Sorting Data larger than the RAM size

Question

This is a Google interview question: Given 2 machines, each having 64 GB RAM, containing all integers (8 byte), sort the entire 128 GB data. You may assume a small amount of add

Answer 1

Each of the 64 GB can be sorted using a quicksort separately and then using the external memory keep pointers at the heads of both 64GB array, lets consider we want RAM1 and RAM2 in that order to have the entire data, keep incrementing pointer at RAM1 if its smaller then the pointer value at RAM2 else swap the value with RAM2 until the pointer reached end of RAM1.

take the same concept to sort all N RAMs. Take pairs of them and sort using above method. You are left with N/2 sorted RAMs. Use the same concept above recursively.