Disk Group Redundancy

If you specify redundancy during the creation of the disk group, then Oracle ASM stores a copy of extent into another failure group in the same disk group. Failure groups are created for NORMAL and HIGH redundancy disk groups. A disk group with EXTERNAL redundancy should use an external file system to manage mirroring (using RAID or any other means).

There are three types of disk groups based on the Oracle ASM redundancy level. The redundancy levels are:

1.   External redundancy: In case of external redundancy, Oracle ASM does not take care of mirroring your data. It’s assumed that the underlying file system has the necessary capability to mirror the blocks (for example, using a RAID configuration or by any other means). But in the case of external redundancy, any write error causes a forced dismount of the disk group.

2.   Normal redundancy: In case of NORMAL REDUNDANCY, Oracle ASM mirrors the extents in the file. So we have two copies of every extent in a file. A loss of one ASM disk does not cause data loss because we have a mirror copy still available. Space requirement in the case of NORMAL REDUNDANCY is double that of the external redundancy, so we have to make sure that we have double the space required by the database.

3.   High redundancy: In case of HIGH redundancy, Oracle ASM makes two copies of the original data, so we have three copies in total. Because of this, loss of disk in two different failure groups is tolerated. If we don’t have enough online failure groups to satisfy the disk group mirroring, Oracle ASM allocates as many mirrors as possible and it allocates the remaining required mirror once the sufficient number of failure groups are available. This redundancy needs thrice the amount of space than data.