Choosing High vs Normal ASM Redundancy with Exadata

Every time I go through an Exadata deployment process with my client, there is a discussion about ASM Redundancy level. As many of you already know that Exadata only supports two ASM redundancy levels (Normal or High) and Oracle Recommends using a High Redundancy level for both DATA and RECO disk groups. Keep in mind that changing the redundancy level will require recreating disk groups.

A brief description about respective redundancy levels is as follows:

*NORMAL redundancy provides protection against a single disk failure or an entire storage server failure.

*HIGH redundancy provides protection against 2 simultaneous disk failures from 2 distinct storage servers or 2 entire storage servers. HIGH redundancy provides redundancy during Exadata storage server rolling upgrades.

Choosing redundancy level for your Exadata machine will depend on your database environment, available capacity, and desired protection level. Some databases are critical and need a HIGH redundancy disk group while most other databases can use NORMAL redundancy disk groups. So if you choose Normal redundancy, it will not be against the norm but you will not be following Oracle recommendations. I have seen clients using Normal Redundancy more often than I want to. Following are some reasons where you should always use High Redundancy level:

  • If it is a production system with no DR in place.
  • If your storage requirement is low and using HP capacity disks
  • If you want to perform storage server rolling upgrades.

Now following are some situations where you can use Normal redundancy:

  • If it is a Dev or UAT system.
  • If you are space constrained.
  • If you have Data Guard in place for production databases.

NOTE: Standard Exadata deployment will create 3 disk groups (DATA, RECO and DBFS_DG), but you can create additional disk groups with different redundancy levels based on your requirement.