Preferred Read Failure Groups

In a multisite cluster, with separate storage at each site, preferred read failure groups allow every node in the cluster to read from its local disks resulting in better performance.

Before Oracle Database 11g, ASM always read the primary copy of a mirrored extent. It may be more efficient for a node to read from a failure group extent that is closest to the node, even if it is a secondary extent. This is especially true in extended (multisite) cluster configurations where reading from a local copy of an extent provides improved performance.

With Oracle Database 11g, you can do this by configuring the preferred mirror read using the new initialization parameter, ASM_PREFERRED_READ_FAILURE_GROUPS. The disks in the identified failure groups become the preferred read disks. Thus, every node can be configured to read from its local extent copy. This results in higher efficiency and performance, and reduced network traffic. The setting for this parameter is instance specific.

If there is more than one mirrored copy and you have set a value for the ASM_PREFERRED_READ_FAILURE_GROUPS parameter, ASM first reads the copy that resides on a preferred read disk. If that read fails, ASM attempts to read from another mirrored copy that might not be on a local preferred read disk.

You can use the PREFERRED_READ column in the V$ASM_DISK view to determine whether a particular disk belongs to a preferred read failure group.

To identify performance issues with the ASM preferred read failure groups, use the V$ASM_DISK_IOSTAT view. This view displays input/output statistics for each ASM client.

+ASM1.ASM_PREFERRED_READ_FAILURE_GROUPS = DG1.F1, DG2.F3

+ASM2.ASM_PREFERRED_READ_FAILURE_GROUPS = DG1.F2, DG2.F4