Its depends, anyway assuming when you say "rac" you're talking about "rack", a good practice is to have part of the hosts that make part of a cluster in different racks and different power sources, because if all resides in the same rack, a failure in that rack can bring the entire cluster down. Putting hosts in different rack also make easy maintenance in racks when you need to for example move the rack to a different location... so if cluster spread across multiple racks, just vMotion virtual machines to host in different rack and than you can make the maintenance in that rack.
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Richardson Porto
Senior Infrastructure Specialist
LinkedIn: http://linkedin.com/in/richardsonporto