An Oracle ASM extended disk group has all the features of a flex disk group, with a specific design to be highly available in an extended (stretch) cluster environment, which contains nodes that span multiple physically separated sites.
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The following are characteristics of an extended disk group:
The redundancy setting of an extended disk group is set to EXTENDED REDUNDANCY and each file group of the extended disk group has its own redundancy property setting.
The redundancy of files and file groups in an extended disk group is specified for each site, rather than for each disk group.
This means that the number of copies determined by the REDUNDANCY setting is multiplied by the number of data sites. If the REDUNDANCY is set to MIRROR, then that specifies two copies, resulting in four copies total for two data sites.
An extended disk group can tolerate the loss of an entire site, plus the loss of up to two failure groups in another site.
This means that an extended disk group can tolerate failures at the site level in addition to the failure group level.
All data sites must have the same number of failure groups during disk group creation.
A quota group is specified for each disk group, rather than for each site.
The quota group limit is the physical space required for all copies across all sites.
For example, with two data sites, a 6 M file with redundancy set to MIRROR uses 24 M of the quota limit.
There must be a total of three sites, two data sites and one quorum site, to create an extended disk group. Each data site should have three failure groups, and the quorum site should have one failure group.
For database files, the redundancy setting is dictated by the redundancy setting of the file groups of the extended disk group.
The COMPATIBLE.ASM and COMPATIBLE.RDBMS disk group attributes must be set to 12.2 or greater.
The minimum allocation unit (AU) size is 4 M.
Converting from another disk group type to an extended disk group is not supported.
The SITE specification for failure groups can be inferred from cells if Oracle Exadata has been configured. Otherwise, the site specification can be specified with the SQL statement when creating a disk group.