The RecordReader class

Unlike InputSplit, the RecordReader class presents a record view of the data to the Map task. RecordReader works within each InputSplit class and generates records from the data in the form of key-value pairs. The InputSplit boundary is a guideline for RecordReader and is not enforced. On one extreme, a custom RecordReader class can be written to read an entire file (though this is not encouraged). Most often, a RecordReader class will have to read from a subsequent InputSplit class to present the complete record to the Map task. This happens when records overlap InputSplit classes.

The reading of bytes from a subsequent InputSplit class happens via the FSDataInputS tream objects. Though this reading does not respect locality in itself, generally, it gathers only a few bytes from the next split and there is not a significant performance overhead. But in some cases where record sizes are huge, this can have a bearing on the performance due to significant byte transfers...