Maintaining identical copies of data on one or more disks. Typically, mirroring is performed on duplicate hard disks at the operating system level, so that if one of the disks becomes unavailable, the other disk can continue to service requests without interruptions.
ASM mirroring is more flexible than operating system mirrored disks because ASM mirroring enables the redundancy level to be specified on a per-file basis. Thus two files can share the same disk group with one file being mirrored while the other is not. Mirroring takes place at the extent level. If a file is mirrored, depending upon redundancy level set for the file, each extent has one or more mirrored copies, and mirrored copies are always kept on different disks in the disk group. the three mirroring options that ASM supports on a per-file basis.
Automatic Storage Management's native mirroring mechanism is an option that can be set in the process of creating a disk group. You can specify the redundancy protection for that group, and if you prefer to use an external mirroring system or you have no need for fault-tolerance, you can set the redundancy to external, in which case ASM will not mirror at all. Unless otherwise specified, redundancy defaults to normal, in which case ASM mirrors each 1 MB extent. For the highest level of fault tolerance, you can specify high redundancy, which provides three-way mirroring. If mirroring is selected for an ASM disk group, failure groups can provide an additional level of data. A failure group is a set of disks in a disk group sharing a common resource whose failure you wish to protect against. Once the DBA has configured a failure group, ASM will intelligently place the redundant copies of the data in separate failure groups to ensure that the loss of one failure group will not result in loss of data. The higher in the stack that one can enable redundancy protection the larger the failures they can survive. The use of failure groups enables larger loss of storage components to not result in any lost data.