What kinds of I/O take place during the Sort and the Merge phases?
Since, during the sort phase, the runs are created using heapsort, I/O is sequential. No performance improvement can ever be gained in this phase.
During the reading step of the merge phase, there are a lot of random accesses (since the buffers containing the different runs get loaded and reloaded at unpredictable times). The number and size of the memory buffers holding the runs determine the number of random accesses. Performance improvements can be made in this step.
The write step of the merge phase, is not influenced by the way in which we organize the runs.