ASM Fast Rebalance

This article talks about the 11g new feature - ASM Fast rebalance and how to implement in 11g ASM Instance

In 11g, you can use the STARTUP RESTRICT command to control access to an ASM instance while you perform maintenance. This startup option is used to implement ASM Fast Rebalance feature. Rebalance operations that occur while a disk group is in RESTRICTED mode eliminate the lock and unlock extent map messaging between ASM instances in Oracle RAC environments.

When an ASM instance is started with 'STARTUP RESTRICT' option and is active in this mode, all of the disk groups are mounted in RESTRICTED mode which prevents databases from connecting to the ASM instance. In addition, the restricted clause of the ALTER SYSTEM statement is disabled for the ASM instance.

Given below is an example to show ASM Fast Rebalance Feature.

Startup ASM instance in Restricted mode

SQL> startup restrict;
ASM instance started
Total System Global Area 92065792 bytes
Fixed Size 1297984 bytes
Variable Size 65601984 bytes
ASM Cache 25165824 bytes

Now in case you wish to mount the Disk group in normal mode , you get following errors

SQL> alter diskgroup dgext11gasm mount;
alter diskgroup dgext11gasm mount
*
ERROR at line 1:
ORA-15251: only restricted mount is allowed in the ASM instance

You need to use restricted clause along with mount option.

SQL> alter diskgroup dgext11gasm mount restricted;
Diskgroup altered.

SQL> alter diskgroup dgext11gasm rebalance;
Diskgroup altered.

If ASM diskgroup is mounted in restricted mode,Database Instance will not be able to access files residing in the ASM Diskgroup

SQL> startup
ORACLE instance started.

Total System Global Area 418484224 bytes
Fixed Size 1300200 bytes
Variable Size 268437784 bytes
Database Buffers 142606336 bytes
Redo Buffers 6139904 bytes
Database mounted.
ORA-01157: cannot identify/lock data file 5 - see DBWR trace file
ORA-01110: data file 5: '+DGEXT11GASM/o11g/datafile/test.256.627345731'

Unlike normal Database Instance, alter system disable restricted session is disabled for ASM Instance.
You need to shutdown and startup ASM Instance in Normal mode to access files reside in the ASM Diskgroup for Database Instance

ASM Instance
=============
SQL> alter system disable restricted session;
alter system disable restricted session
*
ERROR at line 1:
ORA-15000: command disallowed by current instance type

SQL>shutdown immediate
SQL>startup

From Oracle

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