You can use the STARTUP RESTRICT command to control access to an Oracle ASM instance while you perform maintenance. When an Oracle ASM instance is active in this mode, all of the disk groups that are defined in the ASM_DISKGROUPS parameter are mounted in RESTRICTED mode. This prevents databases from connecting to the Oracle ASM instance. In addition, the restricted clause of the ALTER SYSTEM statement is disabled for the Oracle ASM instance. The ALTER DISKGROUP diskgroup MOUNT statement is extended to enable Oracle ASM to mount a disk group in restricted mode.
When you mount a disk group in RESTRICTED mode, the disk group can only be mounted by one instance. Clients of Oracle ASM on that node cannot access that disk group while the disk group is mounted in RESTRICTED mode. The RESTRICTED mode enables you to perform maintenance tasks on a disk group in the Oracle ASM instance without interference from clients.
Rebalance operations that occur while a disk group is in RESTRICTED mode eliminate the lock and unlock extent map messaging that occurs between Oracle ASM instances in an Oracle RAC environment. This improves the overall rebalance throughput. At the end of a maintenance period, you must explicitly dismount the disk group and remount it in normal mode.