1。add some large redo log group to system
alter database add redolog group 10('adfsadf' ,'dfasf') size 100M
alter database add redolog group 11('adfsadf' ,'dfasf') size 100M
alter database add redolog group 12('adfsadf' ,'dfasf') size 100M
2.
alter database switch logfile
select * from v$log
make the new redolog group as current
3.drop small redo log group
alter database drop logfile group ..
4.done
