, as long as that undo tablespace is not currently being used by another instance for undo generation or transaction recovery
Say you're running a 2-node RAC and node 2 dies. The services which were running on node 2 now get re-located to node 1. It is then possible that node 1 will perform transaction rollback/recovery and, when it does so, it will need to be able to read from node 2's undo tablespace (and maybe update the undo segment headers in node 2's undo tablespace, too).