ASM Rebalance

The Rebalance operation provides an even distribution of file extents across all disks in the diskgroup. The rebalance is done on each file to ensure balanced I/O load.

The RBAL background process manages the rebalance activity. It examines the extent map for each file and redistributes the extents to new storage configuration. The RBAL process will calculate estimation time and the work required to perform the rebalance activity and then message the ARBx processes to actually perform the task. The number of ARBx process starts is determined by the parameter ASM_POWER_LIMIT.

There will be one I/O for each ARBx process at a time. Hence the impact of physical movement of file extents will be low. The asm_power_limit parameter determines the speed of the rebalance activity. It can have values between 0 and 11. If the value is 0 no rebalance occurs. If the value is 11 the rebalance takes place at full speed. The power value can also be set for specific rebalance activity using Alter Diskgroup statement.

The rebalance operation has various states, they are

WAIT: No operations are running for the group.
RUN: A rebalance operation is running for the group.
HALT: The DBA has halted the operation.
ERROR: The operation has halted due to errors.

You can query the V$ASM_OPERATION to view the status of rebalance activity.

The rebalance activity is an asynchronous operation, i.e., the operation runs in the background while the users can perform other tasks. In certain situation you need the rebalance activity to finish successfully before performing the other tasks. To make the operation synchronous you add a keyword WAIT while performing the rebalance as shown below.

SQL> Alter diskgroup ASMDB Add Disk ‘/dev/sdc4’ Rebalance power 4 WAIT;

The above statement will not return the control to the user unless the rebalance operation ends.

Thanks