ASM REBALANCING

Dynamic Storage Configuration

ASM enables you to change the storage configuration without having to take the database offline. It automatically rebalances—redistributes file data evenly across all the disks of the disk group—after you add disks to or drop disks from a disk group.

Should a disk failure occur, ASM automatically rebalances to restore full redundancy for files that had extents on the failed disk. When you replace the failed disk with a new disk, ASM rebalances the disk group to spread data evenly across all disks, including the replacement disk.

Tuning Rebalance Operations

The V$ASM_OPERATION view provides information that can be used for adjusting ASM_POWER_LIMIT and the resulting power of rebalance operations. The V$ASM_OPERATION view also gives an estimate in the EST_MINUTES column of the amount of time remaining for the rebalance operation to complete. You can see the effect of changing the rebalance power by observing the change in the time estimate.

Effects of Adding and Dropping Disks from a Disk Group

ASM automatically rebalances whenever disks are added or dropped. For a normal drop operation (without the FORCE option), a disk is not released from a disk group until data is moved off of the disk through rebalancing. Likewise, a newly added disk cannot support its share of the I/O workload until rebalancing completes. It is more efficient to add or drop multiple disks at the same time so that they are rebalanced as a single operation. This avoids unnecessary movement of data.

For a drop operation, when rebalance is complete, ASM takes the disk offline momentarily, and then drops it, setting disk header status to FORMER.

You can add or drop disks without shutting down the database. However, a performance impact on I/O activity may result.