In the shared-disk architecture, only the disks are shared by all processors through the interconnection network. The main memory is not shared: each processor exclusive (non-shared) access to its main memory. Each processor-memory node is under the control of its own copy of the operating system. Since any processor can cache the same disk page, a cache coherency mechanism is necessary.
Shared-disk requires a cache coherency mechanism which allows different nodes to cache a consistent disk page. This function is hard to support and requires some form of distributed lock management. The most notable parallel database system which uses shared-disk is Oracle, with an efficient implementation of a distributed lock manager for cache consistency.
Shared-disk has a number of advantages: lower cost, good extensibility, availability, load balancing, and easy migration from centralized systems. The cost of the interconnection network is significantly less than with...