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. The cluster illustrated in Figure  3-2 is composed of multiple tightly coupled nodes. The IDLM is required. Examples of loosely coupled systems are VAXclusters or Sun clusters.