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POWER In the POWER clause, specify a value from 0 to 11, where 0 stops the rebalance operation and 11 permits Automatic Storage Management to execute the rebalance as fast as possible. T he value you specify in the POWER clause defaults to the value of the ASM_POWER_LIMIT initialization parameter.
ASM uses disks of a disk group in a round robin fashion. ASM works on a file basis and distributes allocation units among all disks in a disk group when data is created or modified.
A rebalance is a separate process that attempts to spread data evenly among all disks in a disk group and also manages hot and cold regions for performance. This is normally automatically triggered when adding or removing disks in a disk group. ASM uses an indexing technique to spread extents on available disks and does not need to re-stripe all data, but only the amount of data so that it gets evenly distributed among all disks.
The affect of setting the rebalance power to 0 (11g) and hence disabling the rebalance process will therefore disable I/O optimization based on device availability in a disk group. In your example, the new disk is still used in a round-robin fashion like any other device in the disk group when creating data, but it won't rebalance existing data and leave the disk group in an unbalanced state.
ASM also maintains data redundancy (fast mirror resync) among failure groups depending on your settings, but as far as I understand, this is not affected by the ASM_POWER_LIMIT setting.
Bottom line is, the new disk will be used by new transactions but existing data will not be rebalanced.