Sunday, May 22, 2011

Global Cache Service Processes in a Oracle Cluster (RAC) with Multiple Databases

Creating a cluster with a single pool of storage managed by Oracle ASM provides the infrastructure to manage multiple databases whether they are single instance databases or Oracle RAC databases.

With Oracle RAC databases, you can adjust the number of instances and which nodes run instances for a given database, based on workload requirements. Features such as cluster-managed services allow you to manage multiple workloads on a single database or across multiple databases. It is important to properly manage the capacity in the cluster when adding work. The processes that manage the cluster—including processes both from Oracle Clusterware and database—must be able to obtain CPU resources in a timely fashion and must be given higher priority in the system.

Oracle recommends that the number of real time Global Cache Service Processes (LMSn) on a server is less than or equal to the number of processors. (Note that this is the number of recognized CPUs that includes cores. For example, a dual-core CPU is considered to be two CPUs.).

Consolidating many small databases into a cluster, you may want to reduce the number of LMSn created by the Oracle RAC instance. By default, Oracle Database calculates the number of processes based on the number of CPUs it finds on the server. This calculation may result in more LMSn processes than is needed for the Oracle RAC instance. One LMS process may be sufficient for up to 4 CPUs.To reduce the number of LMSn processes, set the GC_SERVER_PROCESSES initialization parameter minimally to a value of 1. Add a process for every four CPUs needed by the application. In general, it is better to have few busy LMSn processes. Oracle Database calculates the number of processes when the instance is started, and you must restart the instance if you want to change the value.