External memory algorithm

External memory algorithms, also called out-of-core algorithms, are procedures designed for data sets that are too large to fit in a computer's main memory at one time. Because the data sit on slow bulk storage such as hard drives, tape or a network, the design goal changes: instead of counting arithmetic steps, the designer counts how many times blocks of data must be moved between the slow store and the fast memory. These algorithms are analysed in the external memory model, also called the I/O model or disk access model, which was introduced by Alok Aggarwal and Jeffrey Vitter in 1988. In that model a fast memory of limited size sits in front of an unbounded external memory, both divided into blocks, and the running time is the number of block transfers. The model underlies results such as searching with a B-tree and external sorting by merge or distribution methods, and it suits workloads like large digital elevation data sets.