1) backup and recovery
1@@@@Instance Recovery Case
@@@analysis
Losing an Active Online Redo Log Group
    If the database is still running and the lost active redo log is not the current log, then
  issue the ALTER SYSTEM CHECKPOINT statement. If successful, then the active redo
  log becomes inactive, and you can follow the procedure in "Losing an Inactive Online
  Redo Log Group" on page 19-9. If unsuccessful, or if your database has halted, then
  perform one of procedures in this section, depending on the archiving mode.
    The current log is the one LGWR is currently writing to. If a LGWR I/O fails, then
  LGWR terminates and the instance crashes. In this case, you must restore a backup,
  perform incomplete recovery, and open the database with the RESETLOGS option.

One over one million
~~~~(1)switch logfile by system
~~~~(2)the current logfile group still in noarchived
~~~~(3)then shutdown abort
~~~~(4)restart db, we would miss that logfile which still noarchived. 


@@@Example:
SYS@ocm> startup
ORACLE instance started.
Total System Global Area  536870912 bytes
Fixed Size            2022112 bytes
Variable Size          251659552 bytes
Database Buffers      276824064 bytes
Redo Buffers            6365184 bytes
Database mounted.
ORA-16038: log 2 sequence# 81 cannot be archived
ORA-19809: limit exceeded for recovery files
ORA-00312: online log 2 thread 1: '/u01/app/oracle/oradata/OCM/onlinelog/o1_mf_2_8537yt0d_.log'
ORA-00312: online log 2 thread 1: '/u01/app/oracle/flash_recovery_area/OCM/onlinelog/o1_mf_2_8537ytvh_.log'


SYS@ocm> RECOVER DATABASE UNTIL CANCEL;
Media recovery complete.


SYS@ocm> alter database open;              
alter database open
*
ERROR at line 1:
ORA-01589: must use RESETLOGS or NORESETLOGS option for database open


SYS@ocm> alter database open resetlogs;
Database altered.