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Oracle10g Grid Computing with RAC
Chapter 3 - High Performance and Highly Available
Database Clusters

Avoiding Split Brain 

STOMITH stands for ‘Shoot the Other Machine in the Head’. STOMITH fencing takes a completely different approach. In STOMITH systems, the errant cluster node is simply reset and forced to reboot. When it rejoins the cluster, it acquires resources in the normal way. In many cases, STOMITH operations are performed via smart power switches, which simply remove power from the errant node for a brief period of time. 

However, implementation of avoiding split brain varies from vendor to vendor, and also depends on the type of shared storage in use for the cluster. For example, Sun Cluster avoids split brain by using the majority vote principle coupled with quorum disks and Linux cluster using Polyserve Matrix Server employs fabric fencing.

Let us examine these techniques in detail.

I/O Fencing – exclusion strategy

There will be some situations where the leftover write operations from failed database instances (cluster function failed on the nodes, but the nodes are still running at OS level) reach the storage system after the recovery process starts. Since these write operations are no longer in the proper serial order, they can damage the consistency of the stored data. Therefore, when a cluster node fails, the failed node needs to be fenced off from all the shared disk devices or disk groups. This methodology is called ‘IO Fencing’, sometimes called Disk Fencing or failure fencing.

The main function of the IO fencing includes, a) preventing updates by failed instances, and b) to detect failure and prevent split brain in cluster. Cluster Volume Manager (in association with the shared storage unit) and Cluster File System play a significant role in preventing the failed nodes accessing shared devices.

For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.

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