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Oracle Tips by Burleson |
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.
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