[Oracle ASM] Disk Group Redundancy Levels

 ASM Diskgroup Redundancy

Oracle ASM Diskgroup Redundancy Controls how the data is mirrored to protect against disk or hardware failure failures.

Failure group

Logical groups of disks that share a single failure point, such as a disk controller.
Mirror copies are always placed in different failure groups.

Redundancy Levels

External Redundancy

•  No Mirroring. So there is no redundancy on ASM Disk group
• One Failure Group.
• Uses underlying Hardware Redundancy (RAID 5, 10 etc.)

Normal Redundancy

• 2-Way Mirroring
• Requires Min. of 2 Failure Groups
• Stores 2 copies of every extent across different failure groups
• Can tolerate Failure of 1 failure Groups

High Redundancy

•  3-Way Mirroring
• Requires Min. of 3 Failure Groups
• Stores 3 copies of every extent across different failure groups
• Can tolerate Failure of 2 failure Groups

Flex Redundancy

• Allows mixed redundancy level within one diskgroup
• Allows defining specific redundancy level per file group
• Requires Min. of 3 Failure Groups
• Can tolerate Failure of 2 failure Groups, But if fewer than five are present, the disk group can only tolerate one failure.

File group

A group of files assigned to a database or PDB to manage space quotas and redundancy level independently.

Example

Create External Redundancy:

CREATE DISKGROUP data EXTERNAL REDUNDANCY DISK '/dev/disk1’;

Create Normal Redundancy (requires 2+ groups):

CREATE DISKGROUP data NORMAL REDUNDANCY
  FAILGROUP fg1 DISK '/dev/disk1'
  FAILGROUP fg2 DISK '/dev/disk2’;

Create High Redundancy (requires 3+ groups):

CREATE DISKGROUP data HIGH REDUNDANCY
  FAILGROUP fg1 DISK '/dev/disk1'
  FAILGROUP fg2 DISK '/dev/disk2'
  FAILGROUP fg3 DISK '/dev/disk3';

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