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What is Cluster Interconnect?

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

The cluster interconnect is a high bandwidth, low latency communication facility that connects each node to other nodes in the cluster, and routes messages and data among the nodes.

In general, the cluster-interconnect is used for the following high-level functions:

* Monitor Health, Status, and Synchronize messages

* Transport Distributed lock manager (DLM) messages

* Accessing remote file systems

* Move application-specific traffic

* Provide cluster alias routing

High performance computing involves distribution of the processing across an array of nodes in the clusters, and requires that the cluster interconnect provide high-data rate and low-latency communication between node processes. Also interconnects need to be capable of detecting and isolating faults, and of using alternative paths.

With the significant growth in the power of the processors or SMP servers, for a cluster to be successful and competitive over a large SMP machine, it should be able to move messages and data among the nodes at near the speed of the computational power of the SMP servers. Traditional network systems, using FDDI based on the TCP/IP protocol, are not able to achieve this satisfactorily. This situation has led to many vendors developing their own specialized interconnect products, which are usually a specialized set of devices in terms of switches, hubs, cables and software.

At a moderate performance level requirement, conventional networks such as Ethernet can be used as the cluster-interconnect to build clusters. However, in view of the heavy movement of the data across the nodes through the cluster interconnect; it is always desirable to utilize the specialized high-speed interconnect products as supplied by many vendors.

Essentials for Parallel Database Clusters

Parallel Clusters, much more than the HA clusters, 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 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 case of Failover Clusters, message traffic is usually confined to heart-beat messages and the volume of such messages is small.

In Parallel Databases, there is a great deal of message passing and data blocks or page transfers between the local cache from one node to another node. Much of the functionality and performance depends on the efficiency of the transport medium or methodology. It becomes very critical for overall performance of the cluster and usage of the parallel application that messages and blocks be transferred in as expeditious a time as possible between cluster nodes.

Key Measurements for Scalability

Low Latency: Provides fast (microsecond) passage of small (<512 bytes) lock messages between the nodes in the cluster.

High Bandwidth: Enable high throughput (Tens of GB per second) for larger Cache to Cache transfers (>4Kbytes) between nodes and between nodes and backend storage.

Low CPU Utilization: Minimum CPU and Memory Bus cycles devoted to communications to free up CPU cycles for additional database computing

As the Parallel Databases do not impose any constraints on the node to which users can connect and access, users have a choice to connect to any node in the cluster. Irrespective of the nature of application, OLTP or Data Warehousing databases, the movement of the data blocks from one node to another using the interconnect is widely practiced. The most significant role of the cluster interconnect and vendor provided interconnect software is to provide some kind of extended cache encompassing the data caches from all the nodes participating 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, which reduces the message-passing overhead and avoids unnecessary data copies in the operating system. As a result, these protocols enable lower communication latency and higher throughput.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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