Additionally, instance locks using the IDLM on the Oracle level must be maintained to ensure that all nodes in the cluster see identical data. There is additional overhead in maintaining the locks and ensuring that the data caches are consistent. The performance impact is dependent on the hardware and software components, such as the bandwidth of the high-speed bus through which the nodes communicate, and IDLM performance. Shared Disk Systems Parallel processing advantages and disadvantages of shared disk systems are described in this section. Advantages Shared disk systems permit high availability.
All data is accessible even if one node dies.
These systems have the concept of one database, which is an advantage over shared nothing systems. Shared disk systems provide for incremental growth. If the workload is not partitioned well, there may be high synchronization overhead. There is operating system overhead of running shared disk software. Shared Nothing Systems Shared nothing systems are typically loosely coupled.