In url: http://www.datastax.com/docs/datastax_enterprise3.2/solutions/about_hadoop
"Before starting an analytics/Hadoop node on a production cluster or data center, it is important to disable the virtual node configuration."
What will happen if I enable virtual node in an analytics/Hadoop node?
If you enable virtual nodes on hadoop node, it will lower performance of small Hadoop jobs by raising the number of mappers to at least the number of virtual nodes. E.g. if you use the default 256 vnodes / physical nodes setting, every Hadoop job will launch 257 mappers. Those mappers might have too little data to process and the server would spend most of the time managing those tasks instead of doing useful work.
On a decent hardware, a job with no data and 256 vnodes may take about 5-10 minutes, contrary to the same job requiring only about 20-40 seconds when configured without vnodes.
来源：https://stackoverflow.com/questions/19969329/why-not-enable-virtual-node-in-an-hadoop-node