You will not be able to compress the intermediate data.

You will longer be able to take advantage of a Combiner.

By using multiple reducers with the default HashPartitioner, output files may not be in globally sorted order.

There are no concerns with this approach. It is always advisable to use multiple reduces.

When the types of the reduce operation’s input key and input value match the types of the reducer’s output key and output value and when the reduce operation is both communicative and associative.

When the signature of the reduce method matches the signature of the combine method.

Always. Code can be reused in Java since it is a polymorphic object-oriented programming language.

Always. The point of a combiner is to serve as a mini-reducer directly after the map phase to increase performance.

Never. Combiners and reducers must be implemented separately because they serve different purposes.

Each DataNode receives commands from one designated master NameNode.

DataNodes send periodic heartbeats to all the NameNodes.

Each DataNode registers with all the NameNodes.

DataNodes send periodic block reports to all the NameNodes.