If the POWER clause is not specified in an ALTER DISKGROUP statement, or when rebalance is implicitly run by adding or dropping a disk, then the rebalance power defaults to the value of the ASM_POWER_LIMIT initialization parameter. You can adjust the value of this parameter dynamically. The range of values for the POWER clause is the same for the ASM_POWER_LIMIT initialization parameter. For information about the ASM_POWER_LIMIT initialization parameter, refer to "ASM_POWER_LIMIT".
The higher the power limit, the more quickly a rebalance operation can complete. Rebalancing takes longer with lower power values, but consumes fewer processing and I/O resources which are shared by other applications, such as the database.
Oracle ASM tries to keep a rebalance I/O for each unit of power. Each I/O requires PGA memory for the extent involved in the relocation.
The default value of 1 minimizes disruption to other applications. The appropriate value is dependent on your hardware configuration, performance requirements, and availability requirements. Oracle ASM always adjusts the power to fit available memory.
When the COMPATIBLE.ASM disk group is set to 11.2.0.2 or higher, the rebalance operation may be run as one process using asynchronous I/O. You can check the Oracle ASM alert log for details on the rebalance process.
If a rebalance is in progress because a disk is manually or automatically dropped, then increasing the power of the rebalance shortens the time frame during which redundant copies of that data on the dropped disk are reconstructed on other disks.
You can also affect rebalance behavior with the CONTENT.CHECK and THIN_PROVISIONED disk group attributes. For information about disk group attributes, refer to "Summary of Disk Group Attributes".
The EXPLAIN WORK SQL statement determines the amount of work for a rebalance operation and the resulting calculations are displayed in the V$ASM_ESTIMATE view.
For example:
EXPLAIN WORK FOR ALTER DISKGROUP data DROP DISK data_0000;
Explained.
SELECT est_work FROM V$ASM_ESTIMATE;
EST_WORK
--------
4211
EXPLAIN WORK SET STATEMENT_ID='online' FOR ALTER DISKGROUP data
ONLINE disk data_000;
Explained.
SELECT est_work FROM V$ASM_ESTIMATE WHERE STATEMENT_ID='online';
EST_WORK
--------
421
The V$ASM_ESTIMATE view provides information for adjusting ASM_POWER_LIMIT and the resulting power of rebalance operations. The EST_WORK column provides an estimate of the number of allocation units that have to be moved by the rebalance operation to complete.
The PASS column of V$ASM_OPERATION is updated for resync and rebalance operations. The contents of the column can be RESYNC, REBALANCE, or COMPACT. For example, the following SQL query shows values in the PASS column during a rebalance operation.
SQL> SELECT GROUP_NUMBER, PASS, STATE FROM V$ASM_OPERATION;
GROUP_NUMBER PASS STAT
------------ --------- ----
2 RESYNC WAIT
2 REBALANCE WAIT
2 COMPACT WAIT
For more information about rebalancing disk groups, refer to "Manually Rebalancing Disk Groups".