Could not start database ORA-01092: Oracle instance terminated. Disconnection forced

Oracle11g CRS with an Oracle10g database.

When testing our new RAC installation we disconnected our NetApp storage brutally from the cluster by disconnecting the network cables.
Connected it back again and restarted the both nodes of the cluster. The crs came upp fine but the database failed to start.
The log shiws the following errors:

ORA-00604: error occured at recursive SQL level 1
ORA-00376: file 2 cannot be read at this time
ORA-01110: data file 2: '/oradata/myDb/dbfile/undotbs01.dbf'



$ . oraenv
myDb1

$ sqlplus / as sysdba

SQL> startup mount

SQL> select name, status, file# from gv$datafile;

NAME                                          STATUS                FILE#
----------------------------------   ------------------  -----
/oradata/myDb/dbfile/undotbs01.dbf   OFFLINE             2
/oradata/myDb/dbfile/undotbs02.dbf   ONLINE               3
/oradata/myDb/dbfile/system01.dbf     ONLINE                1

The undo-tablespace has suddenly gone offline!

I tried the following:

SQL> alter database recover automatic datafile 2;

SQL> alter database datafile 2 online;

SQL> alter database open;

database opened


This could also be done from RMAN:


$ rman target / nocatalog

RMAN> startup mount

RMAN> recover datafile 2;

RMAN> sql 'alter database datafile 2 online';

RMAN> alter database open;

database opened

All well again!