Examples of tightly coupled systems include the Pyramid, Sequent, and Sun SparcServer. It does not make sense to run Oracle Parallel Server on a single SMP machine, because the system would incur a great deal of unnecessary overhead from IDLM accesses. Performance is potentially limited in a tightly coupled system by a number of factors. This means that internal synchronization is faster than using the Lock Manager. Shared memory systems are easier to administer than a cluster.
Scalability is limited by bus bandwidth and latency, and by available memory.
Shared Disk Systems Shared disk systems are typically loosely coupled. Memory is not shared between nodes. Communication occurs over a common high-speed bus. Each node has access to the same disks and other resources. A node can be an SMP if the hardware supports it.