The redundancy level that you choose for the Oracle ASM disk group determines how Oracle ASM mirrors files in the disk group, and determines the number of disks and amount of disk space that you require. Except when using external redundancy, Oracle ASM mirrors all Oracle Clusterware files in separate failure groups within a disk group. A quorum failure group, a special type of failure group, contains mirror copies of voting files when voting files are stored in normal or high redundancy disk groups. The disk groups that contain Oracle Clusterware files (OCR and voting files) have a higher minimum number of failure groups than other disk groups because the voting files are stored in quorum failure groups in the Oracle ASM disk group.
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High redundancy
In a high redundancy disk group, Oracle ASM uses three-way mirroring to increase performance and provide the highest level of reliability. A high redundancy disk group requires a minimum of three disk devices (or three failure groups). The effective disk space in a high redundancy disk group is one-third the sum of the disk space in all of its devices.
For Oracle Clusterware files, a high redundancy disk group requires a minimum of five disk devices and provides five voting files and one OCR (one primary and two secondary copies). For example, your deployment may consist of three regular failure groups and two quorum failure groups. Note that not all failure groups can be quorum failure groups, even though voting files need all five disks. With high redundancy, the cluster can survive the loss of two failure groups.
While high redundancy disk groups provide a high level of data protection, you should consider the greater cost of additional storage devices before deciding to select high redundancy disk groups.
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Normal redundancy
In a normal redundancy disk group, to increase performance and reliability, Oracle ASM by default uses two-way mirroring. A normal redundancy disk group requires a minimum of two disk devices (or two failure groups). The effective disk space in a normal redundancy disk group is half the sum of the disk space in all of its devices.
For Oracle Clusterware files, a normal redundancy disk group requires a minimum of three disk devices and provides three voting files and one OCR (one primary and one secondary copy). For example, your deployment may consist of two regular failure groups and one quorum failure group. With normal redundancy, the cluster can survive the loss of one failure group.
If you are not using a storage array providing independent protection against data loss for storage, then Oracle recommends that you select normal redundancy.
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External redundancy
An external redundancy disk group requires a minimum of one disk device. The effective disk space in an external redundancy disk group is the sum of the disk space in all of its devices.
Because Oracle ASM does not mirror data in an external redundancy disk group, Oracle recommends that you use external redundancy with storage devices such as RAID, or other similar devices that provide their own data protection mechanisms.
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Flex redundancy
A flex redundancy disk group is a type of redundancy disk group with features such as flexible file redundancy, mirror splitting, and redundancy change. A flex disk group can consolidate files with different redundancy requirements into a single disk group. It also provides the capability for databases to change the redundancy of its files. A disk group is a collection of file groups, each associated with one database. A quota group defines the maximum storage space or quota limit of a group of databases within a disk group.
In a flex redundancy disk group, Oracle ASM uses three-way mirroring of Oracle ASM metadata to increase performance and provide reliability. For database data, you can choose no mirroring (unprotected), two-way mirroring (mirrored), or three-way mirroring (high). A flex redundancy disk group requires a minimum of three disk devices (or three failure groups).
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Extended redundancy
Extended redundancy disk group has similar features as the flex redundancy disk group. Extended redundancy is available when you configure an Oracle Extended Cluster. Extended redundancy extends Oracle ASM data protection to cover failure of sites by placing enough copies of data in different failure groups of each site. A site is a collection of failure groups. For extended redundancy with three sites, for example, two data sites, and one quorum failure group, the minimum number of disks is seven (three disks each for two data sites and one quorum failure group outside the two data sites). The maximum number of supported sites for extended redundancy is three. In an extended redundancy disk group, each site maintains the user data redundancy as specified by the file group attribute. Each site can host data failure groups and quorum failure groups for a given disk group. For example, if the file group redundancy is specified as 2 or 3, each site has 2 or 3 mirrors respectively, provided there are enough failure groups to accommodate the mirrors. See About Oracle Extended Clusters for more information about selecting redundancy levels for extended clusters.
Note:
You can alter the redundancy level of the disk group after a disk group is created. For example, you can convert a normal or high redundancy disk group to a flex redundancy disk group. Within a flex redundancy disk group, file redundancy can change among three possible values: unprotected, mirrored, or high.