Suppose that a tree is too big to be kept in memory, and thus must be kept on a disk. A disk has large capacity (e.g. a terabyte, 1012 bytes) but slow access (e.g. 10 milliseconds). A computer can execute millions of instructions in the time required for one disk access.

We would like to minimize the number of disk accesses required to get to the data we want. One way to do this is to use a tree with a very high branching factor.

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