Understanding Oracle ASM Disk Group Administration

The disk discovery process locates the operating system names for disks that Oracle ASM can access. Disk discovery finds all of the disks that comprise a disk group to be mounted. The set of discovered disks also includes disks that could be added to a disk group.

An Oracle ASM instance requires an ASM_DISKSTRING initialization parameter value to specify its discovery strings. Only path names that the Oracle ASM instance has permission to open are discovered. The exact syntax of a discovery string depends various factors, such as the platform and whether Oracle Exadata disks are used. The path names that an operating system accepts are always usable as discovery strings.

For information about the ASM_DISKSTRING initialization parameter, refer to "ASM_DISKSTRING". For information about disk discovery, refer to "Oracle ASM Disk Discovery ". For information about Oracle ASM Filter Driver, refer to "Oracle ASM Filter Driver".

A disk group must be mounted by a local Oracle ASM instance before database instances can access the files in the disk group. 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. The disks that you add must be discovered by every Oracle ASM instance using its ASM_DISKSTRING initialization parameter. After you add a disk, Oracle ASM rebalancing operations move data onto the new disk. To minimize the rebalancing I/O, it is more efficient to add multiple disks at the same time.

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. If an error occurs while writing to a disk, then Oracle ASM drops the disk automatically.

For more information about altering disk group membership, see "Altering Disk Groups".

Rebalancing a disk group moves data between disks to ensure that every file is evenly spread across all of the disks in a disk group. When all of the files are evenly dispersed, all of the disks are evenly filled to the same percentage; this ensures load balancing. Rebalancing does not relocate data based on I/O statistics nor is rebalancing started based on I/O statistics. Oracle ASM rebalancing operations are controlled by the size of the disks in a disk group.

Oracle ASM automatically initiates a rebalance after storage configuration changes, such as when you add, drop, or resize disks. The power setting parameter determines the speed with which rebalancing operations occur.

You can manually start a rebalance to change the power setting of a running rebalance. A rebalance is automatically restarted if the instance on which the rebalancing is running stops. Databases can remain operational during rebalancing operations.

You can minimize the impact on database performance with the setting of the ASM_POWER_LIMIT initialization parameter. For more information about the power limit setting, see "ASM_POWER_LIMIT". For more information about disk rebalancing, see "Manually Rebalancing Disk Groups".