Abstract: A method is provided for estimating the size of cache memory required for optimal performance of a network database service, such as a directory service, by means of an iterative process. In the estimation process, the memory size N for best-case performance (i.e., the memory size that avoids any disk I/O operation) is first determined. The allocated memory size is then given a starting value. The probability (p) of cache-miss is then estimated for that memory size. Another probability (q), which is the probability that a record requested by a frequent user of the service is not in the cache, is also estimated for the memory size. The performance impact of the disk I/O rate indicated by p and q is then evaluated. If the performance is not adequate, the cache memory size is adjusted to a different value. The miss probabilities p and q are again estimated, and the performance impact is estimated. This iterative process is continued until the cache memory size is found to provide adequate estimated performance.