How to change Rebalance Power in ASM Instances?

Whenever a new diskgroup is added or dropped, ASM automatically performs REBALANCING operation.Power used by rebalacing defaults to value specified for asm_power_limit initialization parameter.We can check the status of rebalancing from v$asm_operation view.

Increasing the value of ASM_POWER_LIMIT, reduces the estimated time for completion of Rebalance Operation.

ASM power limit can be increased using the below command

alter system set asm_power_limit =11;

To know more about values for this parameter, check this link:

http://www.appsdbadiaries.com/2017/06/asm-features-rebalancing.html

But this does not increase the rebalancing power of current operation. 

To increase the rebalancing power of ongoing operation, use the below command

 alter diskgroup dg1 rebalance power 11;

We can also specify rebalance power at the time of adding or dropping disks.

alter diskgroup dg1 add disk d01 rebalance power 11;

When to use asm_power_limit 0?

Whenever there is a need to add/drop several disks (like migrations), setting the power limit to 0 is highly beneficial.We can avoid the waiting time for rebalance operation to complete,before the next disk is rebalanced.In such cases, set the asm_power_limit to 0, add/drop all the disks and set the value of asm_power_limit to a non zero value.

Rebalance operation will be parallelized and completes faster.

Oracle Database ASM Features: REBALANCING

ASM has the ability to rebalance data across the disks whenever a disk is added,dropped or replaced.
The performance of rebalance operation is controlled by initialization parameter ASM_POWER_LIMIT.

ASM_POWER_LIMIT Parameter

Default value 1

Range of Values 0 to 11 (Prior to 11gR2)

Range of Values 0 to 1024(From 11gR2)

For 10g Databases:

SQL> alter diskgroup DG1 rebalance power 12;
alter diskgroup DG1 rebalance power 12
                                              *
ERROR at line 1:
ORA-15102: invalid POWER expression

SQL> alter diskgroup DG1 rebalance power 11;

Diskgroup altered.


For 11g AND 12c Databases

SQL> alter diskgroup DG1 rebalance power 1025;
alter diskgroup DG1 rebalance power 1025
                                      *
ERROR at line 1:
ORA-15102: invalid POWER expression

SQL> alter diskgroup DG1 rebalance power 1024;

Diskgroup altered.

The higher the value of rebalancing power, faster would be rebalancing operation.