alter system suspend & alter system resume In Oracle Database

If the ALTER SYSTEM SUSPEND statement is entered on one system in an Oracle RAC configuration, then the internal locking mechanisms propagate the halt request across instances, thereby suspending I/O operations for all active instances in a given cluster."

 10:41:18 SQL> select database_status from v$instance;
DATABASE_STATUS
-----------------

SUSPENDED
Elapsed: 00:00:00.01
10:41:26 SQL>
10:41:27 SQL>
10:41:28 SQL>
10:41:28 SQL> alter system resume;
System altered.
Elapsed: 00:00:00.63
10:41:34 SQL>
10:41:35 SQL>
10:41:35 SQL> select database_status from v$instance;
DATABASE_STATUS
-----------------
ACTIVE
Elapsed: 00:00:00.01
10:41:43 SQL>