Oracle ASM rebalance - Turn it up. To 11?

If you've ever seen or heard of the movie This is Spinal Tap, then you have likely heard the phrase "Turn it up to 11." Why bring this up?

When Automatic Storage Management (ASM) was introduced as a method for configuring storage for Oracle, one of the features was the ability to rebalance data across all disks when disks were added or replaced. The value used to control how aggressively Oracle rebalances the disks is the REBALANCE POWER. And yes, the original maximum value for rebalancing was 11, as an homage to the movie.

The Evolution of Rebalance Power Limits

While the "Spinal Tap" limit was a fun bit of trivia, the maximum value for REBALANCE POWER changed significantly with the release of Oracle 11.2.0.2.

As per the documentation for the ASM_POWER_LIMIT parameter, the maximum value is no longer 11, but 1024. I've asked a number of DBAs about this, and it seems that knowledge of this expanded limit is not very well known.

Why Does the Limit Matter?

Imagine that an 11.2.0.4 ASM diskgroup has had disks replaced, and the task took longer than expected. Now you want to speed up the rebalance as much as possible to ensure redundancy is restored quickly.

If you run the following bit of SQL

alter diskgroup data rebalance power 11; 

On Oracle 10g, that would be full power. But on an 11.2.0.4 or 12c database, that sets the power to only 1.07% of the maximum allowed value, having very little effect on how aggressive Oracle will be.

The correct SQL to "turn it up to the actual max" in modern versions would be

alter diskgroup data rebalance power 1024; 

Version Comparison Demonstration

The following demonstration confirms the documented maximum values for REBALANCE POWER across 10.2.0.4, 11.2.0.2, and 12.1.0.2 databases.

Oracle 10g (The 11 Limit

SQL> select version from v$instance; VERSION ----------------- 10.2.0.4.0  SQL> alter diskgroup ASM_COOKED_FS rebalance power 12; * ERROR at line 1: ORA-15102: invalid POWER expression  SQL> alter diskgroup ASM_COOKED_FS rebalance power 11; Diskgroup altered. 

Oracle 11gR2 and 12c (The 1024 Limit

-- Testing Oracle 11.2.0.2 SQL> select version from v$instance; VERSION ----------------- 11.2.0.2.0  SQL> alter diskgroup fra rebalance power 1025; * ERROR at line 1: ORA-15102: invalid POWER expression  SQL> alter diskgroup fra rebalance power 1024; Diskgroup altered.  -- Testing Oracle 12.1.0.2 SQL> select version from v$instance; VERSION ----------------- 12.1.0.2.0  SQL> alter diskgroup data rebalance power 1025; * ERROR at line 1: ORA-15102: invalid POWER expression  SQL> alter diskgroup data rebalance power 1024; Diskgroup altered. 

In summary, if you are running a modern version of Oracle, don't get stuck in the past—ensure your rebalance tasks are running at the appropriate speed for your hardware and maintenance windows.

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