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One of my favorite things about ASM is the ability to add and remove disks/luns from a diskgroup with no down time and an automatic rebalancing of the data on the luns.    This creates all kinds of possibilities.

To remove a lun, use the following command:

alter diskgroup dgroup1 drop disk asmdisk01;

10G R2

The init.ora parameter ASM_POWER_LIMIT is used to influence the throughput and speed of the rebalance operation. The range of values for ASM_POWER_LIMIT is 0–11, where a value of 11 is full throttle and a value of 1 (the default) is low speed.A value of 0, which turns off automatic rebalance, should be used with caution

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If the POWER clause is not specified in an ALTER DISKGROUP command, or when rebalance is implicitly invoked by adding or dropping a disk, the rebalance power defaults to the value of the ASM_POWER_LIMIT initialization parameter. You can adjust this parameter dynamically. The higher the limit, the faster a rebalance operation may complete. Lower values cause rebalancing to take longer, but consume fewer processing and I/O resources. This leaves these resources available for other applications, such as the database. The default value of 1 minimizes disruption to other applications. The appropriate value is dependent upon your hardware configuration as well as performance and availability requirements.

The V$ASM_OPERATION view provides information that can be used for adjusting ASM_POWER_LIMIT and the resulting power of rebalance operations. The V$ASM_OPERATION view also gives an estimate in the EST_MINUTES column of the amount of time remaining for the rebalance operation to complete. You can see the effect of changing the rebalance power by observing the change in the time estimate.

You can manually rebalance the files in a disk group using the REBALANCE clause of the ALTER DISKGROUP statement.This will alter the power limit for any new operations, but not existing ones. To change an existing operation, you must directly alter the power limit on the diskgroup involved in the operation:

The POWER clause of the ALTER DISKGROUP…REBALANCE statement specifies the degree of parallelization, and thus the speed of the rebalance operation. It can be set to a value from 0 to 11. A value of 0 halts a rebalancing operation until the statement is either implicitly or explicitly reinvoked.

If you want the ALTER DISKGROUP…REBALANCE command to wait until the rebalance operation is complete before returning, you can add the WAIT keyword to the REBALANCE clause. This is especially useful in scripts. The command also accepts a NOWAIT keyword, which invokes the default behavior of conducting the rebalance operation asynchronously. You can interrupt a rebalance running in wait mode by typing CTRL-C on most platforms. This causes the command to return immediately with the message ORA-01013: user requested cancel of current operation, and to continue the rebalance operation asynchronously.

11G R2
The ASM_POWER_LIMIT initialization parameter specifies the default power for disk rebalancing in a disk group. The range of values is 0 to 1024. The default value is 1. A value of 0 disables rebalancing. Higher numeric values enable the rebalancing operation to complete more quickly, but might result in higher I/O overhead and more rebalancing processes.

For disk groups that have the disk group ASM compatibility set to less than 11.2.0.2, the operational range of values is 0 to 11 inclusive. If the value for ASM_POWER_LIMIT is larger than 11, a value of 11 is used for these disk groups.

Oracle  recommendation set asm_power_limit value 4 on Exadata Database Machine ,and the value can  impact of the application due to the the ASM rebalance generated Io loss minimization on Exadata.
The big advantage to being able to set the power limit to 0 comes when it is necessary to add/drop several disks at the same time. If the power limit is already set, then each add/drop will be serialized and each operation will have to complete before the next can begin. If, however, the power limit is set to 0 first, then submit all the add/drop operations and finally set the power limit to a non-zero number for the disk group, the operations are paralellized and run at the same time.

references
Jay Caviness‘ article   and oracle document

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