How do I specify external redundancy in an Oracle ASM create diskgroup command? What does external redundancy do?.

Question: How do I specify external redundancy in an Oracle ASM create diskgroup command? What does external redundancy do?. Answer: There are three types of Oracle ASM disk groups: Normal redundancy High redundancy External redundancy With normal and high redundancy, the disk group template specifies the ASM redundancy attributes for all files in the disk group. Configuration of ASM high redundancy provides a greater degree of protection. With external redundancy, ASM does not provide any redundancy for the disk group. In external redundancy, the underlying disks in the disk group must provide redundancy (for example, using a RAID storage array.) The redundancy level or type is specified at the time of creating the disk groups. It is possible to specify The "external redundancy" clause when creating n ASM diskgroup: CREATE DISKGROUP DATA EXTERNAL REDUNDANCY DISK '/dev/rdsk/*'; In the above examples, the EXTERNAL REDUNDANCY clause was used. This means that ASM will provide no extra redundancy support and, therefore, cannot help in the case of disk failure. This is usually an option only when using RAID 1 (mirroring). However, redundancy can be set up within ASM using failure groups. Failure groups allow a DBA to specify two pools of disks that can hold copies of each other's data. ASM allows NORMAL redundancy, in which a diskgroup is composed of two failgroups, or HIGH redundancy, in which a diskgroup is composed of three failgroups. When files are written to the ASM diskgroup under NORMAL redundancy, the files will be written to both failgroups in a round-robin fashion. Files are read from the primary failgroup. For example: CREATE DISKGROUP DATA NORMAL REDUNDANCY FAILGROUP failgrp1 DISK '/dev/sdb1', '/dev/sdb2' FAILGROUP failgrp2 DISK '/dev/sdc1', '/dev/sdc2'; In this example, a file will first go to failgrp1 and be copied to failgrp2. The next file will be written to failgrp2 and copied to failgrp1.