Other Algorithms
External merge sort is not the only external sorting algorithm; there are also distribution sorts, which work by partitioning the unsorted values into smaller "buckets" that can be sorted in main memory. Like merge sort, external distribution sort also has a main-memory sibling; see bucket sort. There is a duality, or fundamental similarity, between merge- and distribution-based algorithms that can aid in thinking about sorting and other external memory algorithms. There are in-place algorithms for external sort, which require no more disk space than the original data.
Read more about this topic: External Sorting