Altering a Disk Group (add, drop, undrop, resize, rebalance)

To reduce the size of all disks under the failgroup, fgcontrol01; modify the command as follows:

ALTER DISKGROUP dskgrp01
RESIZE DISKS IN FAILGROUP fgcontrol01 SIZE 120G;

Manual rebalancing of a disk group is possible with the REBALANCE clause of the ALTER DISKGROUP command. A manual rebalance is seldom needed, but if the DBA feels that the speed of the rebalance operation is not enough, this command is deployed.

Recall what was reviewed regarding asm_power_limit in ASM architecture. This parameter has a profound effect on the rebalance operation as follows. The POWER clause used along with the REBALANCE option specifies the degree of parallelism and speed of the rebalance operation. The POWER value can be from 0 to 11 where 0 stops rebalancing and 11 is the fastest. The speed of an ongoing operation can be changed by altering the POWER with a new level. With a zero value for POWER, the rebalance is stopped until the value is revoked.

The asm_power_limit controls the degree of parallelization for rebalance operations. Even with a value of POWER at eleven (11), the degree of parallelization will not exceed the value specified in asm_power_limit (default = 1). So the rebalance operation is limited by this initialization parameter.

The asm_power_limit can be found in the v$asm_operation view. If the value of the DESIRED_POWER column is less than the ACTUAL_POWER column for a rebalance operation, then asm_power_limit is limiting the speed. The time remaining to finish the operation is found in the EST_MINUTES column of the view.