How ASM Failure Groups and CSS provides high availability

What is ASM Failure Groups

Failure groups are used to store mirror copies of data. When ASM allocates an extent for a normal redundancy file, ASM allocates a primary copy and a secondary copy. ASM chooses the disk on which to store the secondary copy so that it is in a different failure group than the primary copy. Each copy is on a disk in a different failure group so that the simultaneous failure of all disks in a failure group does not result in data loss.

A failure group is a subset of the disks in a disk group, which could fail at the same time because they share hardware. The failure of common hardware must be tolerated.

There are always failure groups even if they are not explicitly created. If you do not specify a failure group for a disk, then Oracle automatically creates a new failure group containing just that disk. A normal redundancy disk group must contain at least two failure groups. A high redundancy disk group must contain at least three failure groups. However, Oracle recommends using several failure groups. A small number of failure groups, or failure groups of uneven capacity, can create allocation problems that prevent full use of all of the available storage.

ASM Fast Disk Resync
Disk loss in ASM can result from a number of reasons, such as loss of controller cards, cable failures, or power-supply errors. In many cases, the disk itself is still intact. To allow for sufficient time to recover from disk failures that do not involve the actual failure of a disk, ASM provides the ASM fast disk resync feature.
By default, when a disk in an ASM disk group fails the disk will be taken offline automatically. The disk will be dropped some 3.6 hours later. As a result, you have only 3.6 hours by default to respond to a disk outage. If you correct the problem and the physical disk media is not corrupted, then ASM fast disk resync will quickly re-synchronize the disk when it comes back online, correcting the problem very quickly.You can change the amount of time that Oracle will wait to automatically drop the disk by setting the disk_repair_time attribute for the individual disk groups using the alter diskgroup command, as shown in this example, where we set the disk_repair_time attribute to 18 hours:
SQL> Alter diskgroup dgroup1 set attribute ‘disk_repair_time’=’10h’;
ASM Preferred Mirror Read

Your ASM configuration may involve remote mirroring to disks that are a fair distance away. When some of your disk mirrors are far away then those disks may not be the best set of disks for a given instance to read from. For example, you might have a Real Application Cluster database with local and remote mirrored disks. In this case, you want to have the RAC instances primarily read from the local disks to ensure the best performance. ASM preferred mirror read is designed to indicate to Oracle which disk fail-group is the preferred read disk group.
ASM preferred mirror read is only available if you are using RAC. Also preferred mirror read is generally used only with clustered ASM instances, but this is not a requirement. To take advantage of ASM preferred mirror read, you should configure each disk failure group with specific geographically located set of disks. Use the Oracle 11g parameter, asm_preferred_read_failure_groups, to configure a database instance with a list of preferred disk failure group names to use when that instance accesses ASM disks. The format of the values of the asm_preferred_read_failure_groups parameter is diskgroup name.failure group name where diskgroup name is the name of the disk group that the failure group belongs to and failure group name is the preferred failure groups name.

In the event ASM cannot read from the preferred disk failure group, then the non-preferred failure groups will be read. To determine if a given disk file group is a preferred disk group you can use the PREFERRED_READ column of the V$ASM_DISK view.

Example of failure groups

Creating diskgroups for high and normal redundancy

CREATE DISKGROUP DATA NORMAL REDUNDANCY
FAILGROUP DATA_FAILURE_group_1 DISK
'/dev/5855d' , '/dev/6476d' ,
FAILGROUP DATA_FAILURE_group_2 DISK
'/dev/5853d' , '/dev/5854d' ;

For two-way mirroring we would expect a diskgroup to contain two failure groups, so individual files are written to two locations.

CREATE DISKGROUP DATA HIGH REDUNDANCY
FAILGROUP DATA_FAILURE_group_1 DISK
'/dev/5851d', '/dev/5852d',
FAILGROUP DATA_FAILURE_group_2 DISK
'/dev/5853d', '/dev/5854d',
FAILGROUP DATA_FAILURE_group_3 DISK
'/dev/5855d', '/dev/5856d';

For three-way mirroring we would expect a diskgroup to contain three failure groups, so individual files are written to three locations.

$ORACLE_HOME/bin/localconfig reset