Closely coupled systems aim at a more efficient communication and cooperation between processing nodes compared to loosely coupled systems. This can be achieved by using globally shared semiconductor memory to speed up the exchange of messages or to store global data structures. For distributed database processing, the database sharing (shared disk) architecture can benefit most from such a close coupling. The author presents a detailed simulation study of closely coupled database sharing systems. A shared store called global extended memory (GEM) was used for system-wide concurrency and coherency control, and to improve input/output (I/O) performance. The performance of such an architecture is evaluated and compared with loosely coupled database sharing systems employing the primary copy approach for concurrency and coherency control. In particular, the impact of different update strategies (FORCE vs. NOFORCE) and workload allocation schemes (random vs. affinity-based routing) is studied. The use of shared disk caches implementing a global database buffer is also considered. Simulation results are presented for synthetically generated debit-credit workloads and a real-life workload represented by a database trace.