If there has ever been the need to remove a disk from an ASM disk group or add, respectively re-add a disk, it should be known that the rebalancing which occurs after such an action can be significant and this should not be done unless absolutely needed. Especially in a clustered ASM configuration, the rebalancing can cause the sending of lots of extent map lock and extent map unlock messages between ASM instances.
If a diskgroup is mounted in restricted mode by one ASM instance in a cluster, no other instance can mount the ASM disk group and access any files there. This eliminates the overhead of sending locking messages, which improves the performance of the rebalancing operation.
An ASM disk group which is mounted in restricted mode on one node of the cluster can be maintained fully like a disk group which is mounted in non-restricted mode, noting that now the maintenance procedure can be finished much faster.
After the maintenance operation is finished, it is necessary to unmount the disk group explicitly and then re-mount it in normal mode. This causes all disk groups to be mounted by this particular instance in restricted mode for maintenance:. It is not allowed to mount disk groups in restricted mode while the ASM cluster is performing a rolling upgrade operation. Preferred mirror read is a feature which is particularly relevant for extended cluster environments.
Such configurations are also called stretched clusters. They are typically used to mirror entire data centers across distances longer than 6 miles. Process flow for ASM rebalancing: 1. On the ASM instance, a disk is added or dropped from a diskgroup.
This invokes the RBAL process to create the rebalance plan and then begin coordination of the redistribution 3. RBAL will calculate estimation time and work required to perform the task and then message the ARBx processes to actually handle the request. The Continuing Operations Directory metadata will be updated to reflect a rebalance activity.
Each extent to be relocated is assigned to an ARBx process. ARBx performs rebalance on these extents. Mounting the disk group requires discovering all of the disks and locating the files in the disk group that is being mounted. You can explicitly dismount a disk group. Oracle reports an error if you attempt to dismount a disk group without the force option when any of the disk group files are open. It is possible to have disks fail in excess of the Oracle ASM redundancy setting. If this happens, then the disk group is forcibly dismounted.
If the disk group is forcibly dismounted, a database cannot access files in the disk group. For more information about disk groups, see " Mounting and Dismounting Disk Groups ". You can add a disk to an existing disk group to add space and to improve throughput. The specified discovery string identifies the disk or disks that you could add. After you add a disk, Oracle ASM rebalancing operations move data onto the new disk.
You can drop a disk from a disk group if it fails or to re-purpose capacity. Use the Oracle ASM disk name to drop a disk, not the discovery string device name.
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