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Introduction to Linux Clustering

servers.

Other fencing devices include fencing at

the SAN level as well as Xen VM fencing,

however it is recommended that power

fencing be used rather than SAN fencing

as it will guarantee that the node is

completely killed and not doing anything

unwanted.

4.2.3 SPLIT-BRAIN

Split-brain is a nasty problem found in clustering, and requires careful thought to prevent.

Consider the following scenario:

1. A two node cluster exists – one server in city A, one server in city B.

2. The internet link between the two cities falls over. Neither server can contact each

other.

3. Each server assumes the other one is down, and both resume activities as the master.

4. When the link comes back online, data corruption occurs.

Other nasty problems can occur if the two nodes are still able to fence each other via the out-

of-band management system, as you may end up with each node repeatedly powering off the

other node.

To prevent this from happening, we have a solution called “Quorum”.

4.2.4 QUORUM

Quorum is effectively a scoring method, where each node in the cluster has a number of

votes (by default one). Each on-line cluster node adds it's votes to the quorum count, and as

long as the quorum count is larger than 50% of the combined votes, the cluster is intact.

If the cluster falls below quorum, the cluster has “lost quorum” and all services will shut-

down and become unavailable.

This is actually a desired feature. Consider a cluster with 10 servers and to maintain quorum,

a score of 6 is needed. In the event of the network suffering a failure and causing the cluster

to spit into two, the smaller half will shut-down and the larger half will continue on. This

prevents split-brain in clusters.

What about situations where there are an even number of nodes, such as 10 nodes? It would

be possible for the network to split into two equally sized clusters. Therefore, any cluster

with an even number of nodes requires at one of the nodes to have an additional vote in the

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Red Hat CLUSTER SUITE FOR ENTERPRISE LINUX 5.1 Manuale Utente Pagina 10