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Donald K. Burleson

Oracle RAC Tips

Essentials for Parallel Database Clusters

Parallel Clusters, much more than the HA cluster, rely on passing messages among the multiple processors located in the cluster nodes. Processors running parallel programs call for data and instructions, and then perform their calculations. Each processor checks periodically with the other nodes or a master node to plan its next move, or to synchronize the delivery of results. These activities rely on message passing software, such as industry-standard MPI. However, in the case of failover clusters, message traffic is usually confined to heartbeat messages, and the volume of these messages is small.

In parallel databases, there is a great deal of message passing and data block or page transfer from the ‘local cache’ between nodes. Much of the functionality and performance depends on the efficiency of the transport medium or methodology. It becomes critical for both overall performance of the cluster and usage of the parallel application that messages and blocks be transferred as expeditiously as possible between cluster nodes.

As the parallel databases do not impose any constraints on which node the user may connect to and access, users may connect to any node in the cluster. Irrespective of the nature of the application or the database (OLTP or data warehouse), use of the interconnect to move data blocks from one node to another is widespread. The most important function of the cluster interconnect and vendor-provided interconnect software is to provide an extended cache, which encompasses the data caches from all participating nodes in the parallel database cluster.

The usual implementation of a message passing system is based on the interconnect communication protocol. For example, a fast Ethernet or gigabit Ethernet interconnect runs on a TCP/IP or UDP/IP protocol-based system. Systems based on the Grand Message (GM) or the Virtual Interface Architecture (VIA) use Myrinet and Emulex (formerly Giganet) interconnects, respectively.

A lightweight communication protocol, such as GM or VIA, is more efficient than the conventional TCP/IP protocol. It allows user programs to communicate with the network interface card (NIC) directly, reducing message-passing overhead and avoiding unnecessary data copies in the operating system. As a result, these protocols enable lower communication latency and higher throughput.

For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.  


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