An Oracle ASM flex disk group is a disk group type that supports Oracle ASM file groups and quota groups.
In general, a flex disk group enables users to manage storage at the granularity of the database, in addition to at the disk group level.
The redundancy of files in a flex disk group is flexible and enables storage management at the database level.
File groups of flex disk groups are used to describe database files.
Each database has its own file group, and storage management can be done at the file group level, in addition to the disk group level. For example, you can specify different redundancy and rebalance settings for different file groups. File groups are associated with a quota group, enabling easy quota management.
The redundancy setting of a flex disk group is set to FLEX REDUNDANCY and each file group of the flex disk group has its own redundancy property setting.
A flex disk group generally tolerates two failures, the same as a HIGH redundancy disk group.
However, if the disk group has fewer than five failure groups, then it can only tolerate one failure. This restriction is the result of the quorum requirement of the disk group.
The failure tolerated by a database is dependent on the redundancy setting of its associated file group in the flex disk group.
A flex disk group requires a minimum of three failure groups.
If a disk group with FLEX REDUNDANCY has only two regular failure groups and one quorum failure group, then only two copies of the data are stored, even if HIGH REDUNDANCY is selected for a database file in the flex disk group. This behavior is a consequence of having only two regular failure groups. However, if an additional regular failure group is added later, then a third copy of the data is created automatically. After the addition of the regular failure group, if one failure group is lost and a rebalance completes successfully, then the disk group can remain mounted after a second failure group goes offline.
Migrating (converting) to a flex disk group can be performed for a NORMAL or HIGH redundancy disk group with the CONVERT REDUNDANCY TO FLEX clause of the ALTER DISKGROUP SQL statement. You cannot migrate an EXTERNAL redundancy disk group to a FLEX redundancy disk group.
Note:
When migrating to a flex disk group, a disk group with NORMAL redundancy must have a minimum of three failure groups and a disk group with HIGH redundancy must have a minimum of five failure groups.
The Virtual Allocation Metadata (VAM) migration disk group attribute must be enabled and the migration completed before migrating without the use of a restrictive mount.
A flex disk group enables the creation of point-in-time database clones.
A point-in-time database clone is a full copy of a specific database, not the underlying storage system. Only data files are copied. Other files are created, or referenced as required to create the clone. For information about flex disk groups and point-in-time database clones, refer to
About Point-In-Time Database Cloning With Oracle ASM Flex Disk Groups.
The default size of the allocation unit (AU) is 4 M.
The Virtually Allocated Metadata (VAM) is enabled and required for a flex disk group.
Flex disk groups require the COMPATIBLE.ASM and COMPATIBLE.RDBMS disk group attributes to be set at 12.2 or greater.
To create an Oracle ADVM volume on a flex disk group, the COMPATIBLE.ADVM disk group attribute must be set to 12.2 or greater.
The REQUIRED_MIRROR_FREE_MB and USABLE_FILE_MB columns in the V$ASM_DISKGROUP view report no values for a flex disk group.
Example 4-14 Using ALTER DISKGROUP to migrate a normal disk group to a flex disk group