A Node is a single computer in Hadoop cluster.
A Rack is a collection of nodes; usually 30 to 40 nodes. All the nodes in a Rack share the same network switch. All the nodes are physically located close to each other. The bandwidth speed among the nodes in a Rack is quick.
A node in a Rack can write or read data quickly from the node located the same rack. A rack when read or write data from a different rack is usually slow. The speed is not noticeable for technically it happens.
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