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Why anyone would need more than two node cluster?

Cluster to my understanding is to provide HA ( not load balancing ). Having multisite two node cluster should provide you enough HA.

I tried searching for answer but could not find any explanation.

Hope someone can shed some light.


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migrated from Jun 23 '11 at 17:18

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  • 3 huge databases
  • One databases per SQL Server Instance) per node
  • The 3 active nodes all fail onto a passive 4th


  • 75% utilisation of nodes
  • 100% of resources per instance per node

Having 3 active/active means you have to allow for the chance of all 3 instance running on one node = less resources can be allocated. There are ways aroound this but KISS.

In addition, you can run the 4th passive node as QA or test etc

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Clustering might provide HA, but it can also be used to provide some degree of load spreading. For instance our Hyper-V cluster is n+1 redundant. If one node goes down the VMs are started up on the other nodes. If we just did clusters of two nodes we would "waste" one whole 'backup' server for every 'primary' server.

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